Re: The "Good" Old Days - Complete Specs for DX-10 Operating System

On Wed, 2 Oct 2024 03:18:23 -0400
"186282@ud0s4.net" <186283@ud0s4.net> wrote:

The 990 series used the TMS-9900 chip and near variants. This was an
odd chip - kept the CPU registers out in ordinary RAM and could
switch quickly between different sets of registers. At that time, the
external RAM and CPU kinda ran at the same speed so little was lost
putting the registers in RAM.
 
I remember fooling with this chip on a TI-99/4a home computer (which,
tragically, horribly under-used the neat new 16-bit CPU). There were
ASM commands for dealing with the register, 'context', shifting. By
that means many users with their own space could be implemented
directly with the hardware.

I've always found the 9900 concept interesting, although its core
assumption about memory speed doesn't really hold up today; much of the
architecture was eventually reincarnated in TI's MSP430 series micro-
controllers, but they ditched the memory-resident register file. But
for the time, context-switching certainly didn't get any faster than
that; only three actual registers to save, but you still got a
comfortably PDP-11ish environment from the programmer's perspective.

But yes, it's astonishing just how bad the TI-99 design was; a cascade
of bad decisions that turned what could've been a real contender in the
home-computer wars into an absolute joke. One of my oneathesedays
projects that I've toyed with for years (though never made any headway
on) is to roll up a homebrew system that does right by the concept...

How it is now AIN'T how it always was.

And may it never be forgotten ;)