[TUHS] Unix & Memory Management Units (MMU)

Noel Chiappa jnc at mercury.lcs.mit.edu
Thu Dec 8 03:51:47 AEST 2016


    > From: "Erik E. Fair" <f

    > One imagines that many pointer mistakes (bugs) in assembly or C were
    > discovered and squashed in that version, modulo the historical
    > unhappiness resulting from address zero containing a zero if
    > dereferenced ("NULL pointers") in process address space.

PS: PDP-11 Unix didn't, I think, do much (anything?) to solve the null pointer
problem. (This is for early C versions; I don't know about the later BSD
ones.)

Location 0 was a usable address for both read and write for everything except
'pure-text' (0410 magic word) processes; in those it was only legal for
read. Address 0 mostly did not contain a 0, either; for C programs using the
stock run-time, it contained a 'setd' instruction, except in split I+D
processes, in which case data space location 0 probably (I'm too busy to spin
up my V6 emulator to check) contained some of the program's initialized data
(unless special arrangements had been made).

	Noel



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