executable_structure.md

The structure of executable files

The executable starts with a header:

• The first byte specifies the architecture (harward, von neumann)
• The $[1;4]$ bytes are the entry point (little endian. The extra 2 bytes were added for possible future changes. For now, if the entry point is greater than 0xffff, it errors)
• The rest of the bytes are the segments

Each segment:

• First 17 bytes are the metadata:
  • The first 4 - where the segment start lies in the address space
  • The next 4 - the size of a single segment byte, in bits
  • The next 4 - the size of the segment in the executable file, in its own bytes
  • The next 4 - the size of the segment in the address space, in its own bytes (can be larger than the disk size - the rest is zero-filled)
  • The next 1 - the flags of the segment:
    • 0 - executable
    • 1 - writable
    • 2 - readable
    • 3 - special (whether should be mapped to the address space at all)
    • 4 - stripped (whether should remain when generating in release mode)
• The rest of the bytes are the segment data (the instructions, the data)