> On Mar 23, 2020, at 5:49 PM, Ray Jewhurst <[email protected]> wrote:
>
> Slightly off topic, could someone explain more about what microcode is and
> how it works? The fact that the CPU instructions are they themselves
> programmed in seems unfathomable.
>
> Ray
It's about a cost vs. performance tradeoff. Many computers have quite complex
instructions. While it's always possible to implement them as big hunks of
logic circuitry, that isn't necessarily the economically best answer. The
PDP-11/20 was done that way. Supercomputers are often done that way because
performance trumps cost. And machines with simple instruction sets (RISC) are
hardwired because the instruction set is specifically designed to make that
efficient.
But a PDP-11, never mind a VAX or x86 PC CPU, has a very hairy instruction set
with many variations. If you think of executing these as a programming
problem, you can decompose each instruction into a sequence of simpler
operations. SIMH does this, of course. But you can also construct a simple
computer that contains a well-chosen set of simple primitives, and construct
complex instruction sets from those.
You can also do what van der Poel did in Holland ca. 1948, which is to expose a
"horizontal microprogramming" instruction set directly to the application
programmer. But that makes the programmer's job rather hard, which is why it
didn't catch on.
paul
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