[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

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).


More information about the TUHS mailing list