[Coco] Bug in VCC ???

[Luis Antoniosi (CoCoDemus)]

One note here: it's the Z-80 code that goes crazy not the cp/m virtual
machine. The cp/m vm still keeps working fetching and running each
instruction.

How the VM works, suppose you have a instruction like INC L in Z-80 I will
use the 6809 counter part, so the implementation is:

INCL inc <L
tfr cc,b
lbsr _inc8
lbra _fetch

_inc8 lda <F
anda #~(SF|ZF|HF|PF|NF) * clean Z80 flags
andb #~(M_CF|M_HF)      * clean M6809 flags
leay FLAGTBL,pcr
ora b,y
sta <F
rts

So FLAGTBL has 256 entries converting each 6809 flag combination to the
Z-80 ones. Some 6809 don't set/unset the same flags on Z-80 so extra code
is needed to set them. I suspect that VCC is setting/unsetting a flag that
wasn't supposed to be set and the next conditional jump that relies on it
goes to the wrong place.


On Wed, Apr 16, 2014 at 12:15 PM, Luis Antoniosi (CoCoDemus) <
retrocanada76 at gmail.com> wrote:

> I noticed the VCC 1.43beta won't run my cpm emulator running zork1.comproperly.
>
> It seem to get a wrong conditional jump and goes crazy scrolling the
> screen up.
>
> The same image DOES work on MESS and a real Coco3. The Mbasic.com
> seemed at beginning to work fine.
>
> I'm using a direct lookup table to convert 6809 flags to Z-80 flags.
> So it could be the case some instruction setting or resetting a wrong
> flag.
>
> I have no clue which instruction is that since I don't use VCC (it
> does not have a debugger it has no purpose for me).
>
> The cpm disks:
>
>
> https://sites.google.com/site/tandycocoloco/dropbox/cpm-os9.zip?attredirects=0&d=1
>
> Mount zork.os9 (80T disk image) and run
>
> /d1/cpm zork1.com
>
> And you will see the bug right away.
>
>
> --
> Long live the CoCo
>



-- 
Long live the CoCo