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The PE File Format by B. Luevelsmeyer

$Id: pe.txt,v 1.9 1999/03/20 23:55:09 LUEVELSMEYER Exp $



The PE file format
==================



Preface
-------

The PE ("portable executable") file format is the format of executable
binaries (DLLs and programs) for MS windows NT, windows 95 and
win32s; in windows NT, the drivers are in this format, too.
It can also be used for object files and libraries.

The format is designed by Microsoft and standardized by the TIS (tool
interface standard) Committee (Microsoft, Intel, Borland, Watcom, IBM
and others) in 1993, apparently based on a good knowledge of COFF, the
"common object file format" used for object files and executables on
several UNIXes and on VMS.

The win32 SDK includes a header file <winnt.h> containing #defines and
typedefs for the PE-format. I will mention the struct-member-names and
#defines as we go.

You may also find the DLL "imagehelp.dll" to be helpful. It is part of
windows NT, but documentation is scarce. Some of its functions are
described in the "Developer Network".



General Layout
--------------

At the start of a PE file we find an MS-DOS executable ("stub"); this
makes any PE file a valid MS-DOS executable. 

After the DOS-stub there is a 32-bit-signature with the magic number
0x00004550 (IMAGE_NT_SIGNATURE).

Then there is a file header (in the COFF-format) that tells on which
machine the binary is supposed to run, how many sections are in it, the
time it was linked, whether it is an executable or a DLL and so on. (The
difference between executable and DLL in this context is: a DLL can not
be started but only be used by another binary, and a binary cannot link
to an executable).

After that, we have an optional header (it is always there but still
called "optional" - COFF uses an "optional header" for libraries but not
for objects, that's why it is called "optional"). This tells us more
about how the binary should be loaded: The starting address, the amount
of stack to reserve, the size of the data segment etc..

An interesting part of the optional header is the trailing array of
'data directories'; these directories contain pointers to data in the
'sections'. If, for example, the binary has an export directory, you
will find a pointer to that directory in the array member
IMAGE_DIRECTORY_ENTRY_EXPORT, and it will point into one of the
sections.

Following the headers we find the 'sections', introduced by the 'section
headers'. Essentially, the sections' contents is what you really need to
execute a program, and all the header and directory stuff is just there
to help you find it.
Each section has some flags about alignment, what kind of data it
contains ("initialized data" and so on), whether it can be shared etc.,
and the data itself. Most, but not all, sections contain one or more
directories referenced through the entries of the optional header's
"data directory" array, like the directory of exported functions or the
directory of base relocations. Directoryless types of contents are, for
example, "executable code" or "initialized data".

    +-------------------+
    | DOS-stub          |
    +-------------------+
    | file-header       |
    +-------------------+
    | optional header   |
    |- - - - - - - - - -|
    |                   |
    | data directories  |
    |                   |
    +-------------------+
    |                   |
    | section headers   |
    |                   |
    +-------------------+
    |                   |
    | section 1         |
    |                   |
    +-------------------+
    |                   |
    | section 2         |
    |                   |
    +-------------------+
    |                   |
    | ...               |
    |                   |
    +-------------------+
    |                   |
    | section n         |
    |                   |
    +-------------------+



DOS-stub and Signature
----------------------

The concept of a DOS-stub is well-known from the 16-bit-windows-
executables (which were in the "NE" format). The stub is used for
OS/2-executables, self-extracting archives and other applications, too.
For PE-files, it is a MS-DOS 2.0 compatible executable that almost
always consists of about 100 bytes that output an error message such as
"this program needs windows NT".
You recognize a DOS-stub by validating the DOS-header, being a
struct IMAGE_DOS_HEADER. The first 2 bytes should be the sequence "MZ"
(there is a #define IMAGE_DOS_SIGNATURE for this WORD).
You distinguish a PE binary from other stubbed binaries by the trailing
signature, which you find at the offset given by the header member
'e_lfanew' (which is 32 bits long beginning at byte offset 60). For OS/2
and windows binaries, the signature is a 16-bit-word; for PE files, it
is a 32-bit-longword aligned at a 8-byte-boundary and having the value
IMAGE_NT_SIGNATURE #defined to be 0x00004550.



File Header
-----------

To get to the IMAGE_FILE_HEADER, validate the "MZ" of the DOS-header
(1st 2 bytes), then find the 'e_lfanew' member of the DOS-stub's header
and skip that many bytes from the beginning of the file. Verify the
signature you will find there. The file header, a struct
IMAGE_FILE_HEADER, begins immediatly after it; the members are described
top to bottom.

The first member is the 'Machine', a 16-bit-value indicating the system
the binary is intended to run on. Known legal values are

    IMAGE_FILE_MACHINE_I386 (0x14c)
        for Intel 80386 processor or better

    0x014d
        for Intel 80486 processor or better

    0x014e
        for Intel Pentium processor or better

    0x0160
        for R3000 (MIPS) processor, big endian

    IMAGE_FILE_MACHINE_R3000 (0x162)
        for R3000 (MIPS) processor, little endian

    IMAGE_FILE_MACHINE_R4000 (0x166)
        for R4000 (MIPS) processor, little endian

    IMAGE_FILE_MACHINE_R10000 (0x168)
        for R10000 (MIPS) processor, little endian

    IMAGE_FILE_MACHINE_ALPHA (0x184)
        for DEC Alpha AXP processor

    IMAGE_FILE_MACHINE_POWERPC (0x1F0)
        for IBM Power PC, little endian

Then we have the 'NumberOfSections', a 16-bit-value. It is the number of
sections that follow the headers. We will discuss the sections later.

Next is a timestamp 'TimeDateStamp' (32 bit), giving the time the file
was created. You can distinguish several versions of the same file by
this value, even if the "official" version number was not altered. (The
format of the timestamp is not documented except that it should be
somewhat unique among versions of the same file, but apparently it is
'seconds since January 1 1970 00:00:00' in UTC - the format used by most
C compilers for the time_t.)
This timestamp is used for the binding of import directories, which will
be discussed later.
Warning: some linkers tend to set this timestamp to absurd values which
are not the time of linking in time_t format as described.

The members 'PointerToSymbolTable' and 'NumberOfSymbols' (both 32 bit)
are used for debugging information. I don't know how to decipher them,
and I've found the pointer to be always 0.

'SizeOfOptionalHeader' (16 bit) is simply sizeof(IMAGE_OPTIONAL_HEADER).
You can use it to validate the correctness of the PE file's structure.

'Characteristics' is 16 bits and consists of a collection of flags, most
of them being valid only for object files and libraries:

    Bit 0 (IMAGE_FILE_RELOCS_STRIPPED) is set if there is no relocation
    information in the file. This refers to relocation information per
    section in the sections themselves; it is not used for executables,
    which have relocation information in the 'base relocation' directory
    described below.

    Bit 1 (IMAGE_FILE_EXECUTABLE_IMAGE) is set if the file is
    executable, i.e. it is not an object file or a library. This flag
    may also be set if the linker attempted to create an executable but
    failed for some reason, and keeps the image in order to do e.g.
    incremental linking the next time.

    Bit 2 (IMAGE_FILE_LINE_NUMS_STRIPPED) is set if the line number
    information is stripped; this is not used for executable files.

    Bit 3 (IMAGE_FILE_LOCAL_SYMS_STRIPPED) is set if there is no
    information about local symbols in the file (this is not used
    for executable files).

    Bit 4 (IMAGE_FILE_AGGRESIVE_WS_TRIM) is set if the operating system
    is supposed to trim the working set of the running process (the
    amount of RAM the process uses) aggressivly by paging it out. This
    should be set if it is a demon-like application that waits most of
    the time and only wakes up once a day, or the like.

    Bits 7 (IMAGE_FILE_BYTES_REVERSED_LO) and 15
    (IMAGE_FILE_BYTES_REVERSED_HI) are set if the endianess of the file is
    not what the machine would expect, so it must swap bytes before
    reading. This is unreliable for executable files (the OS expects
    executables to be correctly byte-ordered).

    Bit 8 (IMAGE_FILE_32BIT_MACHINE) is set if the machine is expected
    to be a 32 bit machine. This is always set for current
    implementations; NT5 may work differently.

    Bit 9 (IMAGE_FILE_DEBUG_STRIPPED) is set if there is no debugging
    information in the file. This is unused for executable files.
    According to other information ([6]), this bit is called "fixed" and
    is set if the image can only run if it is loaded at the preferred
    load address (i.e. it is not relocatable).

    Bit 10 (IMAGE_FILE_REMOVABLE_RUN_FROM_SWAP) is set if the application
    may not run from a removable medium such as a floppy or a CD-ROM. In
    this case, the operating system is advised to copy the file to the
    swapfile and execute it from there.

    Bit 11 (IMAGE_FILE_NET_RUN_FROM_SWAP) is set if the application may
    not run from the network. In this case, the operating system is
    advised to copy the file to the swapfile and execute it from there.

    Bit 12 (IMAGE_FILE_SYSTEM) is set if the file is a system file such
    as a driver. This is unused for executable files; it is also not
    used in all the NT drivers I inspected.

    Bit 13 (IMAGE_FILE_DLL) is set if the file is a DLL.

    Bit 14 (IMAGE_FILE_UP_SYSTEM_ONLY) is set if the file is not
    designed to run on multiprocessor systems (that is, it will crash
    there because it relies in some way on exactly one processor).



Relative Virtual Addresses
--------------------------

The PE format makes heavy use of so-called RVAs. An RVA, aka "relative
virtual address", is used to describe a memory address if you don't know
the base address. It is the value you need to add to the base address to
get the linear address.
The base address is the address the PE image is loaded to, and may vary
from one invocation to the next.

Example: suppose an executable file is loaded to address 0x400000 and
execution starts at RVA 0x1560. The effective execution start will then
be at the address 0x401560. If the executable were loaded to 0x100000,
the execution start would be 0x101560.

Things become complicated because the parts of the PE-file (the
sections) are not necessarily aligned the same way the loaded image is.
For example, the sections of the file are often aligned to
512-byte-borders, but the loaded image is perhaps aligned to
4096-byte-borders. See 'SectionAlignment' and 'FileAlignment' below.

So to find a piece of information in a PE-file for a specific RVA,
you must calculate the offsets as if the file were loaded, but skip
according to the file-offsets.
As an example, suppose you knew the execution starts at RVA 0x1560, and
want to diassemble the code starting there. To find the address in the
file, you will have to find out that sections in RAM are aligned to 4096
bytes and the ".code"-section starts at RVA 0x1000 in RAM and is 16384
bytes long; then you know that RVA 0x1560 is at offset 0x560 in that
section. Find out that the sections are aligned to 512-byte-borders in
the file and that ".code" begins at offset 0x800 in the file, and you
know that the code execution start is at byte 0x800+0x560=0xd60 in the
file.

