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

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.

... 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.  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.

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