Re: Dating a C-64 board
Date: 1999-04-19 07:19:48

CB>On Sun, 18 Apr 1999, [ISO-8859-1] Marko Mäkelä wrote:

CB>> On Sun, 18 Apr 1999, Ethan Dicks wrote:
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>> 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
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