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

[Paul Koning]

> On Mar 23, 2020, at 1:29 PM, Timothe Litt <litt at ieee.org> wrote:
> 
> ...
>> Since the KL10 was DEC's biggest, most expensive machine at the time, it wasn't nearly as cost sensitive as their other CPUs, so there probably wasn't even any consideration given to using PROM for the control store.
> I don't think you could have found fast enough PROMs.  The KL is an ECL machine.  The RAMs are ECL, and the timing is hairy enough that the modules are only populated 1/2 way back - to fill the board would break timing.  The boards also have loops of etch that serve as delay lines.  Manufacturing would short the loops at the right place for each board - depending on how the board (and RAMs) turned out.  Today we take for granted process controls and tolerances that were, if not unattainable, unaffordable at that time.

Was there such a thing as ECL ROM?  I don't remember.

It's interesting to watch the evolution of high speed computers.  There's a continuous back and forth between memory and logic, depending on where the limitations are in the year or two when the design was frozen.  RAM and ROM have generally been different.

One example that comes to mind is the character stroke generator for the CDC 6000 series mainframes.  That has a 10 MHz waveform step clock, so it needs a ROM with an access time comfortably below 100 ns (unless you want to use multiple banks).  In 1964 that was problematic, and the 6000 series display controller instead uses a massive logic chain to produce all the waveform data for all the character codes.  5-ish years later, in the Cyber 170 series, all that was replaced by a ROM.  Same data, but one chip instead of 100 or so plug-in logic modules.

Another example, also from that machine: the memory access and scheduling machinery is quite complex, with 32 memory banks operating concurrently.  That's because many of the instructions complete in far less than a memory cycle time: 300 or 400 ns for the simpler instructions, and even a multiply takes only 1000 ns, while memory cycles in 1000 ns.  By 1964 standards, 1000 ns was amazingly fast, I don't think anyone else came close, and the core memory wiring and circuitry is quite exotic to make it go so fast.

On the VAX 730: as far as I'm aware it's the only VAX built out of standard LSI CPU components.  The guts of the CPU is AMD 2901 bit-slice chips.  All other DEC microprogrammed machines I can think of had their own purpose-designed logic.

2901 bitslices do appear in other DEC products, the UDA comes to mind.  And that has, for running on-board diagnostic tools, a small PDP11-like instruction set implemented in a little bit of 2901 microcode.  By Richie Lary, wizard of compact software...

	paul