Checking for Micro Channel Bus

Assembly Routine

Content by David L. Beem (original HERE). Modified by Tomáš Slavotínek.
Last update: 03 Oct 2021 (synced with David's version dated 25 Sep 2021).


In many of the PS/2 routines you may need to check if the system is a Micro Channel computer. This routine does that check, setting the Carry flag if the system does not have the Micro Channel bus. If it is a Micro Channel system, the Carry flag is clear and the AL register holds the number of Micro Channel 'slots'. Be aware that some of the system planar resources may be identified as a 'slot'.

This routine becomes harder in BASIC or QB (the version included with MS-DOS) with the lack of the CALL INTERRUPT statement. IBM moved the "Get Configuration" table on some of the PS/2s (it started at F000:E6F5h on older IBM systems), otherwise a PEEK at the the particular bit in BIOS could be used. One possible way around this is to POKE an Assembly routine into memory and then execute it.

Assembly Routine

; CheckMCA Assembly routine
; This routine checks whether the system has a
; Micro Channel (MCA) bus.
; Entry:
;     None
; Exit:
;     Carry flag clear if Micro Channel bus
;         AL = Number of MCA slots
;     Carry flag set if not Micro Channel bus
;         AL = 0

CheckMCA	proc	near
		mov	AH,C0h
		int 	15h
		mov	AL,0
		jc	Finish			;Call supported?
		test 	byte ptr ES:[BX+05],02	;MCA flag bit?
		jne 	Finish
		cmp	byte ptr ES:[BX+02],FC	;IBM 286?
		jne	Slots
		cmp	byte ptr ES:[BX+03],06	;7552 Gearbox?
		je	Finish
Slots:		clc
		out 	75h,01
		mov	AL,8Eh
		out 	74h,AL
		in 	AL,76h
		cmp	AL,08
		jbe	Finish
		mov	AL,04
Finish:		sti
CheckMCA	endp


  • The PS/2 Model 50, 50Z, and 55SX, as well as the Industrial Models 7541/7542 (because both are based on a Model 50Z planar) lack the 2 KB Extended CMOS RAM ("NVRAM") that is used for a Micro Channel configuration of more than four adapters.
  • The IBM 7552 "Gearbox" (BIOS Model FCh, Submodel 06h) has Micro Channel adapter interposers that can be installed, but implements the Micro Channel configuration by a different method from later designs, therefore it is excluded by this detection routine.

Content created and/or collected by:
Louis Ohland, Peter Wendt, William Walsh, David Beem, Tatsuo Sunagawa, Jim Shorney, Tim Clarke, Kevin Bowling, Tomáš Slavotínek, and many others.

Ardent Tool of Capitalism - MAD Edition! is maintained by Tomáš Slavotínek.
Last update: 20 Jan 2022 - Changes & Credits | Legal Info & Contact