Re: [hatari-devel] "IKBD buffer full" problem

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

Le 13/09/2012 16:19, Eero Tamminen a écrit :
> Hi,
>
> On torstai 13 syyskuu 2012, Eero Tamminen wrote:
> > I run into a program that causes loads of "IKBD buffer full"
> > messages after moving mouse a bit.
> >
> > After that Hatari refuses to process any information for TOS,
> > *even* if I coldboot the emulation.
> >
> > I.e. there seem to be two problems; the IKBD buffer full issue,
> > and reboot not getting rid of it...
> >
> >
> > This seems a regression, as it doesn't happen with Hatari v1.4.
> >
> > The program causing the issue is attached.
>
> Found another game causing similar issues, called "Downfall".
>
>
> 	- Eero

Hello

the problem is in fact in the $16 command IKBD_Cmd_ReturnJoystick (which 
already has a comment about downfall doing silly things).

This game is sending continuous $16 command to the ikbd without really 
waiting for the 3 returned bytes (so basically, this game is rather 
crappy :) )

From a real ikbd point of view, only one new command should be queued 
until the current command is completed. But IKBD_Cmd_ReturnJoystick has 
an initial 35000 cycles before returning the 1st byte, although command 
$16 takes ~8000 cycles to be sent.

This means that after 8000 cycles, the game will be able to send other 
$16 command and they will eventually fill the buffer ring, since each 
command adds 3 bytes to the buffer that are read much later.

For commands that return several bytes, we should have a "command in 
process" flag to ignore new command suntil the last byte is sent by the 
ikbd (as a real ikbd is not multitasking, he can't process new commands 
while sending back bytes at the same time from a previous command).

I will see if I can modify that, but I'm seriously thinking about 
rewriting completly the whole ACIA emulation ; it should not be so 
merged with the ikbd itself, and some shortcuts were taken to get a 
functional behaviour, but without really emulating the inner real 
behaviour described in the datasheet.

This is why we keep on having problem in the acia/ikbd, and often 
regressions. Patching it again each time a new problem arises is not the 
good solution, rewriting it as a state machine with all the regs / 
latches described in the datasheet will certainly help starting on more 
solid bases.


Nicolas