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

[mjmahon@aol.com (Michael J. Mahon)]

Don Bruder wrote:

>In article <Udzod.178761$df2.88821@edtnps89>,
> "Craig Bower \(Bender MX\)" <NOSPAMnitroacd@telusplanet.NAYSPAM.net> 
> wrote:

<snip>

>> For Example from assembly:
>> 
>> LDA #$00                   Load Accum with #$00 (soft swtich bait)
>> STA $C00A                Enable main ROM from $C300-$C3FF
>> JSR $C300                  PR#3
>> LDA #$00                   Reset Accum in case JSR $C300 scrambled it.
>> <<STA $C00C>>    << Might be optional -try it with & w/out
>> STA $C00F                Turn on Alternate character set.
>
>ERROR! ERROR! ERROR! ERROR!!!
>
>When playing with soft-switches, *ALWAYS* *READ* from them, unless 
>specifically told that writing to activate them is OK. Some of them 
>don't care. Others will toggle twice when written. (A read involves one 
>access, a write involves two accesses - one to "locate" the address, the 
>second to do the actual write) 

*** ERROR: ERRONEOUS ERROR MESSAGE ABOVE! ***  ;-)

Many of the //e softswitches can be written to.  Some _must_ be
written to since they have different functions depending on whether
they are read or written.

$C00F is a switch that should be written to.

The only time a "store" accesses the effective address twice is
in the indexed form.  I think this myth got started because of
the warnings about POKE vs. PEEK, since POKE uses an
indexed store.

Jim Sather has a nice table showing exactly what bus activity
results from the execution of each opcode.

BTW, the read-modify-write instructions reference the effective
address three times (as you might expect  ;-)!

-michael

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