Re: Does an enhanced //e _ALWAYS_ have 80 col. support?

[salfter@salfter.diespammersdie.dyndns.org (Scott Alfter)]

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In article <h%Wod.7358$_3.90267@typhoon.sonic.net>,
Andy McFadden  <fadden@fadden.com> wrote:
>Don Bruder <dakidd@sonic.net> wrote:
>> Right, that's what I was referring to. When in doubt, the "rule" I 
>> always knew was "read unless specifically told otherwise". 
>
>Apple II reference manual (1981 edition), page 20:
>
>  A program can "reference" the address of the special location for the
>  speaker by performing a "read" or "write" operation to that address.
>  The data which are read or written are irrelevant, as it is the
>  _address_ which throws the switch.  Note that a "write" operation on the
>  Apple's 6502 microprocessor actually performs a "read" before the
>  "write", so that if you use a "write" operation to flip any soft
>  switch, you will actually throw that switch _twice_.  For toggle-type
>  soft switches, such as the speaker switch, this means that a "write"
>  operation to the special location controlling the switch will leave
>  the switch in the same state it was in before the operation was
>  performed.
>
>I just now tried a very simple sound loop on a real IIgs.  There was no
>difference for "LDA $C030" vs. "STA $C030".

On the IIe, you should get silence (or something close to it).  With the
read-before-write behavior, the speaker gets toggled twice by STA $C030, so
the speaker (whose frequency response craps out long before 1 MHz :-) )
doesn't make any noise.

OTOH, AppleWin (a IIe emulator) plays the same tone with STA $C030 as it
does with BIT $C030 or LDA $C030.  I'd expect that to be just the kind of
less-than-100%-accurate behavior that you'd get from an emulator.  I've
never tried doing the same thing on the IIGS, though.  Does the 65816 not
exhibit the same read-before-write behavior as the 6502 and 65C02?

  _/_
 / v \ Scott Alfter (remove the obvious to send mail)
(IIGS( http://alfter.us/            Top-posting!
 \_^_/ rm -rf /bin/laden            >What's the most annoying thing on Usenet?

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