Re: DTV software issues

From: MagerValp (
Date: 2005-11-28 11:05:07

>>>>> "UvB" == Ullrich von Bassewitz <> writes:

UvB> After lots of try&error I noticed that this piece of code
UvB> prevents the driver from working. It seems as if reading register
UvB> $d03f doesn't give the last value written to it. Typing in
UvB> ?PEEK(53311) from BASIC proves this: It prints 255. Can this
UvB> really be true?

Yep. Apparently making a simple latch uses fewer transistors than a
read/write register. It's a PITA, but not uncommon - e.g. the SID also
has write only registers.

UvB> There's a similar issue with the register bank: There's no way to
UvB> read the old setting. How are subroutines or drivers supposed to
UvB> work if they cannot read and save the old settings before
UvB> changing anything?

You have to keep a shadow register if you want to read back the value.

    ___          .     .  .         .       . +  .         .      o   
  _|___|_   +   .  +     .     +         .  Per Olofsson, arkadspelare
    o-o    .      .     .   o         +
     -       +            +    .

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