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

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.

... It frees up
the system to optimize page size for just memory management, but it
seems to incur a penalty of requiring two "lookups", one for the
permissions, the other to get the physical address.

>
Most paging systems have multiple levels of page tables, so they're
doing some of that.

Nested paging with 48-bits has 25-levles of page tables (worst case)
5-levels translate each Guest Virtual  to Guest Physical.
5-levles translate each Guest Physical to Host  Physical.
5×5 = 25

                     IBN's zSeries has up to five levels of tables
each with permission bits that apply to lower level tables and some
other complications like an optional bit that says a segment table
pointer isn't to a segment table, it's directly to a 2G or 1M byte
contiguous segment.