Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site kobold.UUCP
Path: utzoo!linus!security!genrad!grkermit!masscomp!kobold!tjt
From: tjt@kobold.UUCP (T.J.Teixeira)
Newsgroups: net.unix-wizards,net.micro.68k
Subject: Re: Dereferencing NULL pointers
Message-ID: <202@kobold.UUCP>
Date: Fri, 25-Nov-83 11:52:38 EST
Article-I.D.: kobold.202
Posted: Fri Nov 25 11:52:38 1983
Date-Received: Sat, 26-Nov-83 07:08:45 EST
References: <1427@rlgvax.UUCP>
Organization: Masscomp, Westford, MA
Lines: 17

The Masscomp 68K port "allows" dereferencing null pointers. i.e. the
address space is the same as UNIX on PDP-11's and VAX's, with the
obvious changes for differences in memory management (our page size is
4K bytes).  The separate mapping for user and kernel space does not
slow up system calls per se, but does make moving data between user and
system space more expensive (Without the 68010 instructions for
accessing different address spaces, portions of the user address space
must be mapped into kernel space first.  This sounds expensive, but is
actually quite simple: just copy one entry from the user page table to
the system page table).

We had considered mapping page 0 out of the address space but decided
not to do that as long as most UNIX systems implicitly rely on the
virtual address space beginning at 0.
-- 
	Tom Teixeira,  Massachusetts Computer Corporation.  Westford MA
	...!{ihnp4,harpo,decvax,ucbcad,tektronix}!masscomp!tjt   (617) 692-6200