Then you disassemble and find an access to a variable at the linear
address 0x1051d0. The linear address will be relocated upon loading the
binary and is given on the assumption that the preferred load address is
used. You find out that the preferred load address is 0x100000, so we
are dealing with RVA 0x51d0. This is in the data section which starts at
RVA 0x5000 and is 2048 bytes long. It begins at file offset 0x4800.
Hence. the veriable can be found at file offset
0x4800+0x51d0-0x5000=0x49d0.


Optional Header
---------------

Immediatly following the file header is the IMAGE_OPTIONAL_HEADER
(which, in spite of the name, is always there). It contains
information about how to treat the PE-file exactly. We'll also have the
members from top to bottom.

The first 16-bit-word is 'Magic' and has, as far as I looked into
PE-files, always the value 0x010b.

The next 2 bytes are the version of the linker ('MajorLinkerVersion' and
'MinorLinkerVersion') that produced the file. These values, again, are
unreliable and do not always reflect the linker version properly.
(Several linkers simply don't set this field.)
And, coming to think about it, what good is the version if you have got
no idea *which* linker was used?

The next 3 longwords (32 bit each) are intended to be the size of the
executable code ('SizeOfCode'), the size of the initialized data
('SizeOfInitializedData', the so-called "data segment"), and the size of
the uninitialized data ('SizeOfUninitializedData', the so-called "bss
segment"). These values are, again, unreliable (e.g. the data segment
may actually be split into several segments by the compiler or linker),
and you get better sizes by inspecting the 'sections' that follow the
optional header.

Next is a 32-bit-value that is a RVA. This RVA is the offset to the
codes's entry point ('AddressOfEntryPoint').
Execution starts here; it is e.g. the address of a DLL's LibMain() or a
program's startup code (which will in turn call main()) or a driver's
DriverEntry(). If you dare to load the image "by hand", you call this
address to start the process after you have done all the fixups and the
relocations.

The next 2 32-bit-values are the offsets to the executable code
('BaseOfCode') and the initialized data ('BaseOfData'), both of them
RVAs again, and both of them being of little interest because you get
more reliable information by inspecting the 'sections' that follow the
headers.
There is no offset to the uninitialized data because, being
uninitialized, there is little point in providing this data in the
image.

The next entry is a 32-bit-value giving the preferred (linear) load
address ('ImageBase') of the entire binary, including all headers. This
is the address (always a multiple of 64 KB) the file has been relocated
to by the linker; if the binary can in fact be loaded to that address,
the loader doesn't need to relocate the file again, which is a win in
loading time.
The preferred load address can not be used if another image has already
been loaded to that address (an "address clash", which happens quite
often if you load several DLLs that are all relocated to the linker's
default), or the memory in question has been used for other purposes
(stack, malloc(), uninitialized data, whatever). In these cases, the
image must be loaded to some other address and it needs to be relocated
(see 'relocation directory' below). This has further consequences if the
image is a DLL, because then the "bound imports" are no longer valid,
and fixups have to be made to the binary that uses the DLL - see 'import
directory' below.

The next 2 32-bit-values are the alignments of the PE-file's sections in
RAM ('SectionAlignment', when the image has been loaded) and in the file
('FileAlignment'). Usually both values are 32, or FileAlignment is 512
and SectionAlignment is 4096. Sections will be discussed later.

The next 2 16-bit-words are the expected operating system version
('MajorOperatingSystemVersion' and 'MinorOperatingSystemVersion' [they
_do_ like self-documenting names at MS]). This version information is
intended to be the operating system's (e.g. NT or Win95) version, as
opposed to the subsystem's version (e.g. Win32); it is often not
supplied, or wrong supplied. The loader doesn't use it, apparently.

The next 2 16-bit-words are the binary's version, ('MajorImageVersion' and
'MinorImageVersion'). Many linkers don't set this information correctly
and many programmers don't bother to supply it, so it is better to rely
on the version-resource if one exists.

The next 2 16-bit-words are the expected subsystem version
('MajorSubsystemVersion' and 'MinorSubsystemVersion'). This should be
the Win32 version or the POSIX version, because 16-bit-programs or
OS/2-programs won't be in PE-format, obviously.
This subsystem version should be supplied correctly, because it *is*
checked and used:
If the application is a Win32-GUI-application and runs on NT4, and the
subsystem version is *not* 4.0, the dialogs won't be 3D-style and
certain other features will also work "old-style" because the
application expects to run on NT 3.51, which had the program manager
instead of explorer and so on, and NT 4.0 will mimic that behaviour as
faithfully as possible.

Then we have a 'Win32VersionValue' of 32 bits. I don't know what it is
good for. It has been 0 in all the PE files that I inspected.

Next is a 32-bits-value giving the amount of memory the image will need,
in bytes ('SizeOfImage'). It is the sum of all headers' and sections'
lengths if aligned to 'SectionAlignment'. It is a hint to the loader how
many pages it will need in order to load the image.

The next thing is a 32-bit-value giving the total length of all headers
including the data directories and the section headers
('SizeOfHeaders'). It is at the same time the offset from the beginning
of the file to the first section's raw data.

Then we have got a 32-bit-checksum ('CheckSum'). This checksum is, for
current versions of NT, only checked if the image is a NT-driver (the
driver will fail to load if the checksum isn't correct). For other
binary types, the checksum need not be supplied and may be 0.
The algorithm to compute the checksum is property of Microsoft, and they
won't tell you. However, several tools of the Win32 SDK will compute
and/or patch a valid checksum, and the function CheckSumMappedFile() in
the imagehelp.dll will do so too.
The checksum is supposed to prevent loading of damaged binaries that
would crash anyway - and a crashing driver would result in a BSOD, so
it is better not to load it at all.

Then there is a 16-bit-word 'Subsystem' that tells in which of the
NT-subsystems the image runs:

    IMAGE_SUBSYSTEM_NATIVE (1)
        The binary doesn't need a subsystem. This is used for drivers.
    
    IMAGE_SUBSYSTEM_WINDOWS_GUI (2)
        The image is a Win32 graphical binary. (It can still open a
        console with AllocConsole() but won't get one automatically at
        startup.)
    
    IMAGE_SUBSYSTEM_WINDOWS_CUI (3)
        The binary is a Win32 console binary. (It will get a console
        per default at startup, or inherit the parent's console.)
    
    IMAGE_SUBSYSTEM_OS2_CUI (5)
        The binary is a OS/2 console binary. (OS/2 binaries will be in
        OS/2 format, so this value will seldom be used in a PE file.)
    
    IMAGE_SUBSYSTEM_POSIX_CUI (7)
        The binary uses the POSIX console subsystem.

Windows 95 binaries will always use the Win32 subsystem, so the only
legal values for these binaries are 2 and 3; I don't know if "native"
binaries on windows 95 are possible.

The next thing is a 16-bit-value that tells, if the image is a DLL, when
to call the DLL's entry point ('DllCharacteristics'). This seems not to
be used; apparently, the DLL is always notified about everything.
    If bit 0 is set, the DLL is notified about process attachment (i.e.
        DLL load).
    If bit 1 is set, the DLL is notified about thread detachments (i.e.
        thread terminations).
    If bit 2 is set, the DLL is notified about thread attachments (i.e.
        thread creations).
    If bit 3 is set, the DLL is notified about process detachment (i.e.
        DLL unload).

The next 4 32-bit-values are the size of reserved stack
('SizeOfStackReserve'), the size of initially committed stack
('SizeOfStackCommit'), the size of the reserved heap
('SizeOfHeapReserve') and the size of the committed heap
('SizeOfHeapCommit').
The 'reserved' amounts are address space (not real RAM) that is reserved
for the specific purpose; at program startup, the 'committed' amount is
actually allocated in RAM. The 'committed' value is also the amount by
which the committed stack or heap grows if necessary. (Other sources
claim that the stack will grow in pages, regardless of the
'SizeOfStackCommit' value. I didn't check this.)
So, as an example, if a program has a reserved heap of 1 MB and a
committed heap of 64 KB, the heap will start out at 64 KB and is
guaranteed to be enlargeable up to 1 MB. The heap will grow in
64-KB-chunks.
The 'heap' in this context is the primary (default) heap. A process can
create more heaps if so it wishes.
The stack is the first thread's stack (the one that starts main()). The
process can create more threads which will have their own stacks.
DLLs don't have a stack or heap of their own, so the values are ignored
for their images. I don't know if drivers have a heap or a stack of
their own, but I don't think so.

After these stack- and heap-descriptions, we find 32 bits of
'LoaderFlags', which I didn't find a useful description of. I only found
a vague note about setting bits that automatically invoke a breakpoint
or a debugger after loading the image; however, this doesn't seem to
work.

Then we find 32 bits of 'NumberOfRvaAndSizes', which is the number of
valid entries in the directories that follow immediatly. I've found this
value to be unreliable; you might wish use the constant
IMAGE_NUMBEROF_DIRECTORY_ENTRIES instead, or the lesser of both.

After the 'NumberOfRvaAndSizes' there is an array of
IMAGE_NUMBEROF_DIRECTORY_ENTRIES (16) IMAGE_DATA_DIRECTORYs.
Each of these directories describes the location (32 bits RVA called
'VirtualAddress') and size (also 32 bit, called 'Size') of a particular
piece of information, which is located in one of the sections that
follow the directory entries.
For example, the security directory is found at the RVA and has the size
that are given at index 4.
The directories that I know the structure of will be discussed later.
Defined directory indexes are:

    IMAGE_DIRECTORY_ENTRY_EXPORT (0)
        The directory of exported symbols; mostly used for DLLs.
        Described below.
    
    IMAGE_DIRECTORY_ENTRY_IMPORT (1)
        The directory of imported symbols; see below.
    
    IMAGE_DIRECTORY_ENTRY_RESOURCE (2)
        Directory of resources. Described below.
    
    IMAGE_DIRECTORY_ENTRY_EXCEPTION (3)
        Exception directory - structure and purpose unknown.
    
    IMAGE_DIRECTORY_ENTRY_SECURITY (4)
        Security directory - structure and purpose unknown.
    
    IMAGE_DIRECTORY_ENTRY_BASERELOC (5)
        Base relocation table - see below.
    
    IMAGE_DIRECTORY_ENTRY_DEBUG (6)
        Debug directory - contents is compiler dependent. Moreover, many
        compilers stuff the debug information into the code section and
        don't create a separate section for it.
    
    IMAGE_DIRECTORY_ENTRY_COPYRIGHT (7)
        Description string - some arbitrary copyright note or the like.
    
