From: MagerValp (MagerValp_at_cling.gu.se)
Date: 2005-11-28 11:05:07
>>>>> "UvB" == Ullrich von Bassewitz <uz@musoftware.de> 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 + MagerValp@cling.gu.se
- + + . http://www.cling.gu.se/~cl3polof/
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