CP/M 3 COM file header
Under CP/M 3, COM files can have a 256-byte header (attached by GENCOM). Assuming the file is loaded at 0100h, then the format of the header is:
0100 DB 0C9h ;Magic number - "RET". This prevents the header ;being executed on older systems which don't know ;about this file format. 0101 DW len ;Length of the original COM file (minus the header and ;any RSXs that have been added to it). This will ;be in memory at 0200h, and should be moved to 0100h ;before it is executed. 0103 RET or JMP init ;Pre-initialisation code, which should return with a ;RET. This will be called before the COM file is ;relocated, and is intended for SCBs: ; GENCOM FILE [SCB=(x,y)] 010D DB l ;Loader flag. 0FFh = no RSX attached. Loader will ;remain active (called with GENCOM FILE [LOADER]). 010F DB n ;Number of valid RSX records, up to 15 0110 DS 10h ;1st RSX record 0120 DS 10h ;2nd RSX record, etc.
Each RSX record is:
01x0 DW offset ;Offset of RSX image from file base (ie, add ;100h to get RSX address) 01x2 DW codelen ;Length of code and initialised data segment 01x4 DB 0 ;Non-banked flag. 0FFh = load this RSX only in ;a non-banked system. 01x5 DB 0 ;Unused 01x6 DB 'NAME ' ;8 bytes original RSX filename 01xE DW 0 ;Unused
John Elliott / Jim Lopushinsky 11 April 1998