    IMAGE_DIRECTORY_ENTRY_GLOBALPTR (8)
        Machine Value (MIPS GP) - structure and purpose unknown.
    
    IMAGE_DIRECTORY_ENTRY_TLS (9)
        Thread local storage directory - structure unknown; contains
        variables that are declared "__declspec(thread)", i.e.
        per-thread global variables.
    
    IMAGE_DIRECTORY_ENTRY_LOAD_CONFIG (10)
        Load configuration directory - structure and purpose unknown.
    
    IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT (11)
        Bound import directory - see description of import directory.
    
    IMAGE_DIRECTORY_ENTRY_IAT (12)
        Import Address Table - see description of import directory.
    
As an example, if we find at index 7 the 2 longwords 0x12000 and 33, and
the load address is 0x10000, we know that the copyright data is at
address 0x10000+0x12000 (in whatever section there may be), and the
copyright note is 33 bytes long.
If a directory of a particular type is not used in a binary, the Size
and VirtualAddress are both 0.



Section directories
-------------------

The sections consist of two major parts: first, a section description
(of type IMAGE_SECTION_HEADER) and then the raw section data. So after
the data directories we find an array of 'NumberOfSections' section
headers, ordered by the sections' RVAs.

A section header contains:

An array of IMAGE_SIZEOF_SHORT_NAME (8) bytes that make up the name
(ASCII) of the section. If all of the 8 bytes are used there is no 0-
terminator for the string! The name is typically something like ".data"
or ".text" or ".bss". There need not be a leading '.', the names may
also be "CODE" or "IAT" or the like.
Please note that the names are not at all related to the contents of the
section. A section named ".code" may or may not contain the executable
code; it may just as well contain the import address table; it may also
contain the code *and* the address table *and* the initialized data.
To find information in the sections, you will have to look it up via the
data directories of the optional header. Do not rely on the names, and
do not assume that the section's raw data starts at the beginning of a
section.

The next member of the IMAGE_SECTION_HEADER is a 32-bit-union of
'PhysicalAddress' and 'VirtualSize'. In an object file, this is the
address the contents is relocated to; in an executable, it is the size of
the contents. In fact, the field seems to be unused; There are linkers
that enter the size, and there are linkers that enter the address, and
I've also found a linker that enters a 0, and all the executables run
like the gentle wind.

The next member is 'VirtualAddress', a 32-bit-value holding the RVA to
the section's data when it is loaded in RAM.

Then we have got 32 bits of 'SizeOfRawData', which is the size of the
secion's data rounded up to the next multiple of 'FileAlignment'.

Next is 'PointerToRawData' (32 bits), which is incredibly useful because
it is the offset from the file's beginning to the section's data. If it
is 0, the section's data are not contained in the file and will be
arbitrary at load time.

Then we have got 'PointerToRelocations' (32 bits) and
'PointerToLinenumbers' (also 32 bits), 'NumberOfRelocations' (16 bits)
and 'NumberOfLinenumbers' (also 16 bits). All of these are information
that's only used for object files. Executables have a special base
relocation directory, and the line number information, if present at
all, is usually contained in a special purpose debugging segment or
elsewhere.

The last member of a section header is the 32 bits 'Characteristics',
which is a bunch of flags describing how the section's memory should be
treated:

    If bit 5 (IMAGE_SCN_CNT_CODE) is set, the section contains
    executable code.
    
    If bit 6 (IMAGE_SCN_CNT_INITIALIZED_DATA) is set, the section
    contains data that gets a defined value before execution starts. In
    other words: the section's data in the file is meaningful.
    
    If bit 7 (IMAGE_SCN_CNT_UNINITIALIZED_DATA) is set, this section
    contains uninitialized data and will be initialized to all-0-bytes
    before execution starts. This is normally the BSS.

    If bit 9 (IMAGE_SCN_LNK_INFO) is set, the section doesn't contain
    image data but comments, description or other documentation. This
    information is part of an object file and may be information for the
    linker, such as which libraries are needed.

    If bit 11 (IMAGE_SCN_LNK_REMOVE) is set, the data is part of an
    object file's section that is supposed to be left out when the
    executable file is linked. Often combined with bit 9.

    If bit 12 (IMAGE_SCN_LNK_COMDAT) is set, the section contains
    "common block data", which are packaged functions of some sort.

    If bit 15 (IMAGE_SCN_MEM_FARDATA) is set, we have far data -
    whatever that means. This bit's meaning is unsure.

    If bit 17 (IMAGE_SCN_MEM_PURGEABLE) is set, the section's data
    is purgeable - but I don't think that this is the same as
    "discardable", which has a bit of its own, see below.
    The same bit is apparently used to indicate 16-bit-information as
    there is also a define IMAGE_SCN_MEM_16BIT for it.
    This bit's meaning is unsure.

    If bit 18 (IMAGE_SCN_MEM_LOCKED) is set, the section should not be
    moved in memory? Perhaps it indicates there is no relocation
    information? This bit's meaning is unsure.

    If bit 19 (IMAGE_SCN_MEM_PRELOAD) is set, the section should be
    paged in before execution starts? This bit's meaning is unsure.

    Bits 20 to 23 specify an alignment that I have no information
    about. There are #defines IMAGE_SCN_ALIGN_16BYTES and the like. The
    only value I've ever seen used is 0, for the default 16-byte-
    alignment. I suspect that this is the alignment of objects in a
    library file or the like.

    If bit 24 (IMAGE_SCN_LNK_NRELOC_OVFL) is set, the section contains
    some extended relocations that I don't know about.

    If bit 25 (IMAGE_SCN_MEM_DISCARDABLE) is set, the section's data is
    not needed after the process has started. This is the case,
    for example, with the relocation information. I've seen it also for
    startup routines of drivers and services that are only executed
    once, and for import directories.

    If bit 26 (IMAGE_SCN_MEM_NOT_CACHED) is set, the section's data
    should not be cached. Don't ask my why not. Does this mean to switch
    off the 2nd-level-cache?

    If bit 27 (IMAGE_SCN_MEM_NOT_PAGED) is set, the section's data
    should not be paged out. This is interesting for drivers.

    If bit 28 (IMAGE_SCN_MEM_SHARED) is set, the section's data is
    shared among all running instances of the image. If it is e.g. the
    initialized data of a DLL, all running instances of the DLL will at
    any time have the same variable contents.
    Note that only the first instance's section is initialized.
    Sections containing code are always shared copy-on-write (i.e. the
    sharing doesn't work if relocations are necessary).

    If bit 29 (IMAGE_SCN_MEM_EXECUTE) is set, the process gets
    'execute'-access to the section's memory.
    
    If bit 30 (IMAGE_SCN_MEM_READ) is set, the process gets
    'read'-access to the section's memory.
    
    If bit 31 (IMAGE_SCN_MEM_WRITE) is set, the process gets
    'write'-access to the section's memory.



After the section headers we find the sections themselves. They are, in
the file, aligned to 'FileAlignment' bytes (that is, after the optional
header and after each section's data there will be padding bytes) and
ordered by their RVAs. When loaded (in RAM), the sections are aligned to
'SectionAlignment' bytes.

As an example, if the optional header ends at file offset 981 and
'FileAlignment' is 512, the first section will start at byte 1024. Note
that you can find the sections via the 'PointerToRawData' or the
'VirtualAddress', so there is hardly any need to actually fuss around
with the alignments.


I will try to make an image of it all:


    +-------------------+
    | DOS-stub          |
    +-------------------+
    | file-header       |
    +-------------------+
    | optional header   |
    |- - - - - - - - - -|
    |                   |----------------+
    | data directories  |                |
    |                   |                |
    |(RVAs to direc-    |-------------+  |
    |tories in sections)|             |  |
    |                   |---------+   |  |
    |                   |         |   |  |
    +-------------------+         |   |  |
    |                   |-----+   |   |  |
    | section headers   |     |   |   |  |
    | (RVAs to section  |--+  |   |   |  |
    |  borders)         |  |  |   |   |  |
    +-------------------+<-+  |   |   |  |
    |                   |     | <-+   |  |
    | section data 1    |     |       |  |
    |                   |     | <-----+  |
    +-------------------+<----+          |
    |                   |                |
    | section data 2    |                |
    |                   | <--------------+
    +-------------------+

There is one section header for each section, and each data directory
will point to one of the sections (several data directories may point to
the same section, and there may be sections without data directory
pointing to them).



Sections' raw data
------------------


general
-------
All sections are aligned to 'SectionAlignment' when loaded in RAM, and
'FileAlignment' in the file. The sections are described by entries in
the section headers: You find the sections in the file via
'PointerToRawData' and in memory via 'VirtualAddress'; the length is in
'SizeOfRawData'.

There are several kinds of sections, depending on what's contained in
them. In most cases (but not in all) there will be at least one
data directory in a section, with a pointer to it in the optional
header's data directory array.


code section
------------
First, I will mention the code section. The section will have, at least,
the bits 'IMAGE_SCN_CNT_CODE', 'IMAGE_SCN_MEM_EXECUTE' and
'IMAGE_SCN_MEM_READ' set, and 'AddressOfEntryPoint' will point somewhere
into the section, to the start of the function that the developer wants
to execute first.
'BaseOfCode' will normally point to the start of this section, but may
point to somewhere later in the section if some non-code-bytes are
placed before the code in the section.
Normally, there will be nothing but executable code in this section, and
there will be only one code section, but don't rely on this.
Typical section names are ".text", ".code", "AUTO" and the like.


data section
------------
The next thing we'll discuss is the initialized variables; this section
contains initialized static variables (like "static int i = 5;"). It will
have, at least, the bits 'IMAGE_SCN_CNT_INITIALIZED_DATA',
'IMAGE_SCN_MEM_READ' and 'IMAGE_SCN_MEM_WRITE' set. Some linkers may
place constant data into a section of their own that doesn't have the
writeable-bit. If part of the data is shareable, or there are other
peculiarities, there may be more sections with the apropriate section-
bits set.
The section, or sections, will be in the range 'BaseOfData' up to
'BaseOfData'+'SizeOfInitializedData'.
Typical section names are '.data', '.idata', 'DATA' and so on.


bss section
-----------
Then there is the uninitialized data (for static variables like "static
int k;"); this section is quite like the initialized data, but will have
a file offset ('PointerToRawData') of 0 indicating its contents is not
stored in the file, and 'IMAGE_SCN_CNT_UNINITIALIZED_DATA' is set
instead of 'IMAGE_SCN_CNT_INITIALIZED_DATA' to indicate that the
contents should be set to 0-bytes at load-time. This means, there is a
section header but no section in the file; the section will be created
by the loader and consist entirely of 0-bytes.
The length will be 'SizeOfUninitializedData'.
Typical names are '.bss', 'BSS' and the like.

