How CPU's work (was Re: Hi, I'm new...)

Chuck Guzis cclist at
Tue Aug 8 18:26:48 CDT 2006

On 8/8/2006 at 3:35 PM Don wrote:

>Having B appear on the address bus is a nice way of combining
>an IN and OUT instruction into a single operation.  I often
>exploit this when decoding key matrixes, etc. (i.e. set
>B to whatever you want to appear on the row drivers, C to
>the IO address for the "keyboard" and do an IN A,C)

Yes!  I prefer to think of it as a 16-bit port address.  But the idea's the
same, instead of latching an index value  into a register file on a device,
one can simply use the value of the B register as the index and do away wit
hthe latch entirely, saving valuable time.

The NSC800 has a configuration port at 0bbh, which can be modified with the
register-indirect form of the OUT instruction, but not the immediate-port


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