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From: bson@rice-chex.ai.mit.edu (Jan Brittenson)
Newsgroups: comp.sys.handhelds
Subject: Re: Questions RE: saturn processor (esp 28, also 48)
Message-ID: <11992@life.ai.mit.edu>
Date: 21 Nov 90 22:55:53 GMT
References: <72330.274a4cb2@vaxc.cc.monash.edu.au>
Sender: news@ai.mit.edu
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In article <72330.274a4cb2@vaxc.cc.monash.edu.au> 
   apm279l@vaxc.cc.monash.edu.au writes:

>(1) According to the now biblical processor notes (thanks Alonzo!)
>    the branch instructions affect the carry. I need to know *how*
>    it is affected. I *guess* that (for example) brgt.f a,c,label
>    does a subtraction, but is it "a-c" or "c-a"?? what about
>    breq, brge, brle, brlt etc. What about brz and brnz??

   The carry flag is set whenever a branch is taken, and cleared when
not taken.

>(3) gosub, gosubvl, gosubl also affect carry - is this because the processor
>    takes PC.a+OFFSET.a ?

   Where did you find your processor notes?! My says nothing about
"gosub" instructions and branches affecting the carry.

>(4) How do the IN and OUT registers get used? (I know they are used somehow
>    for I/O - but what happens, for example when you put something in OUT,
>    how does IN receive input? and from where? - I though the keyboard was
>    bufferred in memory (#F.... something)

   Various bits go to different places. The keyboard is scanned by
software. OUT bit 10 turns the speaker off, 11 turns it on, and the
bits 0-4 are used to output the keyboard column when scanning. Bit 0
is the right-most column. Scanning is performed by putting the column
mask in the OUT register and reading the row pattern with IN. Bit 0 is
the bottom row, and bit 8 the top (menu) row.

I'd love to know how the remaining bits are used!