These were the section data that are *not* pointed to by data
directories. Their contents and structure is supplied by the compiler,
not by the linker.
(The stack-segment and heap-segment are not sections in the binary but
created by the loader from the stacksize- and heapsize-entries in the
optional header.)


copyright
---------
To begin with a simple directory-section, let's look at the data
directory 'IMAGE_DIRECTORY_ENTRY_COPYRIGHT'. The contents is a
copyright- or description string in ASCII (not 0-terminated), like
"Gonkulator control application, copyright (c) 1848 Hugendubel & Cie".
This string is, normally, supplied to the linker with the command line
or a description file.
This string is not needed at runtime and may be discarded. It is not
writeable; in fact, the application doesn't need access at all.
So the linker will find out if there is a discardable non-writeable
section already and if not, create one (named '.descr' or the like). It
will then stuff the string into the section and let the
copyright-directory-pointer point to the string. The
'IMAGE_SCN_CNT_INITIALIZED_DATA' bit should be set.


exported symbols
----------------
(Note that the description of the export directory was faulty in versions
of this text before 1999-03-12. It didn't describe forwarders, exports
by ordinal only, or exports with several names.)

The next-simplest thing is the export directory,
'IMAGE_DIRECTORY_ENTRY_EXPORT'. This is a directory typically found
in DLLs; it contains the entry points of exported functions (and the
addresses of exported objects etc.). Executables may of course also have
exported symbols but usually they don't.
The containing section should be "initialized data" and "readable". It
should not be "discardable" because the process might call
"GetProcAddress()" to find a function's entry point at runtime.
The section is normally called '.edata' if it is a separate thing; often
enough, it is merged into some other section like "initialized data".

The structure of the export table ('IMAGE_EXPORT_DIRECTORY') comprises a
header and the export data, that is: the symbol names, their ordinals
and the offsets to their entry points.

First, we have 32 bits of 'Characteristics' that are unused and normally
0. Then there is a 32-bit-'TimeDateStamp', which presumably should give
the time the table was created in the time_t-format; alas, it is not
always valid (some linkers set it to 0). Then we have 2 16-bit-words of
version-info ('MajorVersion' and 'MinorVersion'), and these, too, are
often enough set to 0.

The next thing is 32 bits of 'Name'; this is an RVA to the DLL name as a
0-terminated ASCII string. (The name is necessary in case the DLL file is
renamed - see "binding" at the import directory.)
Then, we have got a 32-bit-'Base'. We'll come to that in a moment.

The next 32-bit-value is the total number of exported items
('NumberOfFunctions'). In addition to their ordinal number, items may be
exported by one or several names. and the next 32-bit-number is the
total number of exported names ('NumberOfNames').
In most cases, each exported item will have exactly one corresponding
name and it will be used by that name, but an item may have several
associated names (it is then accessible by each of them), or it may have
no name, in which case it is only accessible by its ordinal number. The
use of unnamed exports (purely by ordinal) is discouraged, because all
versions of the exporting DLL would have to use the same ordinal
numbering, which is a maintainance problem.

The next 32-bit-value 'AddressOfFunctions' is a RVA to the list of
exported items. It points to an array of 'NumberOfFunctions'
32-bit-values, each being a RVA to the exported function or variable.

There are 2 quirks about this list: First, such an exported RVA may be 0,
in which case it is unused. Second, if the RVA points into the section
containing the export directory, this is a forwarded export. A forwarded
export is a pointer to an export in another binary; if it is used, the
pointed-to export in the other binary is used instead. The RVA in this
case points, as mentioned, into the export directory's section, to a
zero-terminated string comprising the name of the pointed-to DLL and
the export name separated by a dot, like "otherdll.exportname", or the
DLL's name and the export ordinal, like "otherdll.#19".

Now is the time to explain the export ordinal. An export's ordinal is
the index into the AddressOfFunctions-Array (the 0-based position in
this array) plus the 'Base' mentioned above.
In most cases, the 'Base' is 1, which means the first export has an
ordinal of 1, the second has an ordinal of 2 and so on.

After the 'AddressOfFunctions'-RVA we find a RVA to the array of
32-bit-RVAs to symbol names 'AddressOfNames', and a RVA to the array of
16-bit-ordinals 'AddressOfNameOrdinals'. Both arrays have
'NumberOfNames' elements.
The symbol names may be missing entirely, in which case the
'AddressOfNames' is 0. Otherwise, the pointed-to arrays are running
parallel, which means their elements at each index belong together. The
'AddressOfNames'-array consists of RVAs to 0-terminated export names;
the names are held in a sorted list (i.e. the first array member is the
RVA to the alphabetically smallest name; this allows efficient searching
when looking up an exported symbol by name).
According to the PE specification, the 'AddressOfNameOrdinals'-array has
the ordinal corresponding to each name; however, I've found this array
to contain the actual index into the 'AddressOfFunctions-Array instead.

I'll draw a picture about the three tables:


    AddressOfFunctions
           |
           |
           |
           v
    exported RVA with ordinal 'Base'
    exported RVA with ordinal 'Base'+1
    ...
    exported RVA with ordinal 'Base'+'NumberOfFunctions'-1



    AddressOfNames                  AddressOfNameOrdinals
           |                              |
           |                              |
           |                              |
           v                              v
    RVA to first name           <-> Index of export for first name
    RVA to second name          <-> Index of export for second name
    ...                             ...
    RVA to name 'NumberOfNames' <-> Index of export for name 'NumberOfNames'


Some examples are in order.

To find an exported symbol by ordinal, subtract the 'Base' to get the
index, follow the 'AddressOfFunctions'-RVA to find the exports-array and
use the index to find the exported RVA in the array. If it does not
point into the export section, you are done. Otherwise, it points to a
string describing the exporting DLL and the name or ordinal therein, and
you have to look up the forwarded export there.

To find an exported symbol by name, follow the 'AddressOfNames'-RVA (if
it is 0 there are no names) to find the array of RVAs to the export
names. Search your name in the list. Use the name's index in the
'AddressOfNameOrdinals'-Array and get the 16-bit-number corresponding to
the found name. According to the PE spec, it is an ordinal and you need
to subtract the 'Base' to get the export index; according to my
experiences it is the export index and you don't subtract. Using the
export index, you find the export RVA in the 'AddressOfFunctions'-Array,
being either the exported RVA itself or a RVA to a string describing a
forwarded export.


imported symbols
----------------
When the compiler finds a call to a function that is in a different
executable (mostly in a DLL), it will, in the most simplistic case, not
know anything about the circumstances and simply output a normal
call-instruction to that symbol, the address of which the linker will
have to fix, like it does for any external symbol.
The linker uses an import library to look up from which DLL which symnol
is imported, and produces stubs for all the imported symbols, each of
which consists of a jump-instruction; the stubs are the actual
call-targets. These jump-instructions will actually jump to an address
that's fetched from the so-called import address table. In more
sophisticated applications (when "__declspec(dllimport)" is used), the
compiler knows the function is imported, and outputs a call to the
address that's in the import address table, bypassing the jump.

Anyway, the address of the function in the DLL is always necessary and
will be supplied by the loader from the exporting DLL's export directory
when the application is loaded. The loader knows which symbols in what
libraries have to be looked up and their addresses fixed by searching
the import directory.

I will better give you an example. The calls with or without
__declspec(dllimport) look like this:

    source:
        int symbol(char *);
        __declspec(dllimport) int symbol2(char*);
        void foo(void)
        {
            int i=symbol("bar");
            int j=symbol2("baz");
        }
    
    assembly:
        ...
        call _symbol                 ; without declspec(dllimport)
        ...
        call [__imp__symbol2]        ; with declspec(dllimport)
        ...

In the first case (without __declspec(dllimport)), the compiler didn't
know that '_symbol' was in a DLL, so the linker has to provide the
function '_symbol'. Since the function isn't there, it will supply a
stub function for the imported symbol, being an indirect jump. The
collection of all import-stubs is called the "transfer area" (also
sometimes called a "trampoline", because you jump there in order to jump
to somewhere else).
Typically this transfer area is located in the code section (it is not
part of the import directory). Each of the function stubs is a jump to
the actual function in the target DLLs. The transfer area looks like
this:

    _symbol:        jmp  [__imp__symbol]
    _other_symbol:  jmp  [__imp__other__symbol]
    ...


This means: if you use imported symbols without specifying
"__declspec(dllimport)" then the linker will generate a transfer area
for them, consisting of indirect jumps. If you do specify
"__declspec(dllimport)", the compiler will do the indirection itself and
a transfer area is not necessary. (It also means: if you import
variables or other stuff you must specify "__declspec(dllimport)",
because a stub with a jmp instruction is appropriate for functions
only.)

In any case the adress of symbol 'x' is stored at a location '__imp_x'.
All these locations together comprise the so-called "import address
table", which is provided to the linker by the import libraries of the
various DLLs that are used. The import address table is a list of
addresses like this:

   __imp__symbol:   0xdeadbeef
   __imp__symbol2:  0x40100
   __imp__symbol3:  0x300100
   ...

This import address table is a part of the import directory, and it is
pointed to by the IMAGE_DIRECTORY_ENTRY_IAT directory pointer (although
some linkers don't set this directory entry and it works nevertheless;
apparently, the loader can resolve imports without using the directory
IMAGE_DIRECTORY_ENTRY_IAT).
The addresses in this table are unknown to the linker; the linker
inserts dummies (RVAs to the function names; see below for more
information) that are patched by the loader at load time using the
export directory of the exporting DLL. The import address table, and how
it is found by the loader, will be described in more detail later in
this chapter.

Note that this description is C-specific; there are other application
building environments that don't use import libraries. They all need to
generate an import address table, though, which they use to let their
programs access the imported objects and functions. C compilers tend to
use import libraries because it is convenient for them - their linkers
use libraries anyway. Other environments use e.g. a description file
that lists the necessary DLL names and function names (like the "module
definition file"), or a declaration-style list in the source.


This is how imports are used by the program's code; now we'll look how
an import directory is made up so the loader can use it.


The import directory should reside in a section that's "initialized
data" and "readable".
The import directory is an array of IMAGE_IMPORT_DESCRIPTORs, one for
each used DLL. The list is terminated by a IMAGE_IMPORT_DESCRIPTOR
that's entirely filled with 0-bytes.
An IMAGE_IMPORT_DESCRIPTOR is a struct with these members:

    OriginalFirstThunk
        An RVA (32 bit) pointing to a 0-terminated array of RVAs to
        IMAGE_THUNK_DATAs, each describing one imported function. The
        array will never change.

