William Levak wrote: > The VIC20, on the other hand, uses the old scheme from the PET. The > screen is updated every thirtieth of a second, using a system interrupt. > This is fast enough for the disk drive, but not for the cassette, hence > the screen is blanked while accessing the cassette. I don't know about the VIC20, but the PET does not refresh the screen every thirtieth of a second. Instead video memory is always read during Phi1, the clock phase where the CPU does not need the bus. I.e. video memory read is interleaved with CPU access and does not interfere in any way in the PET. (The 80 columns PET even read two bytes during Phi1, which is possible due to parallel RAM chips (8032) or faster DRAMs (8296)) Andre -- Email address may be invalid. Use "fachat AT physik DOT tu-chemnitz DOT de" ------Fight SPAM - join CAUCE http://www.cauce.org------Thanks, spammers... Andre Fachat, Institute of physics, Technische Universität Chemnitz, FRG http://www.tu-chemnitz.de/~fachat - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail firstname.lastname@example.org.
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