Re: cbm 8032 io area $e800-$e8ff decoding

Yeah, as Steve said $E880-$E88F (mirrored through $E8FF) is the CRTC in the newer PETs.

They simply used the address lines A4 through A7 as chip selects (within the $E8xx range) for the I/O chips (KBD+ PIA, IEEE PIA, VIA and CRTC), which means each chip is only mirrored within that particular address range (unlike the older PETs); the PIAs have positive chip selects which makes it even easier.

That leaves E800-E80F (NOT(A4.A5.A6.A7)) available.

m

----- Original Message ----- 
From: "Ethan Dicks" <ethan.dicks@gmail.com>
To: <cbm-hackers@musoftware.de>
Sent: Thursday, November 10, 2016 1:53 PM
Subject: Re: cbm 8032 io area $e800-$e8ff decoding


>I was curious so I checked...
> 
>> I don't see anything at $E800 - $E80F; what uses it, if anything?
> 
> I used TIM on my 2001-N/3032 (no CTRC) to look for locations that
> aren't returning "real" values, and I see two places that read back
> with a value of $E8 ("dead" locations will read back the upper address
> byte).
> 
> $E800-$E80F
> $E880-$E88F
> 
> Every other address in the $E800 block read back with some value, with
> lots of obvious mirroring.
> 
> I didn't have a handy CTRC machine to run the same check.
> 
> -ethan
> 
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