    TimeDateStamp
        A 32-bit-timestamp that has several purposes. Let's pretend that
        the timestamp is 0, and handle the advanced cases later.

    ForwarderChain
        The 32-bit-index of the first forwarder in the list of imported
        functions. Forwarders are also advanced stuff; set to all-bits-1
        for beginners.
        
    Name
        A 32-bit-RVA to the name (a 0-terminated ASCII string) of the
        DLL.
        
    FirstThunk
        An RVA (32 bit) to a 0-terminated array of RVAs to
        IMAGE_THUNK_DATAs, each describing one imported function. The
        array is part of the import address table and will change.

So each IMAGE_IMPORT_DESCRIPTOR in the array gives you the name of the
exporting DLL and, apart from the forwarder and timestamp, it gives you
2 RVAs to arrays of IMAGE_THUNK_DATAs, using 32 bits. (The last member
of each array is entirely filled with 0-bytes to mark the end.)
Each IMAGE_THUNK_DATA is, for now, an RVA to a IMAGE_IMPORT_BY_NAME
which describes the imported function.
The interesting point is now, the arrays run parallel, i.e.: they point
to the same IMAGE_IMPORT_BY_NAMEs.

No need to be desparate, I will draw another picture. This is the
essential contents of one IMAGE_IMPORT_DESCRIPTOR:

     OriginalFirstThunk      FirstThunk
            |                    |
            |                    |
            |                    |
            V                    V

            0-->    func1     <--0
            1-->    func2     <--1
            2-->    func3     <--2
            3-->    foo       <--3
            4-->    mumpitz   <--4
            5-->    knuff     <--5
            6-->0            0<--6      /* the last RVA is 0! */

where the names in the center are the yet to discuss
IMAGE_IMPORT_BY_NAMEs. Each of them is a 16-bit-number (a hint) followed
by an unspecified amount of bytes, being the 0-terminated ASCII name of
the imported symbol.
The hint is an index into the exporting DLL's name table (see export
directory above). The name at that index is tried, and if it doesn't
match then a binary search is done to find the name.
(Some linkers don't bother to look up correct hints and simply specify
1 all the time, or some other arbitrary number. This doesn't harm, it
just makes the first attempt to resolve the name always fail, enforcing
a binary search for each name.)

To summarize, if you want to look up information about the imported
function "foo" from DLL "knurr", you first find the entry
IMAGE_DIRECTORY_ENTRY_IMPORT in the data directories, get an RVA, find
that address in the raw section data and now have an array of
IMAGE_IMPORT_DESCRIPTORs. Get the member of this array that relates to
the DLL "knurr" by inspecting the strings pointed to by the 'Name's.
When you have found the right IMAGE_IMPORT_DESCRIPTOR, follow its
'OriginalFirstThunk' and get hold of the pointed-to array of
IMAGE_THUNK_DATAs; inspect the RVAs and find the function "foo".

Ok, now, why do we have *two* lists of pointers to the
IMAGE_IMPORT_BY_NAMEs? Because at runtime the application doesn't need
the imported functions' names but the addresses. This is where the
import address table comes in again. The loader will look up each
imported symbol in the export-directory of the DLL in question and
replace the IMAGE_THUNK_DATA-element in the 'FirstThunk'-list (which
until now also points to the IMAGE_IMPORT_BY_NAME) with the linear
address of the DLL's entry point.
Remember the list of addresses with labels like "__imp__symbol"; the
import address table, pointed to by the data directory
IMAGE_DIRECTORY_ENTRY_IAT, is exactly the list pointed to by
'FirstThunk'. (In case of imports from several DLLs, the import address
table comprises the 'FirstThunk'-Arrays of all the DLLs. The directory
entry IMAGE_DIRECTORY_ENTRY_IAT may be missing, the imports will still
work fine.)
The 'OriginalFirstThunk'-array remains untouched, so you can always look
up the original list of imported names via the
'OriginalFirstThunk'-list.

The import is now patched with the correct linear addresses and looks
like this:

     OriginalFirstThunk      FirstThunk
            |                    |
            |                    |
            |                    |
            V                    V

            0-->    func1        0-->  exported func1
            1-->    func2        1-->  exported func2
            2-->    func3        2-->  exported func3
            3-->    foo          3-->  exported foo
            4-->    mumpitz      4-->  exported mumpitz
            5-->    knuff        5-->  exported knuff
            6-->0            0<--6

            
This was the basic structure, for simple cases. Now we'll learn about
tweaks in the import directories.

First, the bit IMAGE_ORDINAL_FLAG (that is: the MSB) of the
IMAGE_THUNK_DATA in the arrays can be set, in which case there is no
symbol-name-information in the list and the symbol is imported purely by
ordinal. You get the ordinal by inspecting the lower word of the
IMAGE_THUNK_DATA.
The import by ordinals is discouraged; it is much safer to import by
name, because the export ordinals might change if the exporting DLL is
not in the expected version.

Second, there are the so-called "bound imports".

Think about the loader's task: when a binary that it wants to execute
needs a function from a DLL, the loader loads the DLL, finds its export
directory, looks up the function's RVA and calculates the function's
entry point. Then it patches the so-found address into the 'FirstThunk'-
list.
Given that the programmer was clever and supplied unique preferred load
addresses for the DLLs that don't clash, we can assume that the
functions' entry points will always be the same. They can be computed
and patched into the 'FirstThunk'-list at link-time, and that's what
happens with the "bound imports". (The utility "bind" does this; it is
part of the Win32 SDK.)

Of course, one must be cautious: The user's DLL may have a different
version, or it may be necessary to relocate the DLL, thus invalidating
the pre-patched 'FirstThunk'-list; in this case, the loader will still
be able to walk the 'OriginalFirstThunk'-list, find the imported symbols
and re-patch the 'FirstThunk'-list. The loader knows that this is
necessary if a) the versions of the exporting DLL don't match or b) the
exporting DLL had to be relocated.

To decide whether there were relocations is no problem for the loader,
but how to find out if the versions differ? This is where the
'TimeDateStamp' of the IMAGE_IMPORT_DESCRIPTOR comes in. If it is 0, the
import-list has not been bound, and the loader must fix the entry points
always. Otherwise, the imports are bound, and 'TimeDateStamp' must match
the 'TimeDateStamp' of the exporting DLL's 'FileHeader'; if it doesn't
match, the loader assumes that the binary is bound to a "wrong" DLL and
will re-patch the import list.

There is an additional quirk about "forwarders" in the import-list. A DLL
can export a symbol that's not defined in the DLL but imported from
another DLL; such a symbol is said to be forwarded (see the export
directory description above).
Now, obviously you can't tell if the symbol's entry point is valid by
looking into the timestamp of a DLL that doesn't actually contain the
entry point. So the forwarded symbols' entry points must always be fixed
up, for safety reasons. In the import list of a binary, imports of
forwarded symbols need to be found so the loader can patch them.

This is done via the 'ForwarderChain'. It is an index into the thunk-
lists; the import at the indexed position is a forwarded export, and the
contents of the 'FirstThunk'-list at this position is the index of the
*next* forwarded import, and so on, until the index is "-1" which
indicates there are no more forwards. If there are no forwarders at all,
'ForwarderChain' is -1 itself.

This was the so-called "old-style" binding.

At this point, we should sum up what we have had so far :-)

Ok, I will assume you have found the IMAGE_DIRECTORY_ENTRY_IMPORT and you have
followed it to find the import-directory, which will be in one of the
sections. Now you're at the beginning of an array of
IMAGE_IMPORT_DESCRIPTORs the last of which will be entirely 0-bytes-
filled.
To decipher one of the IMAGE_IMPORT_DESCRIPTORs, you first look into the
'Name'-field, follow the RVA and thusly find the name of the exporting
DLL. Next you decide whether the imports are bound or not;
'TimeDateStamp' will be non-zero if the imports are bound. If they are
bound, now is a good time to check if the DLL version matches yours by
comparing the 'TimeDateStamp's.
Now you follow the 'OriginalFirstThunk'-RVA to go to the
IMAGE_THUNK_DATA-array; walk down this array (it is be 0-terminated),
and each member will be the RVA of a IMAGE_IMPORT_BY_NAME (unless the
hi-bit is set in which case you don't have a name but are left with a
mere ordinal). Follow the RVA, and skip 2 bytes (the hint), and now
you have got a 0-terminated ASCII-string that's the name of the imported
function.
To find the supplied entry point addresses in case it is a bound import,
follow the 'FirstThunk' and walk it parallel to the
'OriginalFirstThunk'-array; the array-members are the linear addresses
of the entry points (leaving aside the forwarders-topic for a moment).

There is one thing I didn't mention until now: Apparently there are
linkers that exhibit a bug when they build the import directory (I've
found this bug being in use by a Borland C linker). These linkers set
the 'OriginalFirstThunk' in the IMAGE_IMPORT_DESCRIPTOR to 0 and create
only the 'FirstThunk'-array. Obviously, such import directories cannot
be bound (else the necessary information to re-fix the imports were
lost - you couldn't find the function names). In this case, you will
have to follow the 'FirstThunk'-array to get the imported symbol names,
and you will never have pre-patched entry point addresses. I have found
a TIS document ([6]) describing the import directory in a way that is
compatible to this bug, so that paper may be the origin of the bug.
The TIS document specifies:
    IMPORT FLAGS
    TIME/DATE STAMP
    MAJOR VERSION - MINOR VERSION
    NAME RVA
    IMPORT LOOKUP TABLE RVA
    IMPORT ADDRESS TABLE RVA
as opposed to the structure used elsewhere:
    OriginalFirstThunk
    TimeDateStamp
    ForwarderChain
    Name
    FirstThunk

The last tweak about the import directories is the so-called "new style"
binding (it is described in [3]), which can also be done with the
"bind"-utility. When this is used, the 'TimeDateStamp' is set to
all-bits-1 and there is no forwarderchain; all imported symbols get their
address patched, whether they are forwarded or not. Still, you need to
know the DLLs' version, and you need to distinguish forwarded symbols
from ordinary ones. For this purpose, the
IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT directory is created. This will, as
far as I could find out, *not* be in a section but in the header, after
the section headers and before the first section. (Hey, I didn't invent
this, I'm only describing it!)
This directory tells you, for each used DLL, from which other DLLs there
are forwarded exports.
The structure is an IMAGE_BOUND_IMPORT_DESCRIPTOR, comprising (in this
order):
A 32-bit number, giving you the 'TimeDateStamp' of the DLL;
a 16-bit-number 'OffsetModuleName', being the offset from the beginning
of the directory to the 0-terminated name of the DLL;
a 16-bit-number 'NumberOfModuleForwarderRefs' giving you the number of
DLLs that this DLL uses for its forwarders.

