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From: "Brian G. Lucas" <bagel99@gmail.com>
Newsgroups: comp.arch
Subject: Re: Why VAX Was the Ultimate CISC and Not RISC
Date: Mon, 3 Mar 2025 11:04:55 -0500
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On 3/2/25 5:27 PM, John Levine wrote:
> According to BGB  <cr88192@gmail.com>:
>> I had thought it apparently used a model similar to the 65C816.
>>
>> Namely, that you could address 64K code + 64K data at a time, but then
>> load a value into a special register to access different RAM banks.
> 
> Not really.  The low end PDP-11's were 16 bit, 64K was it.
> 
> The larger ones had memory mapping with 8K pages, a size carefully
> chosen to be too large for paging, but too small to map whole programs.
> There were three modes, user, supervisor, and kernel, with 64K instruction
> and data in each.  The kernel changed the maps by poking values into
> I/O addresses, so it's not something a normal program could do.
> 
> Unix only used user and kernel so for our early bitmap terminals, I
> mapped the screen's video memory into supervisor data and set the
> mode bits so you could access it with MOVE TO/FROM PREVIOUS DATA
> SPACE. C didn't generate those so we had some little assembler
> routines.
> 
> Given the way the PDP-11 was set up, it's hard to think of a memory
> expansion scheme that wasn't a grotesque kludge so I think it was
> the right decision for VAX to have a new instruction set with a mode
> to run PDP-11 code, sort of like the 386's virtual 86 mode for
> real mode 8086 code.
> 
>>> That was not what customers were interested in.  There were various
>>> Unix variants available for the PC, but the customers preferred using
>>> DOS, which was preinstalled and did not cost extra. ...
> 
> Yup.  PC/IX was a really nice Unix port for the IBM PC and nobody was interested.
> 
As this (the kernel part) was my project, it was very disappointing.  I think 
IBM priced such that with DOS being "free", it had no chance.

Brian