.OBJ

Introduction

OMF, the Object Module Format, is the object file format Intel defined for the 8086. Most compilers and assemblers for 16-bit DOS and Windows wrote it, including Borland’s Turbo C, Turbo Assembler and Borland C++, and Microsoft’s early C compilers and MASM. A DOS linker such as Microsoft LINK or Borland TLINK joined OMF files into an MZ or NE program. Some 32-bit compilers, such as Watcom C and Borland’s 32-bit tools, kept using it with 32-bit extensions.

Libraries of OMF object files also use the .lib extension, in a format of their own that starts with a library header record. Modern Windows compilers write COFF object files instead. See all the .OBJ variations.

Structure

An OMF file is a sequence of records. Each starts with a one-byte record type and a 16-bit little-endian length, which counts the rest of the record including a final checksum byte. The checksum makes the sum of all the record’s bytes zero. Many record types come in pairs: the even number is the 16-bit form and the odd number the 32-bit form.

A module begins with a THEADR record (0x80) holding its name, usually the source file name, and ends with a MODEND record (0x8A or 0x8B). Between them, LNAMES (0x96) lists names, SEGDEF (0x98) and GRPDEF (0x9A) define segments and groups, EXTDEF (0x8C) and PUBDEF (0x90) list the symbols the module needs and provides, LEDATA (0xA0) and LIDATA (0xA2) carry the code and data, and FIXUPP (0x9C) records say which addresses the linker must fill in. COMENT records (0x88) carry compiler notes and extensions.

Also used for

• Wavefront OBJ .obj

Links