Fake 60Hz with ECS Agnus?

[mark_k]

A few games use a CIA interrupt to write to VPOSW to achieve a "fake 60Hz" display. That works even with OCS Agnus. [The resulting refresh rate can be quite far from 60Hz depending on how many scanlines the programmer chose to skip.]

With the ECS Agnus it would be possible to have the copper write to VPOSW instead. Just wondering, did any games or demos do that? Adjusting the refresh rate would just require changing a wait line in the copper list.

[ross]

Originally Posted by mark_k

A few games use a CIA interrupt to write to VPOSW to achieve a "fake 60Hz" display. That works even with OCS Agnus. [The resulting refresh rate can be quite far from 60Hz depending on how many scanlines the programmer chose to skip.]

With the ECS Agnus it would be possible to have the copper write to VPOSW instead. Just wondering, did any games or demos do that? Adjusting the refresh rate would just require changing a wait line in the copper list.

Hi mark_k, some misconception here.
You can fake 60Hz display in OCS with the method you prefer (CPU, Copper) writing to VPOSW.

In ECS Agnus you can directly setup a 60Hz display writing to BEAMCON0 (a real fixed 60Hz mode).

Remember: in PAL machine this is PAL60 and not really NTSC.
Vice-versa for NTSC if you force PAL: is really a NTSC50 mode.

This is due to different main base frequency.

[Toni Wilen]

IMHO it is perfectly valid question: is there (stupid) programs that don't bother with BEAMCON0 but want to use VPOSW using method that is not OCS compatible

You can't write to VPOSW with OCS Copper. On OCS all addresses below $dff040 count as CDANG being always set.

[ross]

Originally Posted by Toni Wilen

You can't write to VPOSW with OCS Copper. On OCS all addresses below $dff040 count as CDANG being always set.

Forgot it

[ross]

However I would never use a CIA IRQ to set VPOSW, too much time drift!

Is not better a copper IRQ?

[mark_k]

CIA interrupt is level 6 whereas copper is level 3 (along with vblank and blitter done). Audio and disk sync interrupts are higher priority than copper, maybe that explains why programmers used CIA??

[Toni Wilen]

Does it really use cia interrupts or just trigger level 6 by copper move to INTREQ?

[ross]

During game, if you do not use CIA IRQ6 for music routine or DSKSYN or AUD# or BLIT, Copper IRQ can be alone during last video lines.
CIA IRQ6 drift during music replay (so is not perfect 50Hz/60Hz but not really a problem, the time is constant)..

I've not checked this CIA VPOSW routine, i'm now curious

[ross]

Originally Posted by Toni Wilen

or just trigger level 6 by copper move to INTREQ?

Yes, this definitely better

[Toni Wilen]

CIA-B TOD should also work, it counts hsyncs.

[mark_k]

At least 60HzEmulator on Aminet uses a vertical blanking interrupt routine to set the CIA timer, then the CIA interrupt routine writes to VPOSW.

[ross]

Originally Posted by mark_k

At least 60HzEmulator on Aminet uses a vertical blanking interrupt routine to set the CIA timer, then the CIA interrupt routine writes to VPOSW.

This make sense. Every VBL CIA timer is reset so refresh time is constant and don't drift.
Just a bit over complicated.

Originally Posted by Toni Wilen

CIA-B TOD should also work, it counts hsyncs.

Another forgotten property! (is this unexplored for variable refresh rate?)
And can work without VBL IRQ assistance.