Re: Dating a C-64 board

From: William Levak (
Date: 1999-04-19 19:33:13

On Mon, 19 Apr 1999, [ISO-8859-1] Marko Mäkelä wrote:

> Actually sprite data is fetched on every line the corresponding sprite is
> being displayed.  For details, see the document
> <URL:>.
> Fetching sprite data steals 2 cycles of CPU per sprite.  What happens in
> the first line of each text row is that the character pointers and colors
> are fetched.  This will halt the CPU for 40 cycles.  And in order to
> circumvent the bug/feature of NMOS 6502 processors (they do not obey the
> RDY line during write cycles), the video chip will halt the processor 3
> cycles in advance.  (The maximum number of consecutive writes is 3, that's
> in the beginning of an interrupt sequence.)

If sprite data is fetched every raster line, then every eigth raster line
the Video chip fetches character AND sprite data, which is where the
problem occurs.

> Actually the serial bus routines in the C64 (at least in the PAL C64) do
> not compensate enough for the lost cycles.  If you enable all sprites and
> try to load something from the disk drive, it will hang.

I have loaded from the drive with sprites still on the screen with no

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