[Coco] Apparent discrepancy between MAME and XRoar re: SBCB instruction

[Barry Nelson]

> Pierre Sarrazin
> Tue Oct 17 19:22:57 EDT 2017
> 
> Hi folks,
> 
> I seem to have found a difference in the way the SBCB instruction
> executes on MAME (0.158) and XRoar (0.34.7), unless I'm doing something
> wrong without realizing it.
> 
> I would like to know which emulator is right. I don't have easy access
> to my real CoCo, so I wondered if anyone here could run the following
> program and tell me which number appears on the line before the final
> OK prompt (on the 32x16 screen).
> 
> 10 DATA5F,1C,00,1A,01,C2,FF
> 20 DATA34,01,35,02,B7,04,00,39
> 30 FORI=0TO14:READH$
> 40 POKE&H3800+I,VAL("&H"+H$)
> 50 NEXT
> 60 CLS:PRINT
> 70 EXEC&H3800
> 80 PRINTPEEK(1024)AND15
> 
> On MAME 0.158, it's 7 while on XRoar, it's 5. This value shows the N, Z,
> V and C flags that result from the SBCB instruction. The two values mean
> that MAME's SBCB set the V (overflow) flag to 1, while XRoar set it to 0.
> 
> I also tested on MAME 0.140, and it gave 5, interestingly.
> 
> A test on the latest MAME would also be useful.
> 
> The assembly source is the following:
> 
>         org     $3800
> 
> start
>         clrb
>         andcc   #$00    ; clear all flags
>         orcc    #$01    ; set carry
>         sbcb    #$FF    ; $00 - $ff - 1 = $00
>         pshs    cc
>         puls    a
>         sta     $400    ; store resulting CC on 32x16 screen
>         rts
>         
>         end     start
> 
> It can be assembled with LWTOOLS like this:
> lwasm -fdecb -o sbc.bin sbc.asm
> 
> I also prepared a disk image with the Basic program on it:
> http://sarrazip.com/dev/sbc.dsk <http://sarrazip.com/dev/sbc.dsk>
> 
> Thanks.

On my CoCo 1 I get an inverse E and below that, a result of 5, but on MAME 0.187 I get an inverse G and below that, a result of 7, so yes, MAME has another bug.