Immediatly following this struct you find 'NumberOfModuleForwarderRefs'
structs that tell you the names and versions of the DLLs that this DLL
forwards from. These structs are 'IMAGE_BOUND_FORWARDER_REF's:
A 32-bit-number 'TimeDateStamp';
a 16-bit-number 'OffsetModuleName', being the offset from the beginning
of the directory to the 0-terminated name of the forwarded-from DLL;
16 unused bits.

Following the 'IMAGE_BOUND_FORWARDER_REF's is the next
'IMAGE_BOUND_IMPORT_DESCRIPTOR' and so on; the list is terminated by an
all-0-bits-IMAGE_BOUND_IMPORT_DESCRIPTOR.


Sorry for the inconvenience, but that's what it looks like :-)


Now, if you have a new-bound import directory, you load all the DLLs,
use the directory pointer IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT to find the
IMAGE_BOUND_IMPORT_DESCRIPTOR, scan through it and check if the
'TimeDateStamp's of the loaded DLLs match the ones given in this
directory. If not, fix them in the 'FirstThunk'-array of the import
directory.



resources
---------
The resources, such as dialog boxes, menus, icons and so on, are stored
in the data directory pointed to by IMAGE_DIRECTORY_ENTRY_RESOURCE. It
is in a section that has, at least, the bits
'IMAGE_SCN_CNT_INITIALIZED_DATA' and 'IMAGE_SCN_MEM_READ' set.

A resource base is a 'IMAGE_RESOURCE_DIRECTORY'; it contains several
'IMAGE_RESOURCE_DIRECTORY_ENTRY's each of which in turn may point to a
'IMAGE_RESOURCE_DIRECTORY'. This way, you get a tree of
'IMAGE_RESOURCE_DIRECTORY's with 'IMAGE_RESOURCE_DIRECTORY_ENTRY's as
leafs; these leafs point to the actual resource data.

In real life, the situation is somewhat relaxed. Normally you won't find
convoluted trees you can't possibly sort out.
The hierarchy is, normally, like this: one directory is the root. It
points to directories, one for each resource type. These directories
point to subdirectories, each of which will have a name or an ID and
point to a directory of the languages provided for this resource; for
each language you will find one resource entry, which will finally point
to the data. (Note that multi-language-resources don't work on
Win95, which always uses the same resource if it is available in several
languages - I didn't check which one, but I guess it's the first it
encounters. They do work on NT.)

The tree, without the pointer to the data, may look like this:

                           (root)
                              |
             +----------------+------------------+
             |                |                  |
            menu            dialog             icon
             |                |                  |
       +-----+-----+        +-+----+           +-+----+----+
       |           |        |      |           |      |    |
    "main"      "popup"   0x10   "maindlg"    0x100 0x110 0x120
       |            |       |      |           |      |    | 
   +---+-+          |       |      |           |      |    |
   |     |     default   english   default    def.   def.  def.
german english


A IMAGE_RESOURCE_DIRECTORY comprises:
32 bits of unused flags called 'Characteristics';
32 bits 'TimeDateStamp' (again in the common time_t representation),
giving you the time the resource was created (if the entry is set);
16 bits 'MajorVersion' and 16 bits 'MinorVersion', thusly allowing you
to maintain several versions of the resource;
16 bits 'NumberOfNamedEntries' and another 16 bits 'NumberOfIdEntries'.

Immediatly following such a structure are
'NumberOfNamedEntries'+'NumberOfIdEntries' structs which are of the
format 'IMAGE_RESOURCE_DIRECTORY_ENTRY', those with the names coming first.
They may point to further 'IMAGE_RESOURCE_DIRECTORY's or they point to
the actual resource data.
A IMAGE_RESOURCE_DIRECTORY_ENTRY consists of:
32 bits giving you the id of the resource or the directory it describes;
32 bits offset to the data or offset to the next sub-directory.

The meaning of the id depends on the level in the tree; the id may be a
number (if the hi-bit is clear) or a name (if the hi-bit is set). If it
is a name, the lower 31 bits are the offset from the beginning of the
resource section's raw data to the name (the name consists of 16 bits
length and trailing wide characters, in unicode, not 0-terminated).

If you are in the root-directory, the id, if it is a number, is the
resource-type:
    1: cursor
    2: bitmap
    3: icon
    4: menu
    5: dialog
    6: string table
    7: font directory
    8: font
    9: accelerators
    10: unformatted resource data
    11: message table
    12: group cursor
    14: group icon
    16: version information
Any other number is user-defined. Any resource-type with a type-name is
always user-defined.

If you are one level deeper, the id is the resource-id (or resource-
name).

If you are another level deeper, the id must be a number, and it is the
language-id of the specific instance of the resource; for example, you
can have the same dialog in australian english, canadian french and
swiss german localized forms, and they all share the same resource-id.
The system will choose the dialog to load based on the thread's locale,
which in turn will usually reflect the user's "regional setting".
(If the resource cannot be found for the thread locale, the system will
first try to find a resource for the locale using a neutral sublanguage,
e.g. it will look for standard french instead of the user's canadian
french; if it still can't be found, the instance with the smallest
language id will be used. As noted, all this works only on NT.)
To decipher the language id, split it into the primary language id and
the sublanguage id using the macros PRIMARYLANGID() and SUBLANGID(),
giving you the bits 0 to 9 or 10 to 15, respectivly. The values are
defined in the file "winresrc.h".
Language-resources are only supported for accelerators, dialogs, menus,
rcdata or stringtables; other resource-types should be
LANG_NEUTRAL/SUBLANG_NEUTRAL.

To find out whether the next level below a resource directory is another
directory, you inspect the hi-bit of the offset. If it is set, the
remaining 31 bits are the offset from the beginning of the resource
section's raw data to the next directory, again in the format
IMAGE_RESOURCE_DIRECTORY with trailing IMAGE_RESOURCE_DIRECTORY_ENTRYs.

If the bit is clear, the offset is the distance from the beginning of
the resource section's raw data to the resource's raw data description,
a IMAGE_RESOURCE_DATA_ENTRY. It consists of 32 bits 'OffsetToData' (the
offset to the raw data, counting from the beginning of the resource
section's raw data), 32 bits of 'Size' of the data, 32 bits 'CodePage'
and 32 unused bits.
(The use of codepages is discouraged, you should use the 'language'-
feature to support multiple locales.)


The raw data format depends on the resource type; descriptions can be
found in the MS SDK documentation. Note that any string in resources is
always in UNICODE except for user defined resources, which are in the
format the developer chooses, obviously.


relocations
-----------
The last data directory I will describe is the base relocation
directory. It is pointed to by the IMAGE_DIRECTORY_ENTRY_BASERELOC entry
in the data directories of the optional header. It is typically
contained in a section if its own, with a name like ".reloc" and the
bits IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_DISCARDABLE and
IMAGE_SCN_MEM_READ set.

The relocation data is needed by the loader if the image cannot be
loaded to the preferred load address 'ImageBase' mentioned in the
optional header. In this case, the fixed addresses supplied by the
linker are no longer valid, and the loader has to apply fixups for
absolute addresses used for locations of static variables, string
literals and so on.

The relocation directory is a sequence of chunks. Each chunk contains
the relocation information for 4 KB of the image. A chunk starts with a
'IMAGE_BASE_RELOCATION' struct. It consists of 32 bits 'VirtualAddress'
and 32 bits 'SizeOfBlock'. It is followed by the chunk's actual
relocation data, being 16 bits each.
The 'VirtualAddress' is the base RVA that the relocations of this chunk
need to be applied to; the 'SizeOfBlock' is the size of the entire chunk
in bytes.
The number of trailing relocations is
('SizeOfBlock'-sizeof(IMAGE_BASE_RELOCATION))/2
The relocation information ends when you encounter a
IMAGE_BASE_RELOCATION struct with a 'VirtualAddress' of 0.

Each 16-bit-relocation information consists of the relocation position
in the lower 12 bits and a relocation type in the high 4 bits. To get
the relocation RVA, you need to add the IMAGE_BASE_RELOCATION's
'VirtualAddress' to the 12-bit-position. The type is one of:
    IMAGE_REL_BASED_ABSOLUTE (0)
        This is a no-op; it is used to align the chunk to a 32-bits-
        border. The position should be 0.
    IMAGE_REL_BASED_HIGH (1)
        The high 16 bits of the relocation must be applied to the 16
        bits of the WORD pointed to by the offset, which is the high
        word of a 32-bit-DWORD.
    IMAGE_REL_BASED_LOW (2)
        The low 16 bits of the relocation must be applied to the 16
        bits of the WORD pointed to by the offset, which is the low
        word of a 32-bit-DWORD.
    IMAGE_REL_BASED_HIGHLOW (3)
        The entire 32-bit-relocation must be applied to the entire 32
        bits in question. This (and the no-op '0') is the only
        relocation type I've actually found in binaries.
    IMAGE_REL_BASED_HIGHADJ (4)
        This is one for the tough. Read yourself (from [6]) and make
        sense out of it if you can:
        "Highadjust. This fixup requires a full 32-bit value. The high
        16-bits is located at Offset, and the low 16-bits is located in
        the next Offset array element (this array element is included in
        the Size field). The two need to be combined into a signed
        variable. Add the 32-bit delta. Then add 0x8000 and store the
        high 16-bits of the signed variable to the 16-bit field at
        Offset."
    IMAGE_REL_BASED_MIPS_JMPADDR (5)
        Unknown
    IMAGE_REL_BASED_SECTION (6)
        Unknown
    IMAGE_REL_BASED_REL32 (7)
        Unknown

As an example, if you find the relocation information to be
    0x00004000      (32 bits, starting RVA)
    0x00000010      (32 bits, size of chunk)
    0x3012          (16 bits reloc data)
    0x3080          (16 bits reloc data)
    0x30f6          (16 bits reloc data)
    0x0000          (16 bits reloc data)
    0x00000000      (next chunk's RVA)
    0xff341234
you know the first chunk describes relocations starting at RVA 0x4000 and
is 16 bytes long. Because the header uses 8 bytes and one relocation
uses 2 bytes, there are (16-8)/2=4 relocations in the chunk.
The first relocation is to be applied to the DWORD at 0x4012, the next
to the DWORD at 0x4080, and the third to the DWORD at 0x40f6. The last
relocation is a no-op.
The next chunk has a RVA of 0 and finishes the list.

Now, how do you do a relocation?
You know that the image *is* relocated to the preferred load address
'ImageBase' in the optional header; you also know the address you did
load the image to. If they match, you don't need to do anything.
If they don't match, you calculate the difference
actual_base-preferred_base
and add that value (signed, it may be negative) to the relocation
positions, which you will find with the method described above.


