Hi! |
A|A
(n n)
\_/
I've been away quite a bit from the list. Anyway,
I read this, in a reply by Alwin Henseler
([EMAIL PROTECTED]) to Shevek ([EMAIL PROTECTED]):
>> (...) I never tried to write to register 14, but if it
>> is not hazardous, that is only good.
>
> Sure you can't destroy MSX hardware by a simple
> programming trick! (prove me wrong) (...)
Okay, I'll try: have you read documentation about
PSG register #7? It follows:
7 6 5 4 3 2 1 0
joystick C B A C B A
control -- noise -- -- tone --
bits
The joystick control bits are connected to some
active electronic components (transistors or ICs). Bit
1 means output, 0 is input. They must be ever set to
"10xxxxxx". However, if it is made something like
"11xxxxxx", you will have both signals set to output,
causing an internal (PSG) and external (coupling
circuitry) short-circuit! I have heard from half a dozen
people that they tried it home and nothing happened. On
the other hand, I have read A LOT of complaints like
"can't use joystick in game X" or "my MSX resets after
loading opening" or even "it takes half an hour to restart
my MSX if I run game Z". Funny? Not at all. Some of that
problems are caused by wrong set-up of PSG R7. Some MSX
don't have any protection against stupid programmers that
haven't read MSX data books before starting to set-up PSG
registers. The result is that there are many, and I mean
MANY, games that set those two bits wrong! Most MSX won't
complain if the joystick control bits are set wrong (at
least, not immediately - it takes some time to stress the
circuitry to the point of hardware failure), but it's not
worth to risk burning some MSX circuits running programs
that does that. Once my turbo R restarted and got a
"strange behaviour" after testing one of such games. I
debugged it and found the "hazardous bug": it set both
bits to 1 when restarting the music routine.
There is a similar problem with the PPI, about the
keyboard control bits, but I cannot remember it by heart.
Some say it requires that the user types something to
actually cause the short-circuit, but what if there are
active circuits, like multiplexers [the Brazilian Expert
does!], connected...? And they are just programming
"tricks"!
... Cyberknight...
<Over>
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