Axel Muhr <axelAtgeekdotDotcom> wrote:

> Hi fellow group members,
> 
> (I dared to crosspost my mail from c.s.a2, as my original post there seems
> to grow wild and created many parallel threads. This is the try to
> concentrate this effort)
> 
> For those comming new to this issue:
> While trying to port Contiki to the A2 I ran into the problem of a missing
> "system tick counter" (e.g.. ever increasing counter @60Hz) on the Apple II
> system. Such things are available on other 6502 systems (e.g. C64) at a
> specific address.

No problem on the IIc or IIgs, as you can use VBL interrupts: tied into
the mouse hardware support on the IIc, and can be accessed through the
mouse firwmare on the IIgs, provided that the appropriate slot is set to
have the mouse firmware enabled.

Note that on a ROM 3 IIgs, the user can disable the built-in mouse
firmware and still use the mouse in GS/OS applications, but not ProDOS-8
(with the end result being that slot 4 is available for your own card).
This would also prevent 8-bit software from accessing VBL through the
mouse firmware, but VBL is still available through other IIgs-specific
interrupt mechanisms.

On a ROM 1 IIgs, if you set slot 4 to "Your Card", the ADB mouse is
completely disabled (you could install a mouse card and use that
instead).

The IIe and earlier machines are more of a problem, because they do not
have built-in hardware support for a VBL interrupt (in fact, they have
no built-in hardware interrupt sources).  The II+ doesn't even have a
software VBL detection scheme.

In the IIe, the usual solution is to install a mouse card, which is able
to derive accurate VBL interrupts (I think it does this by monitoring
the timing signals which are visible on the slot bus to determine where
the video frame starts).  As mentioned on csa2, there was also a plain
VBL-only interrupt card which was supplied with GEOS, but I don't know
much about this such as whether it is compatible with the mouse firwmare
or whether software needs to be written specially to support it.

The mouse card will also work in the II+, and I assume the GEOS
interrupt card will.

-- 
David Empson
dempson@actrix.gen.nz