PDP-11 pages/segments/etc (Was: PDP-12 at the RICM)

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue Jul 14 20:13:31 CDT 2015

    > From: Johnny Billquist

    > While the pages are variable in length, each page starts at an 8K
    > virtual address boundary.

Which is another difference between PDP-11 'pages', and real pages as used on
every other machine of the period which had virtual memory: normally, page
sizes were small, ranging from 512 bytes to 1K words. No other machine was
even _close_ to 8KB.

In other machines, the page was the fundamental unit of memory allocation; on
PDP-11's, that unit is the click (0100 bytes).

    > If you only had one page, and that page covered the full virtual
    > address space, then I would agree that it was a segment model.

That's only true in the brain-damaged x86 model, in which 'segments' were
added as a kludge to expand the address space.

If you study _real_ segmented architecture machines, like the GE645, NS32K,
etc you will discover that in them, a segment is a fixed-size chunk of the
total (much larger) address space, and they start at fixed offsets in that
space. A segment which is less than full-sized leaves a gap in the address
space before the (fixed) start of the next segment.

All of which sounds just like the 'pages' in the PDP-11...


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