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

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.  There are different "limits" involved.  One is the maximum addressable by a single program, another is the maximum memory the architecture can support, another is the maximum size of a section.  This last is related to the design of the ISA, i.e., register size, number of bits used for displacement, etc. These limits don't have to be the same.  And a use of base and bounds can make the first as large as you want, provided you don't want to address all of it "simultaneously".  That is if you have a way for the user program to, within appropriate limits change the contents of the base and bounds registers.   As I said before, it is somewhat awkward, but certainly doable.
--
  - Stephen Fuld
(e-mail address disguised to prevent spam)