On Mon, 19 Apr 1999, Andre Fachat wrote: > 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)) As everyone has pointed out, I have confused the VIC20 with the C64 cassette. Read and write to PET video memory is crontrolled by an interrupt generated by the clock on one of the interface chips. This same signal (among others) is used to controll screen memory read by the display circuitry. This signal separates read and write of screen memory so that basic can only access screen memory on it's half of each cycle and the display circuitry accesses it the other half (thirty times per second). It is possible to disable this interrupt and speed up the PET. However, when this is done, it results in "snow" on the screen. Simply put, the cirtuitry is not quite as fast as the numbers say it should be, and it is necessary to resort to this scheme to produce an acceptable display. - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tcm.hut.fi.
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