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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: Address bits again, Article on new mainframe use
Date: Wed, 4 Sep 2024 14:33:52 -0000 (UTC)
Organization: Taughannock Networks
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It appears that Lawrence D'Oliveiro  <ldo@nz.invalid> said:
>It is true that, while the x86 segmentation system sucked, the PDP-11’s 
>address limitations made it look good by comparison.

I don't think anyone at DEC thought it was an expansion mechanism.  The 11/45 and /70
had eight 8K pages in each address space of which there were six, two each for
user, supervisor, and kernel.  I think they thought it was paging, but of course
8K pages were way too large.  So they overreacted and the Vax pages were 512 bytes
which were too small.

>> VAX stood for Virtual Address Extension.  The key improvement was the 32
>> bit addresses.  Everything else was a detail.
>
>Hey, don’t forget the kitchen-sink instruction set. ;)

We can try to forget, but it's hard.

>> Also don't forget that back in that era everyone who had disks used
>> overlays.
>
>Or multiple passes, as separate executables. One of our lecturers got hold 
>of a copy of a Pascal compiler for our PDP-11. That consisted of two 
>separate programs, and it wasn’t quite a complete Pascal implementation. ...

Good point.  The dmr C compiler was four passes, a front end that turned source
code into trees, a code generator that wrote assembler, an optional optimizer
that rewrote the assembler, and the assembler which created the object file,
Each pass was a separate program.

>> The IBM mainframe linkers had complicated ways to build
>> overlays and squeeze programs into 64K or whatever. ...

>I never quite learned how to build overlaid programs. ...

When I was working on the DOS version of Javelin we used a linker that had overlays
just like the mainframe linkers.  I got it to work and squeezed the code into
about 1/3 the space it'd take otherwise but it wasn't pleasant.

-- 
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly