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From: merlin@hqda-ai.UUCP
Newsgroups: comp.os.minix
Subject: Re: Memory handling in MINIX ?
Message-ID: <254@hqda-ai.UUCP>
Date: Sat, 24-Jan-87 18:02:17 EST
Article-I.D.: hqda-ai.254
Posted: Sat Jan 24 18:02:17 1987
Date-Received: Sun, 25-Jan-87 05:47:50 EST
References: <5490004@nucsrl.UUCP>
Organization: Army AI Center, Pentagon
Lines: 34
Summary: /dev/mem and /dev/kmem exist, but no memory protection.

In article <5490004@nucsrl.UUCP>, naim@nucsrl.UUCP (Naim Abdullah) writes:
> Does anybody know whether /dev/mem and/or /dev/kmem exist in MINIX ?

     Yes, they exist.  /dev/mem reads physical memory starting at
address 0, where the interrupt vectors are located.  /dev/kmem
reads physical memory, also.  However, to simulate "kernel virtual
space", /dev/kmem begins reading at address 0x600 (1536 decimal).
That's where MINIX loads.  Other than the 1536-byte offset, the
two are identical.

> The consensus seemed to be
> that multi tasking was hard because there is no memory protection
> hardware. A process could presumably overwrite the kernel. How did
> Andy solve this problem ?

     He didn't, really.  A process is allocated as much text space
as it needs, and (normally) 64k for data and stack space.
[Well-behaved programs can be allocated less.]  There is a
"gentlemen's agreement" that no one writes anywhere but in their
data segment, which is referenced by the DS or SS segment
registers.  The C compiler abides by this.  However, someone
writing in assembler *could* overwrite anything they chose.

> Is a MMU a standard part of a pc ?

     No.  One of the major failings of the PC architecture.  On
the AT (80286), there is some help here, but not much.  The Compaq
Deskpro 386 (80386) does have the requisite hardware as part of
the '386 processor chip.
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