On Sun, 18 Apr 1999, [ISO-8859-1] Marko Mäkelä wrote: > On Sun, 18 Apr 1999, Ethan Dicks wrote: > > > > The NTSC VIC-20 runs at the same speed at the NTSC C64... > > > > I don't think that's correct. In my original 1541 manual, it mentions the > > U+ and U- commands to change the IEEE speed to match VIC-20 or C-64 speeds. > > Aren't they UI- and UI+? Anyway, Richard is correct. The reason why > those commands were introduced is that the C64 has bad lines (video DMA > that will occasionally interrupt the processor) while the VIC-20 does not. > If you blank the screen of an NTSC C64, it will run at the same speed as > an NTSC VIC-20. I can't say that the timing problem wasn't caused by the DRAM, but it doesn't seem likely as those are separate lines that access RAM throught different multiplexers than the CPU uses. I can say that at the end of every eight raster lines, the VIC chip needs to fetch character and sprite data from RAM. It cannot always do this in the time allocated to it (especially if sprites are active) and must interrupt the main CPU in order to keep the screen updated. This means that the CPU on the C64 may not respond as fast as the VIC20 CPU to interrupts from the drive. 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. - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tcm.hut.fi.
Archive generated by hypermail 2.1.1.