Re: Oh why did Apple dump IBM....

In article <1i8vpgf.mb2egwjc72jjN%real-address-in-sig@xxxxxxxxxxxxxxx>,
Rowland McDonnell <real-address-in-sig@xxxxxxxxxxxxxxx> wrote:

"Load A; Store A; Load A; Store A" is not elegant.

Oh yes it is - in the context of primitive 8 bit microprocessors. The
6502 used a very efficient design that didn't have any fancy
instructions and was missing some that you'd've expected to see - LSR,
but no LSL, for example (I hope - I could check, but I've decided to see
if my memory's working the more dangerous way) - and it turned out that
it could get jobs done rather a lot quicker than a Z-80 at the same
clock speed.

Certainly, if they'd used microcode as the competition did, they could
have written a `memory location to memory location' command - but all
that would have done would have been a microcode equivalent of `Load A;
Store A', perhaps with a dedicated register for such operations to avoid
clobbering the main ALU register. But that would have added cost and
slowed things down - not the 6502 philosophy.

My objection was not really to the lack of memory-to-memory
instructions, but to the fact that more or less everything had to go
through one register, or use a particular register for, say, indexing.
If it had had 8 registers that could be used for all instructions and
addressing modes, it would have been a lot less tedious.

-- Richard
--
:wq
.

Relevant Pages

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