Concise Atari ST 68000 Programmer's Reference Guide


GEM BIOS calls

To access the GEM BIOS functions, push the parameters in the order given onto the stack and then call Trap #13. Reply or status is returned in register D0 and the data placed on the stack trashed.

Typical use might be:
move.w	driveA,-(sp)	* push device code 
move.w	record,-(sp)	* push record to start
move.w	count, -(sp)	* push no, of sectors
move.1	address,-(sp)	* push buffer address 
move.w	#0, -(sp)	* push read data
eove.w	#4, -(sp)	* push swabs function call
trap	#13		* call the function
add.w	#14,sp		* tidy the stack
tst.w	D0		* test for error

It is the programmers responsibility to tidy the stack after the call. The BIOS, accessible from user mode, is re-entrant to three levels of calls, users are advised that this non-standard feature should be used wisely where program portability is required.