[Coco] 6809 CC Reg Overflow bit

[Neil Cherry]

On 11/06/2018 06:24 PM, Gene Heskett wrote:
> On Tuesday 06 November 2018 17:58:22 Brendan Donahe wrote:
> 
>> Ok, dumb followup question then - if those 100s and 200s are decimal
>> (I see no $) then why is this an overflow?  Is overflow for a signed
>> byte?
> 
> I think so, v is set (I think) if the result is >127, for 2's complement
> math. This also explains the need for the SEX, or sign extend
> instruction, something the C compiler goes crazy with.

I think I had it wrong:


.a400
0400 0000       NEG   $00	lda #100
0402 0000       NEG   $00	adda #100
0404 0000       NEG   $00	adda #100
0406 0000       NEG   $00	nop
0407 0000       NEG   $00	nop
0408 0000       NEG   $00	.
.r
X=0000 Y=0000 U=0000 S=0400 A=00 B=00 D=00 C=00
P=0400 LDA   #$64
.p
X=0000 Y=0000 U=0000 S=0400 A=64 B=00 D=00 C=00
P=0402 ADDA  #$64
.p
X=0000 Y=0000 U=0000 S=0400 A=C8 B=00 D=00 C=0A
P=0404 ADDA  #$64
.p
X=0000 Y=0000 U=0000 S=0400 A=2C B=00 D=00 C=01
P=0406 NOP
.

This is from the sbc09 emulator, I grabbed a copy and tossed it on my
Linux box. C=0A (Negative set, Overflow set) then C=01 (Negative cleared,
Overflow cleared).

Sorry too many asm languages. :-)

-- 
Linux Home Automation         Neil Cherry       ncherry at linuxha.com
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