[TUHS] Unix & Memory Management Units (MMU)

Diomidis Spinellis dds at aueb.gr
Thu Dec 8 03:31:22 AEST 2016


On 07/12/2016 18:46, Erik E. Fair wrote:
> 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.

I remember Jan-Simon Pendry telling me in the 1980s, that the NULL 
dereference check was first introduced in SunOS with much pain due to 
the resulting crashes. In BSD Unix, which preceded it, the VAX MMU was 
setup with a page in virtual address 0 that would contain the value 0 at 
that address.  (BSD Unix offered memory protection. I assume it had this 
setup for backward compatibility.)


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