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From: mo@messy.bellcore.com (Michael O'Dell)
Newsgroups: comp.arch
Subject: Re: Multiprogramming the Microcode: The B1700
Message-ID: <27843@bellcore.bellcore.com>
Date: 13 Oct 90 13:47:03 GMT
References: <1990Oct4.001346.4139@Stardent.COM> <8052@scolex.sco.COM> <2926@sequent.cs.qmw.ac.uk> <1990Oct11.164904.12550@zoo.toronto.edu>
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Reply-To: mo@messy.UUCP (Michael O'Dell)
Organization: Center for Chaotic Repeatabilty
Lines: 25

I think the Burroughs B1700 was multiprogramming at the microcode
level before the Alto.  For those not familiar with it, the B1700 was,
in some sense, the ultimate CISC.  Main memory was BIT addressed
(actually the HOLES between the bits were address because the
field at address "85-up" was disjoint with the field at address "85-down"),
and each language processor on the machine generated code for an
idealized machine (complete with Huffman coding for incredibly dense
code) and when you context switched between user programs, the instruction
interpretation changed too (sort of like in Unix where the kernel stack
segment switches on a context switch).  There was a microkernel (in microcode)
which implemented the basic part of processes, including the S-level
(traditional machine code) interpreter.  All the system utilities
(OS, compilers, tools, etc) were coded in a language called SDL,
rather reminiscent of XPL in some ways, but with interesting extensions.
So, when people start talking about bit addressing and super-dense
memory images using Huffman codes and such, just remember that it's
all be done before.... Oh yes, the internal bus of the B1700 was
24-bits wide, but there were some wonderful things in the S-memory
address registers to make iterating over chunks of larger operands
much easier to program in the microcode (which was written in a language
called MIL, if memory serves me right.)   I think the people at SUNY
Buffalo, circa 1974, lead the charge in using the machine in CS teaching
and research.

	-Mike