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

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:
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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.

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Multics had segments of variable size, with each segment paged.
That worked, give or take the inevitable problem of running out of
address bits.
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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.

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Who was it that said:: The worst error a computer architect
can make is to specify insufficient number of address bits ??
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Gordon Bell ?
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Base and bunds simply doubles down on avoiding this rule.

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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.
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"}}

                                        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.
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