Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Stephen Fuld <sfuld@alumni.cmu.edu.invalid>
Newsgroups: comp.arch
Subject: Re: base and bounds, Why I've Dropped In
Date: Mon, 16 Jun 2025 10:56:40 -0700
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On 6/16/2025 10:42 AM, MitchAlsup1 wrote:
> On Mon, 16 Jun 2025 4:02:26 +0000, Stephen Fuld wrote:
> 
>> On 6/15/2025 1:09 PM, MitchAlsup1 wrote:
>>> On Sun, 15 Jun 2025 19:44:41 +0000, John Levine wrote:
>>>
>>>> According to Stephen Fuld  <sfuld@alumni.cmu.edu.invalid>:
>>>>> [ base and bounds vs paging
>>>>> An interesting question is whether it is worthwhile to do both.
>>>>
>>>> Multics had segments of variable size, with each segment paged.
>>>> That worked, give or take the inevitable problem of running out of
>>>> address bits.
>>>>
>>>> I think that is an insoluble problem with base and bounds.  However
>>>> you divide up the address space into sections, some sections will
>>>> be too small.
>>>
>>> Who was it that said:: The worst error a computer architect
>>> can make is to specify insufficient number of address bits ??
>>>
>>> Gordon Bell ?
>>>
>>> Base and bunds simply doubles down on avoiding this rule.
>>
>> I am not sure what you are saying here.
> 
> Often base ends up with too few address bits,
> almost always bounds ends up with too few size bits.

But since these can be/should be/are hidden from the user, they can be 
expanded at will.

> On a machine today, wanting byte alignment and bite sizing of
> memory areas, you need 64-bits of base, 64-bits of bounds and
> about 5 more bits to control things. {{Now while I would not
> call someone doing base-bounds sane today--I would claim that
> knocking a couple bits off the top of bounds and another couple
> of bits of bounds to make the control point fit a control register
> container might be considered "sane"}}

:-)



-- 
  - Stephen Fuld
(e-mail address disguised to prevent spam)