Re: VIC-registers

From: Andrew Vardy (
Date: 1998-03-03 19:39:06

On Tue, 3 Mar 1998, [ISO-8859-1] Marko Mäkelä wrote:

> Hello Ruud,
> > But my idea has one big unsolved problem yet: how can I read data? Maybe
> > one of you knows how the SID behaves when you read a write-only
> > register. 
> Why don't you experiment yourself?  If I remember correctly, the SID is
> quite special, because the write-only registers (at least some of them) 
> hold the newly written value for some time.  

Yes, all of them work the same.  $D400 - $D418 ; $D41D - D41F

>Another possibility would be
> that it returns "random" data, i.e. the same as $de00-$dfff when no device
> is connected.  But I vaguely remember writing a test program that wrote
> and read back the write-only SID registers.

They retain the written value for a few microseconds.  Something like


STA $D400
LDA $D400

Always works.  Acc. does not change.  Same too:

STA $D400

Note, when the SID is not producing any sound, these registers all always
return $00.

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