CS 241 - MIPS Executable Relocatable Linkable (.merl) File Format

A .merl file is an executable MIPS binary file that is augmented by a table containing relocation and linking information. A .merl file has three components:

Header: Three words consisting of
- Cookie: a word containing 0x10000002 (which happens to be the binary encoding of the MIPS instruction beq $0,$0,2)
- EndModule: the length (in bytes) of the .merl file
- EndCode: the length (in bytes) of the header plus the MIPS program (see below)
MIPS program: a MIPS binary program encoded so as to execute correctly when loaded at RAM address 0xc (immediately following the header)
Relocation and External Symbol Table: zero or more table entries, each having one of the following format:
- relocation entry: each relocation entry contains two words:
  - REL format code: a word containing 0x01
  - location: the location in the .merl file where the relocatable value is encoded
- external symbol definition:
  - ESD format code: a word containing 0x05
  - location: a 32 bit word encoding the (relocatable) value of the defined symbol (that is, the location the symbol refers to)
  - name: a 32 bit word encoding n, the number of characters in the symbol name, followed by n words, each encoding one of the characters in ASCII
- external symbol reference: a relocatable value whose encoding is imported from some other .merl file, consisting of
  - ESR format code: a word containing 0x11
  - location: the location in the .merl file where the value is to be encoded, once known
  - name: a 32 bit word encoding n, the number of characters in the symbol name, followed by n words, each encoding one of the characters in ASCII

A .merl file may be executed at location 0 as a .mips binary, or may be used as input to a loader or linker which creates a .mips binary to be executed at some other location, or which creates another .merl file.