X86 Disassembly/Mac OS X

Mach-O format overview

MacOS (Previously OS X) uses the Mach-O file format to encode executables, object files, and shared libraries (.dylib files). Here, we will be looking at the 64-bit version of the Mach-O format. The majority of data in Mach-O files are 'segments' and 'sections', where Segments are containers for Sections, and store information about each Section. The Sections themselves are containers for data. Mach-O files have five primary structures:

Structure	Description
Header	Contains information about the purpose, and size of the file's structures
Load Commands	Declaration of all Segments and Sections
Data	The actual contents of the file (e.g. Data section, Text section).
Symbol table	Says where each symbol is located in the file
String table	Contains the name of each symbol

Note that when each Structure is gone over, they are all an unbroken sequence of bytes, and there is no empty space between them.

Header

Information

The header is the very first thing in the file, and it has 8 unsigned 32-bit integers:

Name	Purpose	Endianness	Typical Value
Magic Number	The File's magic number	Big-Endian	0xFEEDFACF for 64-bit architecture
CPU Type	The Intended CPU type for the executable	Little-Endian	0x01000007 for x86_64
CPU subtype	The specific kind of CPU used	Little-Endian	0x00000003 for all x64 CPUs
File type	The purpose of the file	Little-Endian	0x00000001 for object file, 0x00000002 for executable
Number of Load Commands	The quantity of Load commands (does not include section headers)	Little-Endian	Variable
Size of Load Commands	The number of bytes occupied by the Load Commands	Little-Endian	Variable
Flags	Extra file information	Little-Endian	0x00000000
Reserved	No practical use	Little-Endian	0x00000000