[TUHS] /dev/drum

Noel Chiappa jnc at mercury.lcs.mit.edu
Sun May 6 23:07:38 AEST 2018


    > From: Johnny Billquist

    >> "A logical segment is a piece of contiguous memory, 32 to 32K 16-bit
    >> words long [which can grow in increments of 32 words]"

    > But then it is not actually giving programs direct access and
    > manipulation of the hardware. It is a software construct and service
    > offered by the OS, and the OS might fiddly around with various hardware
    > to give this service.

I don't understand how this is significant: most time-sharing OS's don't give
the users access to the memory management control hardware?

    > So the hardware is totally invisible after all.

Not quite - the semantics available for - and _visible_ to - the user are
constrained by the mechanisms of the underlying hardware.

Consider a machine with a KT11-B - it could not provide support for very small
segments, or be able to adjust the segment size with such small quanta. On the
other side, the KT11-B could support starting a 'software segment' at any
512-byte boundary in the virtual address space, unlike the KT11-C which only
supports 8KB boundaries.

    Noel


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