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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: Address bits again, Article on new mainframe use
Date: Mon, 2 Sep 2024 10:11:48 -0000 (UTC)
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According to Lawrence D'Oliveiro  <ldo@nz.invalid>:
>On Sun, 1 Sep 2024 00:52:34 +0000, MitchAlsup1 wrote:
>
>> Imagine trying to fit LLVM or GCC into a PDP/11 address space.
>
>Pretty much from the moment the PDP-11 range was introduced, it was 
>obvious the 16-bit address space was going to be a significant limitation.

I dunno, when I first programmed a PDP-11 in 1970, 64K seemed like a
lot. The machine I used had as I recall 8K bytes (which they called 4K
words) which was plenty for what I was doing.

It is true that the -11 died for lack of address space, but nobody I
know has ever come up with a good design where the address size is
bigger than the word size.  You end up with segments as on the 286
or bank switching which is what later -11's did.

VAX stood for Virtual Address Extension.  The key improvement was the
32 bit addresses.  Everything else was a detail.  Some of those details
were unfortunate but that's a different argument.

Also don't forget that back in that era everyone who had disks used
overlays. The IBM mainframe linkers had complicated ways to build
overlays and squeeze programs into 64K or whatever.  Even though the
address space was 16MB it was a long time before machines had that much
RAM and by then they'd added paging to make the physical memory size less
relevant.

-- 
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