VBL Interrupts - more findings

[Yves McDonald <""yves.mcdonald\"@NO SPAM sympatico.ca>]

The plot thickens...  The more I read Apple 2e Tech. Ref manual, the 
less I think there will be an universal timer interrupt source for all 
apple 2s prior to the GS.  The Apple 2C has handy soft-switches 
(addresses tied to I/O operations) to control VBL interrupts.  It also 
has more soft-switches than the //e.  The enhanced Apple //e handles VBL 
interrupts only if there's a memory card in its auxiliary slot.  I have 
not found any way to control this interrupt source yet - perhaps it's 
turned on by default in the boot process or when 80 column display is 
enabled.  One way to test this is to write a small interrupt handler in 
assembler which toggle the speaker output.  We should hear a 60 Hz tone 
if interrupts are present and handled.  Another way is to probe IRQ* pin 
on the 65C02 with a logic probe - a more reliable test.  The Apple Mouse 
card also generates VBL interrupts for smooth cursor movements but 
little is said about mouse firmware in the manual so I won't speculate 
about it.  The mouse card could be a requirement for ][+ users.  Mouse 
support would be cool for the user interface anyway :).

I don't have my //e clone at hand right now so I couldn't try anything 
now and the GS (my development machine) is too different when it comes 
to interrupt support.

(Incidentally, my Sprite Card does generate an interrupt on vertical 
retrace.  I had it enabled by accident and it created havoc... on the 
Pascal program I was writing to control it!  I'm not proposing this as 
an interrupt source to support Contiki :)

Hope this help, this is not an easy one to solve.

Yves