Strange piece of ASM

From: Baltissen, GJPAA (Ruud) (ruud.baltissen_at_abp.nl)
Date: 2003-10-15 15:15:29

hallo allemaal,


At this moment I'm commenting the BIOS of the PC20-III and I run into a
strange piece of code. The first routine is run on several places in the
ROM. The first time after initialising the DRAM:

		call	sub_FA39  
		jnz	loc_E2D2		; error, ->

E2D2 is the start of an error routine. This means that to continue, the
Zeroflag must be set. The routine itself:

;
;                              SUBROUTINE
;

sub_FA39	proc	near
		push	dx
		mov	dx,62h
		in	al,dx			; port 62h, 8255 C -
dipswitch, etc.
		xchg	ah,al
		call	ResetParityFF
		and	ah,0C0h
		shl	ah,1	
		shl	ah,1	
		or	al,ah
		out	dx,al	
		test	ah,30h
		pop	dx
		retn
sub_FA39	endp


;
;                              SUBROUTINE
;

;**  Resets the two flipflops that hold the onboard and external party error
;[FA52]
ResetParityFF	proc	near
		mov	dx,61h
		in	al,dx			; 8255 port B, read
		xor	al,30h
		out	dx,al			; 8255 B - spkr, etc
		xor	al,30h
		out	dx,al			; 8255 B - spkr, etc
		retn
ResetParityFF	endp


I removed my own comment so you won't be influenced by it. The main question
is: what is the content of AH at the end of the first subroutine? IMHO it is
always zero. But that would mean that the routine makes no sense or that I'm
stupid.

Thanks for any comment, even if you have to tell me I'm stupid :)


--
    ___
   / __|__
  / /  |_/     Groetjes, Ruud
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   \___|       http://Ruud.C64.org



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