[stella] 3F 74LS173 ... how?

Subject: [stella] 3F 74LS173 ... how?
From: Adam Wozniak <adam@xxxxxxxxxxxxxxxx>
Date: Wed, 1 Jun 2005 09:12:09 -0400
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Thought I'd post some notes re: 3F and the 74LS173, since it took
me a little headscratching to figure out.

datasheet at http://sipi.usc.edu/~weber/ee459/datasheets/74LS173.pdf

74LS173 "M" is tied to ground.
74LS173 "N" is tied to A11.
74LS173 "/G1" is tied to A7.
74LS173 "/G1" is tied to A6.
74LS173 "CLK" is tied to enable/A12.

When A7 and A6 are low, followed by a high transition on A12, the 74LS173
loads its flip-flops.  This means we can't access the TIA at 00-3F, we
must use 40-7F.  This also means we can't switch banks from code running
in RAM.

It seems that, with one 74LS173, we could access 16 2K segments for
a total of 32K.  With two 74LS173 in parallel, we could access 256 2K
segments for 512K.

- -- 
adam@xxxxxxxxxxxxxxxx        http://cuddlepuddle.org/~adam
KG6GZR                       http://cuddlepuddle.org/~adam/resume.html


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