[Simh] Is it possible to simulate the first Vaxen I ever used?

[Paul Koning]

> On Mar 23, 2020, at 5:49 PM, Ray Jewhurst <raywjewhurst at gmail.com> 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