Re: base and bounds, Why I've Dropped In

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)