Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!elroy.jpl.nasa.gov!jarthur!uunet!microsoft!alonzo
From: alonzo@microsoft.UUCP (Alonzo GARIEPY)
Newsgroups: comp.sys.handhelds
Subject: Re: Questions RE: saturn processor (esp 28, also 48)
Message-ID: <59467@microsoft.UUCP>
Date: 2 Dec 90 03:08:55 GMT
References: <72330.274a4cb2@vaxc.cc.monash.edu.au>
Reply-To: alonzo@microsoft.UUCP (Alonzo GARIEPY)
Organization: Microsoft Corp., Redmond WA
Lines: 58

Here are the answers to your questions, with some help from a higher
authority (HA).  Please repost to comp.sys.handhelds and save me the effort.

Alonzo

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??

If the branch is taken (i.e., the test is true) the carry bit is set.

| (2) What about neg.f X - how is carry altered? It could either take
|     a two's complement by inverting and THEN incrementing, or by
|     decrementing, THEN inverting.

Seems to me that the NEG instruction will set carry according to whether
the result is 0.  This is the only case where decrementing before inversion
or incrementing after inversion will cause overflow.  The order of operations
is therefore irrelevant.  Try it and see.  This command performs a 2's 
complement, or 9's complement, depending on the DEC/BIN mokde.

The NOT instruction performs a 1's complement, or 10's complement, depending 
on the DEC/BIN mode.  It clears the carry bit.  

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

I am told that this is absolutely untrue.  They do not affect 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)

It's a cool idea to add stuff to the bus and do IO, but I'm not a
hardware junkie.  The instructions OUT=C and OUT=CS send the X or least
significant nibble of C.  The instructions A=IN copies IN into nibbles
0 through 3 of A, and C=IN does likewise to C.  --HA

| (5) What's known about the BUS commands? (BUSCB, BUSCC, BUSCD - do these put
|     the contents of B,C or D on the bus? if so, what field is put?)
|     CON/UNCONFIG, MOVE.A ID,C, SREQ, RESET. - I have the brief descriptions
|     of these from Alonzo's notes, but what are the chips in question? What
|     are the ID's, what happens when you config/unconfig/reset the chip?

The bus commands issue a sequence that is intrepreted by the chips on
the bus - they do not send register contents. Unless you are adding
Saturn chips to the bus that are smart enough to interpret bus commands
and not torch the 48's configuration, there's no reason to support these
instructions.  They were most useful on the 71, but since the 48's
address space is spoken for, they're just PLA wasters now.  --HA


Alonzo Gariepy
alonzo@microsoft