Re: Apple emulators that support "floating bus" video sensing?

["Charlie" <charlieDOTd@verEYEzon.net>]

"mdj" <mdj.mdj@gmail.com> wrote in message 
4c0d6218-3524-4319-9039-264c24a8bede@d62g2000hsf.googlegroups.com">news:4c0d6218-3524-4319-9039-264c24a8bede@d62g2000hsf.googlegroups.com...
> On Mar 18, 5:20 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:
>
>> The objective is not to do something sophisticated, like avoid tearing
>> of the animation (though that would be nice), but simply to cause it to
>> run at its intended speed, regardless of the speed of the processor
>> (emulated or real).
>>
>> Polling VBL works on an Apple //e, but *only* on a //e, since later
>> machines de-implemented VBL polling in favor of VBL interrupts.
>
> Actually, this isn't strictly true. On a IIe, the $C019 softswitch
> will read with the high order bit set during (and only during) the
> vertical blanking interval. On a IIc, the same softswitch will read
> with the high order bit set if a VBL interrupt has occurred since the
> last time it was read.
>
> For practical purposes, being either waiting for VBL to draw, or using
> it as a real time event source, software that polls the softswitch
> will behave identically on both machines, assuming you read the
> softswitch at least every 1/60th of a second.
>
> The catch is, $C019 is also used to indicate mouse button and movement
> activity whilst the mouse firmware is active, so you cannot reliably
> poll it on a IIc if your program is also using the mouse.
>
> I'm not 100% sure since I don't have a IIc handy that I can test with,
> but I also have an inkling that VBL interrupts must be active for this
> to work on a IIc ($C05A disable, $C05B enable), and I'm relatively
> sure they're disabled by default. Of course, you also have to ensure
> (processor) interrupts are disabled or the firmware is going to handle
> them for you...
>
> I guess Apple decided that the relatively short amount of time the IIe
> spent in the wild before the IIc appeared meant it was OK to change
> the behaviour? Most software would have used the floating bus method
> to remain II/II+ compatible so the impact on developers was minor.
>
> And for the IIgs? I have no idea... Can someone shed some light?

Apple's technical people seem to have been confused about this.  In my copy 
of the Firmware reference it says for $C019; "bit 7 = 1 if not VBL".

In Apple IIgs Technical Note #40 it says:

"On the IIGS, the screen is blanked when the most significant bit of $C019 
is high (greater than 127 or $7F), while on the IIe, the screen is blanked 
when the bit is low (less than 128 or $80)."

So from that I get that, you *can* read bit 7 of $C019 on a IIGS to poll the 
VBL.  I'm just unsure whether the bit should be high or low :-)


There is also a VBL interrupt available on the IIGS (in the Miscellaneous 
Tool Set or the mouse firmware).

On the IIGS the VBL begins on scan line 192 regardless of the display mode.

Charlie