Re: Disk drive timing

From: Marko Mäkelä (
Date: 1998-08-22 17:41:51

On Sat, 22 Aug 1998, Stephen Judd wrote:

> I am considering fixing up a PAL fastloading routine.  Could someone
> explain the timing issues, i.e. why are PAL and NTSC fastloading
> routines incompatible? 

It's mainly a matter of clock speed.  The 1541 runs at exactly 1 MHz (and
the 1581 and the 1571 in its fast mode run at exactly 2 MHz).  But the C64
runs either at 17734472/18 Hz (PAL) or 14318181/14 Hz (NTSC) or twice that
frequency in the C128 fast mode.  (In the C128's fast mode, there are 5
slow clock cycles on each raster line.)

Some fast loaders don't blank the screen while loading.  They usually
check from $d012 whether a bad line is coming, and don't initiate a
transfer when it is.

> How does this work out in terms of machine cycles?

Well, the PAL C64 is slower than the drive, and the NTSC C64 is faster. 
You have to remove delay from the drive code or add some delay to the C64
code.  Or use a protocol that is immune to the speed difference.


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