Linards Ticmanis wrote:
Michael J. Mahon wrote:The Mockingboard makes this pretty easy, since it contains *two* 6522 VIA chips, each of which has two timers with interrupt capability!And on a //e and above, if you program the Mockingboard with the right number of cycles and then wait for VBL ($C019) before actually enabling the counter, it makes a decent-enough VBL interrupt source too. That way you can switch your double buffer in the blanking interval, preventing tearing.
You can also program one timer to create an interrupt repeatedly, with a period of one vertical frame. You then have to poll VBL (or use "vapor lock" technique) to get the phase synchronized. This will stay in sync, once established, because the Mockingboard is locked by the Apple II clock. That way, once sync is established, you never have to wait for VBL again. -michael NadaNet networking for Apple II computers! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it's seriously underused."