User`s guide

4.4.1 The RSX Prefix

The first 27 bytes of an RSX program contain a standard data structure called the

RSX prefix. The RSX prefix has the following format:

serial:

d b 0,0,0,0,0,0

start:.

jmp ftest ; start of Program

n e x t :

db 0c3h ; jump instruction to

dw 0 ; next module in line

dw 0 ; Previous module

remove:

db 0ffh ; remove flag

nonbank:

db 0 ; nonbank flag

name:

db '123456713' ; any B-character name

loade r :

db 0 ; loader flag

db 0,0 ; reserved area

The only fields of the RSX prefix that you must initialize are the remove: flag, the

nonbank: flag, and the name: of the RSX.

For compatibility with previous releases of CP/M, the serial: field of the prefix is

set to the serial number of the operating system by the LOADER module when the

RSX is loaded into memory. Thus, the address in location 6 locates the byte follow-

ing the serial number of the operating system with or withou; RSXs in memory.

The start: field contains a jump instruction to the beginning of the RSX code

where the RSX tests to see if this BDOS function call is to be intercepted or passed

on to the next module in line.

The next: field contains a jump instruction to the next module in the chain or the

LOADER module if the RSX is the oldest one in memory. The RSX program must

make its own BDOS function calls by calling the next: entry point.

4-21

4.4 Construction of an RSX Program CP/M 3 Programmer's Guide