Relocatable Object Module Format (OMF) Specification
Relocatable Object Module Format (OMF) Specification
Relocatable Object Module Format (OMF) Specification
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<strong>Relocatable</strong> <strong>Object</strong> <strong>Module</strong> <strong>Format</strong><br />
The <strong>Object</strong> Record <strong>Format</strong><br />
Record <strong>Format</strong><br />
All object records conform to the following format:<br />
<br />
1 2 1<br />
Record Type Record Length Record Contents Checksum or 0<br />
The Record Type field is a 1-byte field containing the hexadecimal number that identifies the type of object record.<br />
The format is determined by the least significant bit of the Record type field. An odd Record Type indicates that<br />
certain numeric fields within the record contain 32-bit values; an even Record Type indicates that those fields<br />
contain 16-bit values. The affected fields are described with each record. Note that this principle does not govern<br />
the Use32/Use16 segment attribute (which is set in the ACBP byte of SEGDEF records); it simply specifies the<br />
size of certain numeric fields within the record. It is possible to use 16-bit <strong>OMF</strong> records to generate 32-bit<br />
segments, or vice versa.<br />
The Record Length field is a 2-byte field that gives the length of the remainder of the object record in bytes<br />
(excluding the bytes in the Record Type and Record Length fields). The record length is stored with the low-order<br />
byte first. An entire record occupies 3 bytes plus the number of bytes in the Record Length field.<br />
The Record Contents field varies in size and format, depending on the record type.<br />
The Checksum field is a 1-byte field that contains the negative sum (modulo 256) of all other bytes in the record.<br />
In other words, the checksum byte is calculated so that the low-order byte of the sum of all the bytes in the record,<br />
including the checksum byte, equals 0. Overflow is ignored. Some compilers write a 0 byte rather than computing<br />
the checksum, so either form should be accepted by programs that process object modules.<br />
Note: The maximum size of the entire record (unless otherwise noted for specific record types) is<br />
1024 bytes.<br />
Tool Interface Standards (TIS) <strong>OMF</strong> <strong>Specification</strong>, Version 1.1 1