25 Jul
2014
25 Jul
'14
4:01 p.m.
On Fri, Jul 25, 2014 at 2:53 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
The PERQ is one of the few machines that makes no hardwae assumptions
about the machine code instruciton set. So it is equally (in)efficient
and implementing just about anything.
Well, that's true in a sense -- the PERQ's hardware *is* optimized for
building microcode to execute bytecode (hence the system's heavy use of
Pascal), the byte-wide opcode file and dispatch mechanisms are provided in
the hardware to make this fairly quick. You *could* make it implement just
about anything, though. (I've often toyed with the idea of building a
68000 in PERQ microcode, though I've never had time to actually do anything
about it. An 8-bit microprocessor would be pretty easy, though...)
- Josh
The VAX-11/780
introduced the concept of patchable control store,
where most of the control store was PROM but there was a small amount
of patch RAM. Costs of high speed RAM declined quickly enough that
THe 11/730 uses 200ns DRAMs (!) for the stnadard microcode store. I
didn't beleive it either, but it's in the printset and the tech manual
(both on bitsavers). Ridiculously slow _and_ the CPU has to be halted
every few ms to refresh the control store (the nature fof microdcode is
that it epends a lot of time in small loops, so you can't assume that hte
normal CPU instrucion exectuion will acess enough to the control store to
refresh it). Ho-hum...
-tony