CB>On Sun, 18 Apr 1999, [ISO-8859-1] Marko Mäkelä wrote: CB>> On Sun, 18 Apr 1999, Ethan Dicks wrote: CB>> CB>> > > The NTSC VIC-20 runs at the same speed at the NTSC C64... CB>> > CB>> > I don't think that's correct. In my original 1541 manual, it mentions t CB>> > U+ and U- commands to change the IEEE speed to match VIC-20 or C-64 spee CB>> CB>> Aren't they UI- and UI+? Anyway, Richard is correct. The reason why CB>> those commands were introduced is that the C64 has bad lines (video DMA CB>> that will occasionally interrupt the processor) while the VIC-20 does not. CB>> If you blank the screen of an NTSC C64, it will run at the same speed as CB>> an NTSC VIC-20. CB>I can't say that the timing problem wasn't caused by the DRAM, but it CB>doesn't seem likely as those are separate lines that access RAM throught CB>different multiplexers than the CPU uses. CB>I can say that at the end of every eight raster lines, the VIC chip needs CB>to fetch character and sprite data from RAM. It cannot always do this in CB>the time allocated to it (especially if sprites are active) and must CB>interrupt the main CPU in order to keep the screen updated. This means CB>that the CPU on the C64 may not respond as fast as the VIC20 CPU to CB>interrupts from the drive. CB>The VIC20, on the other hand, uses the old scheme from the PET. The CB>screen is updated every thirtieth of a second, using a system interrupt. CB>This is fast enough for the disk drive, but not for the cassette, hence CB>the screen is blanked while accessing the cassette. You have that backwards. The VIC-20's screen stays on durring cassette loads but the C=64 blanks its screen durring cassette loads. I was a long time VIC user before I got my 64 and the first time I tried to load from a tape I about had a heart attack when the screen blanked, I thought I had broken my New expensive computer. I hadn't read the manuel since the 64 was supposed to be just like the VIC with more of everything. An early speed loader for the Cc=64 blanked the screen and put the 1541 in 1540 mode. - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail email@example.com.
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