[Coco] Graphics on NitrOS9 Level 1 v3.3.0 (CoCo 1)

[Arthur Flexser]

I'm not an OS-9 user, but I can tell a certain amount from the code and the
comments/labels.

It appears that the routine is supposed to be entered with one of the bits
of A set, and with B equal to zero.

The routine determines which particular address from a table of 2-byte
addresses is to be used.  If bit n of A (n = 0 to 7) is set on entry, then
the loop will execute n+1 times and exit with 2 x n in the B register.
That number will be used as an offset from the start of the table of 2-byte
addresses.

One plausible guess for the source of the bug is that instead of B being
set to zero prior to entering the loop, A got set to zero instead, causing
the loop never to exit since the set bit of A that is being searched for
does not exist.  So you might look and see if there is a CLRA shortly
before the loop that is supposed to be CLRB.

Art


On Tue, Feb 6, 2018 at 6:00 PM, David Ladd <davidwladd at gmail.com> wrote:

> Rogelio,
>
> So using MAME's debugger I was able to find that where we think it gets
> locked up is actually it is stuck in a loop that it never gets out of.
> After doing some digging through GrfDrv and VTIO for level1 I did find the
> only routine that matched the routine it is forever locked in a loop is in
> VTIO
>
> Here is the code
> {code}
> AddrFind
>          bita  #$01       ; Done all shifts ?
>          bne   AddrDone
>          addb  #$2        ; increment addr offset ptr
>          lsra
>          bra   AddrFind   ; Test again
> AddrDone
> {code}
>
> So from what MAME is showing me A register is always 0.  So hopefully
> someone more versed in 6809 assembly and the VTIO for level1 could possible
> look into this that understands how this routine should be working.
>
>
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> | David Ladd a.k.a. PacoOtaktay a.k.a. Drencor                          |
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>
>
> On Mon, Feb 5, 2018 at 10:12 AM, Rogelio Perea <os9dude at gmail.com> wrote:
>
> > On Mon Feb 5 2018, David Ladd wrote:
> >
> > So after some looking at NitrOS-9 L1 it does seem there is a issue with
> > > VTIO and possible GrfDrv as well.
> > >
> > > So far VTIO does not seem to automatically load GrfDrv as it should.
> If
> > > you merge GrfDrv in the OS9Boot file on startup so it is in memory then
> > > when VTIO tries to call GrfDrv the system locks up.
> > >
> > > Graphics display codes that start with $15 to $1F are directed to go
> > > through GrfDrv.  Which this functionality seems to be currently
> broken...
> > >
> >
> > Thanks for the reply and the tech insight David, will take care of
> > reporting suspected bugs at the Sourceforge page as noted.
> >
> >
> > -- RP
> >
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