# Portable Executable

The Portable Executable (PE) format is a file format for executables, object code, dynamic-link-libraries (DLLs), and binary files used on 32-bit and 64-bit Windows operating systems, as well as in UEFI environments. It is the standard format for executables on Windows NT-based systems, including files such as .exe, .dll, .sys (for system drivers), and .mui. At its core, the PE format is a structured data container that gives the Windows operating system loader everything it needs to properly manage the executable code it contains. This includes references for dynamically linked libraries, tables for importing and exporting APIs, resource management data and thread-local storage (TLS) information.
According to the Unified Extensible Firmware Interface (UEFI) specification, the PE format is also the accepted standard for executables in EFI environments. On Windows NT systems, it currently supports a range of instruction sets, including IA-32, x86-64 (AMD64/Intel 64), IA-64, ARM and ARM64. Before the advent of Windows 2000, Windows NT (and by extension the PE format) also supported MIPS, Alpha, and PowerPC architectures. Moreover, thanks to its use in Windows CE, PE has maintained compatibility with several MIPS, ARM (including Thumb), and SuperH variants.
Functionally, the PE format is similar to other platform-specific executable formats, such as the ELF format used in Linux and most Unix-like systems, and the Mach-O format found in macOS and iOS.

## History

Microsoft first introduced the PE format with Windows NT 3.1, replacing the older 16-bit New Executable (NE) format. Soon after, Windows 95, 98, ME, and the Win32s extension for Windows 3.1x, all adopted the PE structure. Each PE file includes a DOS executable header, which generally displays the message "This program cannot be run in DOS mode". However, this DOS section can be replaced by a fully functional DOS program, as demonstrated in the Windows 98 SE installer. Developers can add such a program using the /STUB switch with Microsoft's linker, effectively creating a fat binary.
Over time, the PE format has grown with the Windows platform. Notable extensions include the .NET PE format for managed code, PE32+ for 64-bit address space support, and a specialized version for Windows CE.
To determine whether a PE file is intended for 32-bit or 64-bit architectures, one can examine the Machine field in the IMAGE_FILE_HEADER. Common machine values are 0x014c for 32-bit Intel processors and 0x8664 for x64 processors. Additionally, the Magic field in the IMAGE_OPTIONAL_HEADER reveals whether addresses are 32-bit or 64-bit. A value of 0x10B indicates a 32-bit (PE32) file, while 0x20B indicates a 64-bit (PE32+) file.

## Technical details
### Layout

A PE file consists of several headers and sections that instruct the dynamic linker about on how to map the file into memory. An executable image consists of several different regions, each requiring different memory protection attributes. To ensure proper alignment, the start of each section must align to a page boundary. For instance, the .text section, which contains program code, is typically mapped as an execute/read-only. Conversely, the .data section, which holds global variables, is mapped as no-execute/read write. However, to conserve space, sections are not aligned on disk in this manner. The dynamic linker maps each section to memory individually and assigns the correct permissions based on the information in the headers.

### Import table

The import address table (IAT) is used as a lookup table when the application calls a function in a different module. The imports can be specified by ordinal or by name. Because a compiled program cannot know the memory locations of its dependent libraries beforehand, an indirect jump is necessary for API calls. As the dynamic linker holds modules and resolves dependancies, it populates the IAT slots with actual addresses of the corresponding library functions. Although this adds an extra jump, incurring a performance penalty compared to intermodular calls, it minimizes the number of memory pages that that require copy-on-write changes, thus conserving memory and disk I/O. If a call is known to be intermodular beforehand (if indicated by a dllimport attribute), the compiler can generate optimized code with a simple indirect call opcode.

### Address Space Layout Randomization (ASLR)

PE files aren't position-independent by default; they are compiled to run at a specific, fixed memory address. Modern operating systems use Address Space Layout Randomization (ASLR) to make it harder for attackers to exploit memory-related vulnerabilities. ASLR works by randomly changing the memory address of important parts of the program every time it's loaded. This includes the base address of the program itself, shared libraries (DLLs), and memory areas like the heap and stack. ASLR rearranges the address space positions of key data areas of a process, including the base of the executable and the positions of the stack, heap and libraries. By randomizing these memory addresses each time the process an application is loaded, ASLR prevents attackers from being able to reliably predict memory locations.

## .NET, metadata, and the PE format

In a .NET executable, the PE code section contains a stub that invokes the CLR virtual machine startup entry, _CorExeMain or _CorDllMain in mscoree.dll, much like it was in Visual Basic executables. The virtual machine then makes use of .NET metadata present, the root of which, IMAGE_COR20_HEADER (also called "CLR header") is pointed to by IMAGE_DIRECTORY_ENTRY_COMHEADER (the entry was previously used for COM+ metadata in COM+ applications, hence the name) entry in the PE header's data directory. IMAGE_COR20_HEADER strongly resembles PE's optional header, essentially playing its role for the CLR loader.
The CLR-related data, including the root structure itself, is typically contained in the common code section, .text. It is composed of a few directories: metadata, embedded resources, strong names and a few for native-code interoperability. Metadata directory is a set of tables that list all the distinct .NET entities in the assembly, including types, methods, fields, constants, events, as well as references between them and to other assemblies.

## Use on other operating systems

The PE format is also used by ReactOS, an open-source operating system created to be binary-compatible with Windows. Historically, it has also been used by other operating systems such as SkyOS and BeOS R3. However, both SkyOS and BeOS eventually moved to ELF.
The Mono development platform, which aims to be binary compatible with the Microsoft .NET Framework, uses the same PE format as the Microsoft implementation. The same goes for Microsoft's own cross-platform .NET Core.
On x86(-64) Unix-like operating systems, Windows binaries (in PE format) can be executed using Wine. The HX DOS Extender also uses the PE format for native DOS 32-bit binaries, and can execute some Windows binaries in DOS, thus acting like an equivalent of Wine for DOS.
Mac OS X 10.5 has the ability to load and parse PE files, although it does not maintain binary compatibility with Windows.
UEFI and EFI firmware use PE files as well as the Windows ABI x64 calling convention for applications.