Acknowledgments
---------------
Thanks go to David Binette for his debugging and proof-reading.
(The remaining errors are entirely mine.)
Also thanks to wotsit.org for letting me put the file on their site.


Copyright
---------
This text is copyright 1999 by B. Luevelsmeyer. It is freeware, and you
may use it for any purpose but on your own risk. It contains errors and
it is incomplete. You have been warned.


Bug reports
-----------
Send any bug reports (or other comments) to
bernd.luevelsmeyer@iplan.heitec.net


Versions
--------
You find the date of the current release at the top of the file.

1998-04-06
  First public release

1998-07-29
  Changed wrong "byte" to "word" for image version and subsystem version
  Corrected error "stack is limited to 1 MB" (in fact it is not limited)
  Corrected some typos

1999-03-15
  Corrected export directory description, which was very incomplete
  Reworded import directory description, which had been unclear
  Corrected typos and did some rewording in other sections
  

Literature
----------

[1]
"Peering Inside the PE: A Tour of the Win32 Portable Executable File
Format" (M. Pietrek), in: Microsoft Systems Journal 3/1994

[2]
"Why to Use _declspec(dllimport) & _declspec(dllexport) In Code", MS
Knowledge Base Q132044

[3]
"Windows Q&A" (M. Pietrek), in: Microsoft Systems Journal 8/1995

[4]
"Writing Multiple-Language Resources", MS Knowledge Base Q89866

[5]
"The Portable Executable File Format from Top to Bottom" (Randy Kath),
in: Microsoft Developer Network

[6]
Tool Interface Standard (TIS) Formats Specification for Windows Version
1.0 (Intel Order Number 241597, Intel Corporation 1993)


Appendix: hello world
---------------------
In this appendix I will show how to make programs by hand. The example
will use Intel-assembly, because I don't speak DEC Alpha.

The program will be the equivalent of

    #include <stdio.h>
    int main(void)
    {
        puts(hello,world);
        return 0;
    }

First, I translate it to use Win32 functions instead of the C runtime:

    #define STD_OUTPUT_HANDLE -11UL
    #define hello "hello, world\n"

    __declspec(dllimport) unsigned long __stdcall
    GetStdHandle(unsigned long hdl);

    __declspec(dllimport) unsigned long __stdcall
    WriteConsoleA(unsigned long hConsoleOutput,
                    const void *buffer,
                    unsigned long chrs,
                    unsigned long *written,
                    unsigned long unused
                    );

    static unsigned long written;

    void startup(void)
    {
        WriteConsoleA(GetStdHandle(STD_OUTPUT_HANDLE),hello,sizeof(hello)-1,&written,0);
        return;
    }

Now I will fumble out the assembly:
    startup:
                ; parameters for WriteConsole(), backwards
    6A 00                     push      0x00000000
    68 ?? ?? ?? ??            push      offset _written
    6A 0D                     push      0x0000000d
    68 ?? ?? ?? ??            push      offset hello
                ; parameter for GetStdHandle()
    6A F5                     push      0xfffffff5
    2E FF 15 ?? ?? ?? ??      call      dword ptr cs:__imp__GetStdHandle@4
                ; result is last parameter for WriteConsole()
    50                        push      eax
    2E FF 15 ?? ?? ?? ??      call      dword ptr cs:__imp__WriteConsoleA@20
    C3                        ret       

    hello:
    68 65 6C 6C 6F 2C 20 77 6F 72 6C 64 0A   "hello, world\n"
    _written:
    00 00 00 00

That was the compiler part. Anyone can do that. From now on we play
linker, which is much more interesting :-)

I need to find the functions WriteConsoleA() and GetStdHandle(). They
happen to be in "kernel32.dll". (That was the 'import library' part.)

Now I can start to make the executable. Question marks will take the
place of yet-to-find-out values; they will be patched afterwards.

First the DOS-stub, starting at 0x0 and being 0x40 bytes long:
    00 | 4d 5a 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    10 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    20 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    30 | 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 00
As you can see, this isn't really a MS-DOS program. It's just the header
with the signature "MZ" at the beginning and the e_lfanew pointing
immediatly after the header, without any code. That's because it isn't
intended to run on MS-DOS; it's just here because the specification
requires it.

Then the PE signature, starting at 0x40 and being 0x4 bytes long:
        50 45 00 00

Now the file-header, which will start at byte 0x44 and is 0x14 bytes long:
    Machine                     4c 01       ; i386
    NumberOfSections            02 00       ; code and data
    TimeDateStamp               00 00 00 00 ; who cares?
    PointerToSymbolTable        00 00 00 00 ; unused
    NumberOfSymbols             00 00 00 00 ; unused
    SizeOfOptionalHeader        e0 00       ; constant
    Characteristics             02 01       ; executable on 32-bit-machine

And the optional header, which will start at byte 0x58 and is 0x60 bytes long:
    Magic                       0b 01       ; constant
    MajorLinkerVersion          00          ; I'm version 0.0 :-)
    MinorLinkerVersion          00          ;
    SizeOfCode                  20 00 00 00 ; 32 bytes of code
    SizeOfInitializedData       ?? ?? ?? ?? ; yet to find out
    SizeOfUninitializedData     00 00 00 00 ; we don't have a BSS
    AddressOfEntryPoint         ?? ?? ?? ?? ; yet to find out
    BaseOfCode                  ?? ?? ?? ?? ; yet to find out
    BaseOfData                  ?? ?? ?? ?? ; yet to find out
    ImageBase                   00 00 10 00 ; 1 MB, chosen arbitrarily
    SectionAlignment            20 00 00 00 ; 32-bytes-alignment
    FileAlignment               20 00 00 00 ; 32-bytes-alignment
    MajorOperatingSystemVersion  04 00      ; NT 4.0
    MinorOperatingSystemVersion  00 00      ;
    MajorImageVersion           00 00       ; version 0.0
    MinorImageVersion           00 00       ;
    MajorSubsystemVersion       04 00       ; Win32 4.0
    MinorSubsystemVersion       00 00       ;
    Win32VersionValue           00 00 00 00 ; unused?
    SizeOfImage                 ?? ?? ?? ?? ; yet to find out
    SizeOfHeaders               ?? ?? ?? ?? ; yet to find out
    CheckSum                    00 00 00 00 ; not used for non-drivers
    Subsystem                   03 00       ; Win32 console
    DllCharacteristics          00 00       ; unused (not a DLL)
    SizeOfStackReserve          00 00 10 00 ; 1 MB stack
    SizeOfStackCommit           00 10 00 00 ; 4 KB to start with
    SizeOfHeapReserve           00 00 10 00 ; 1 MB heap
    SizeOfHeapCommit            00 10 00 00 ; 4 KB to start with
    LoaderFlags                 00 00 00 00 ; unknown
    NumberOfRvaAndSizes         10 00 00 00 ; constant

As you can see, I plan to have only 2 sections, one for code and one for
all the rest (data, constants and import directory). There will be no
relocations and no other stuff like resources. Also I won't have a BSS
segment and stuff the variable 'written' into the initialized data.
The section alignment is the same in the file and in RAM (32 bytes);
this helps to keep the task easy, otherwise I'd have to calculate RVAs
back and forth too much.

Now we set up the data directories, beginning at byte 0xb8 and being 0x80 bytes long:
    Address        Size
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_EXPORT (0)
    ?? ?? ?? ??    ?? ?? ?? ??         ; IMAGE_DIRECTORY_ENTRY_IMPORT (1)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_RESOURCE (2)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_EXCEPTION (3)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_SECURITY (4)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_BASERELOC (5)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_DEBUG (6)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_COPYRIGHT (7)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_GLOBALPTR (8)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_TLS (9)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_LOAD_CONFIG (10)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT (11)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_IAT (12)
    00 00 00 00    00 00 00 00         ; 13
    00 00 00 00    00 00 00 00         ; 14
    00 00 00 00    00 00 00 00         ; 15
Only the import directory is in use.

Next are the section headers. First we make the code section, which will
contain the above mentioned assembly. It is 32 bytes long, and so will
be the code section. The header begins at 0x138 and is 0x28 bytes long:

    Name            2e 63 6f 64 65 00 00 00     ; ".code"
    VirtualSize         00 00 00 00             ; unused
    VirtualAddress      ?? ?? ?? ??             ; yet to find out
    SizeOfRawData       20 00 00 00             ; size of code
    PointerToRawData    ?? ?? ?? ??             ; yet to find out
    PointerToRelocations 00 00 00 00            ; unused
    PointerToLinenumbers 00 00 00 00            ; unused
    NumberOfRelocations  00 00                  ; unused
    NumberOfLinenumbers  00 00                  ; unused
    Characteristics     20 00 00 60             ; code, executable, readable

The second section will contain the data. The header begins at 0x160 and
is 0x28 bytes long:

    Name            2e 64 61 74 61 00 00 00     ; ".data"
    VirtualSize         00 00 00 00             ; unused
    VirtualAddress      ?? ?? ?? ??             ; yet to find out
    SizeOfRawData       ?? ?? ?? ??             ; yet to find out
    PointerToRawData    ?? ?? ?? ??             ; yet to find out
    PointerToRelocations 00 00 00 00            ; unused
    PointerToLinenumbers 00 00 00 00            ; unused
    NumberOfRelocations  00 00                  ; unused
    NumberOfLinenumbers  00 00                  ; unused
    Characteristics     40 00 00 c0             ; initialized, readable, writeable

The next byte is 0x188, but the sections need to be aligned to 32 bytes
(because I chose so), so we need padding bytes up to 0x1a0:

    00 00 00 00 00 00       ; padding
    00 00 00 00 00 00
    00 00 00 00 00 00
    00 00 00 00 00 00


Now the first section, being the code section with the above mentioned
assembly, *does* come. It begins at byte 0x1a0 and is 0x20 bytes long:
    6A 00                    ; push      0x00000000
    68 ?? ?? ?? ??           ; push      offset _written
    6A 0D                    ; push      0x0000000d
    68 ?? ?? ?? ??           ; push      offset hello_string
    6A F5                    ; push      0xfffffff5
    2E FF 15 ?? ?? ?? ??     ; call      dword ptr cs:__imp__GetStdHandle@4
    50                       ; push      eax
    2E FF 15 ?? ?? ?? ??     ; call      dword ptr cs:__imp__WriteConsoleA@20
    C3                       ; ret       

Because of the previous section's length we don't need any padding
before the next section (data), and here it comes, beginning at 0x1c0:

    68 65 6C 6C 6F 2C 20 77 6F 72 6C 64 0A  ; "hello, world\n"
    00 00 00                                ; padding to align _written
    00 00 00 00                             ; _written

Now all that's left is the import directory. It will import 2 functions
from "kernel32.dll", and it's immediatly following the variables in the
same section. First we will align it to 32 bytes:

    00 00 00 00 00 00 00 00 00 00 00 00     ; padding

It begins at 0x1e0 with the IMAGE_IMPORT_DESCRIPTOR:
    OriginalFirstThunk      ?? ?? ?? ??     ; yet to find out
    TimeDateStamp           00 00 00 00     ; unbound
    ForwarderChain          ff ff ff ff     ; no forwarders
    Name                    ?? ?? ?? ??     ; yet to find out
    FirstThunk              ?? ?? ?? ??     ; yet to find out

We need to terminate the import-directory with a 0-bytes-entry (we are at 0x1f4):
    OriginalFirstThunk      00 00 00 00     ; terminator
    TimeDateStamp           00 00 00 00     ;
    ForwarderChain          00 00 00 00     ;
    Name                    00 00 00 00     ;
    FirstThunk              00 00 00 00     ;

Now there's the DLL name left, and the 2 thunks, and the thunk-data, and
the function names. But we will be finished real soon now!

