Re: [hatari-devel] Sync byte on Falcon

["Roger Burrows" <anodyne@xxxxxxxxxxxx>]

On 18 Nov 2023 at 7:43, Thomas Huth wrote:

> Am Wed, 15 Nov 2023 22:35:42 -0500
> schrieb "Roger Burrows" <anodyne@xxxxxxxxxxxx>:
> > The following results were all gathered using TOS 4.04.l
> > 
> > With both an RGB monitor and plain old VGA, reading 0xff820a on my Falcon 
> > returns 0x00, which indicates 60Hz.  Looking at the Hatari source 
> > (VIDEL_SyncMode_WriteByte() in Hatari 2.3.0), if there isn't a mono monitor
> > connected, whenever you write to 0xff820a, Hatari kindly ORs it with 0x02,
> > which is then always returned to a program reading that byte.
> > 
> > I think Hatari is clearly wrong: bit 1 on a Falcon means exactly the same as
> it 
> > does on an ST(e) and should be handled in a similar fashion.
> 
> Ok, that really sounds like the documentation (and thus Hatari) was wrong.
> 
> I can change the source code in Hatari, but I'd like to have a better
> understanding how this register really behaves.
> 
> So if you've got some spare minutes, could you write a little test program
> that:
> 
> - Reads ff820a to back up the original value
> - Write 0xfc to ff820a and read back the value from the register
> - Write 0x02 to ff820a and read back the value from the register
> - Write 0x01 to ff820a and read back the value from the register
> - Restore the original value to the register
> - Print out the three values that have been read back after altering the
>   register
> 
> and then run it with your RGB monitor (or let me know if you want me to
> write such a program instead).

OK, values returned are 0x00, 0x02, 0x01 respectively.

> I think it shouldn't harm the monitor if ff820a is just changed for some
> few cycles (but to be sure, you could also switch the monitor off before
> starting the program with a double click).

I took this precaution, my monitor is very elderly.

 If you're sure that your monitor
> could also handle 50 Hz, you could also write 0x02 or 0x01 to ff820a and
> leave it there for some frames to see whether it makes a difference in the
> output at all...
> 
Sorry, didn't want to risk that.  I only have 1 old-style colour monitor.

Roger