The DLL name, 0-terminated, beginning at 0x208:
    6b 65 72 6e 65 6c 33 32 2e 64 6c 6c 00  ; "kernel32.dll"
    00 00 00                                ; padding to 32-bit-boundary

The original first thunk, starting at 0x218:
    AddressOfData   ?? ?? ?? ??             ; RVA to function name "WriteConsoleA"
    AddressOfData   ?? ?? ?? ??             ; RVA to function name "GetStdHandle"
                    00 00 00 00             ; terminator

The first thunk is exactly the same list and starts at 0x224:
(__imp__WriteConsoleA@20, at 0x224)
    AddressOfData   ?? ?? ?? ??             ; RVA to function name "WriteConsoleA"
(__imp__GetStdHandle@4, at 0x228)
    AddressOfData   ?? ?? ?? ??             ; RVA to function name "GetStdHandle"
                    00 00 00 00             ; terminator

Now what's left is the two function names in the shape of an
IMAGE_IMPORT_BY_NAME. We are at byte 0x230.
    01 00                                      ; ordinal, need not be correct
    57 72 69 74 65 43 6f 6e 73 6f 6c 65 41 00  ; "WriteConsoleA"
    02 00                                      ; ordinal, need not be correct
    47 65 74 53 74 64 48 61 6e 64 6c 65 00     ; "GetStdHandle"

Ok, that's about all. The next byte, which we don't really need, is
0x24f. We need to fill the section with padding up to 0x260:
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; padding
    00

------------

We are done. Now that we know all the byte-offsets, we can apply fixups
to all those addresses and sizes that were indicated as "unknown" with
'??'-marks.
I won't force you to read that step-by-step (it's quite
straightforward), and simply present the result:

------------

DOS-header, starting at 0x0:
    00 | 4d 5a 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    10 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    20 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    30 | 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 00

signature, starting at 0x40:
        50 45 00 00

file-header, starting at 0x44:
    Machine                     4c 01       ; i386
    NumberOfSections            02 00       ; code and data
    TimeDateStamp               00 00 00 00 ; who cares?
    PointerToSymbolTable        00 00 00 00 ; unused
    NumberOfSymbols             00 00 00 00 ; unused
    SizeOfOptionalHeader        e0 00       ; constant
    Characteristics             02 01       ; executable on 32-bit-machine

optional header, starting at 0x58:
    Magic                       0b 01       ; constant
    MajorLinkerVersion          00          ; I'm version 0.0 :-)
    MinorLinkerVersion          00          ;
    SizeOfCode                  20 00 00 00 ; 32 bytes of code
    SizeOfInitializedData       a0 00 00 00 ; data section size
    SizeOfUninitializedData     00 00 00 00 ; we don't have a BSS
    AddressOfEntryPoint         a0 01 00 00 ; beginning of code section
    BaseOfCode                  a0 01 00 00 ; RVA to code section
    BaseOfData                  c0 01 00 00 ; RVA to data section
    ImageBase                   00 00 10 00 ; 1 MB, chosen arbitrarily
    SectionAlignment            20 00 00 00 ; 32-bytes-alignment
    FileAlignment               20 00 00 00 ; 32-bytes-alignment
    MajorOperatingSystemVersion  04 00      ; NT 4.0
    MinorOperatingSystemVersion  00 00      ;
    MajorImageVersion           00 00       ; version 0.0
    MinorImageVersion           00 00       ;
    MajorSubsystemVersion       04 00       ; Win32 4.0
    MinorSubsystemVersion       00 00       ;
    Win32VersionValue           00 00 00 00 ; unused?
    SizeOfImage                 c0 00 00 00 ; sum of all section sizes
    SizeOfHeaders               a0 01 00 00 ; offset to 1st section
    CheckSum                    00 00 00 00 ; not used for non-drivers
    Subsystem                   03 00       ; Win32 console
    DllCharacteristics          00 00       ; unused (not a DLL)
    SizeOfStackReserve          00 00 10 00 ; 1 MB stack
    SizeOfStackCommit           00 10 00 00 ; 4 KB to start with
    SizeOfHeapReserve           00 00 10 00 ; 1 MB heap
    SizeOfHeapCommit            00 10 00 00 ; 4 KB to start with
    LoaderFlags                 00 00 00 00 ; unknown
    NumberOfRvaAndSizes         10 00 00 00 ; constant

data directories, starting at 0xb8:
    Address        Size
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_EXPORT (0)
    e0 01 00 00    6f 00 00 00         ; IMAGE_DIRECTORY_ENTRY_IMPORT (1)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_RESOURCE (2)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_EXCEPTION (3)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_SECURITY (4)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_BASERELOC (5)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_DEBUG (6)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_COPYRIGHT (7)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_GLOBALPTR (8)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_TLS (9)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_LOAD_CONFIG (10)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_BOUND_IMPORT (11)
    00 00 00 00    00 00 00 00         ; IMAGE_DIRECTORY_ENTRY_IAT (12)
    00 00 00 00    00 00 00 00         ; 13
    00 00 00 00    00 00 00 00         ; 14
    00 00 00 00    00 00 00 00         ; 15

section header (code), starting at 0x138:
    Name            2e 63 6f 64 65 00 00 00     ; ".code"
    VirtualSize         00 00 00 00             ; unused
    VirtualAddress      a0 01 00 00             ; RVA to code section
    SizeOfRawData       20 00 00 00             ; size of code
    PointerToRawData    a0 01 00 00             ; file offset to code section
    PointerToRelocations 00 00 00 00            ; unused
    PointerToLinenumbers 00 00 00 00            ; unused
    NumberOfRelocations  00 00                  ; unused
    NumberOfLinenumbers  00 00                  ; unused
    Characteristics     20 00 00 60             ; code, executable, readable

section header (data), starting at 0x160:
    Name            2e 64 61 74 61 00 00 00     ; ".data"
    VirtualSize         00 00 00 00             ; unused
    VirtualAddress      c0 01 00 00             ; RVA to data section
    SizeOfRawData       a0 00 00 00             ; size of data section
    PointerToRawData    c0 01 00 00             ; file offset to data section
    PointerToRelocations 00 00 00 00            ; unused
    PointerToLinenumbers 00 00 00 00            ; unused
    NumberOfRelocations  00 00                  ; unused
    NumberOfLinenumbers  00 00                  ; unused
    Characteristics     40 00 00 c0             ; initialized, readable, writeable

(padding)
    00 00 00 00 00 00       ; padding
    00 00 00 00 00 00
    00 00 00 00 00 00
    00 00 00 00 00 00

code section, starting at 0x1a0:
    6A 00                    ; push      0x00000000
    68 d0 01 10 00           ; push      offset _written
    6A 0D                    ; push      0x0000000d
    68 c0 01 10 00           ; push      offset hello_string
    6A F5                    ; push      0xfffffff5
    2E FF 15 28 02 10 00     ; call      dword ptr cs:__imp__GetStdHandle@4
    50                       ; push      eax
    2E FF 15 24 02 10 00     ; call      dword ptr cs:__imp__WriteConsoleA@20
    C3                       ; ret       

data section, beginning at 0x1c0:
    68 65 6C 6C 6F 2C 20 77 6F 72 6C 64 0A  ; "hello, world\n"
    00 00 00                                ; padding to align _written
    00 00 00 00                             ; _written
padding:
    00 00 00 00 00 00 00 00 00 00 00 00     ; padding
IMAGE_IMPORT_DESCRIPTOR, starting at 0x1e0:
    OriginalFirstThunk      18 02 00 00     ; RVA to orig. 1st thunk
    TimeDateStamp           00 00 00 00     ; unbound
    ForwarderChain          ff ff ff ff     ; no forwarders
    Name                    08 02 00 00     ; RVA to DLL name
    FirstThunk              24 02 00 00     ; RVA to 1st thunk
terminator (0x1f4):
    OriginalFirstThunk      00 00 00 00     ; terminator
    TimeDateStamp           00 00 00 00     ;
    ForwarderChain          00 00 00 00     ;
    Name                    00 00 00 00     ;
    FirstThunk              00 00 00 00     ;
The DLL name, at 0x208:
    6b 65 72 6e 65 6c 33 32 2e 64 6c 6c 00  ; "kernel32.dll"
    00 00 00                                ; padding to 32-bit-boundary
original first thunk, starting at 0x218:
    AddressOfData   30 02 00 00             ; RVA to function name "WriteConsoleA"
    AddressOfData   40 02 00 00             ; RVA to function name "GetStdHandle"
                    00 00 00 00             ; terminator
first thunk, starting at 0x224:
    AddressOfData   30 02 00 00             ; RVA to function name "WriteConsoleA"
    AddressOfData   40 02 00 00             ; RVA to function name "GetStdHandle"
                    00 00 00 00             ; terminator
IMAGE_IMPORT_BY_NAME, at byte 0x230:
    01 00                                      ; ordinal, need not be correct
    57 72 69 74 65 43 6f 6e 73 6f 6c 65 41 00  ; "WriteConsoleA"
IMAGE_IMPORT_BY_NAME, at byte 0x240:
    02 00                                      ; ordinal, need not be correct
    47 65 74 53 74 64 48 61 6e 64 6c 65 00     ; "GetStdHandle"
(padding)
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ; padding
    00
First unused byte: 0x260

--------------

Alas, this works on NT but didn't on windows 95. windows95 can't run
applications with a section alignment of 32 bytes, it needs an
alignment of 4 KB and, apparently, a file alignment of 512 bytes. So for
windows95 you'll have to insert a large number of 0-bytes (for padding)
and adjust the RVAs. Thanks go to D. Binette for testing on windows95.


        -- end of text --





						
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