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From: leo@krabat.marco.de (Matthias Pfaller)
Newsgroups: comp.os.minix
Subject: Re: Virtual addresses
Message-ID: <864@krabat.marco.de>
Date: 21 May 91 11:57:08 GMT
References: <53697@nigel.ee.udel.edu>
Organization: marco GmbH, 8047 Karlsfeld, West-Germany
Lines: 31

In article <53697@nigel.ee.udel.edu>, tsarna@polar.bowdoin.edu (Tyler Sarna) writes:
> CvW writes:
> 
> > I do not know what exactly has been done around -- but:
> 
> Do you know if anything has been done?
> 	
> > Future versions of 68K MINIX with MMU have the following features:
> > - all processes have virtual addresses starting at zero (or, near zero.
> >   It is sensible to leave the first 8K or so unused to trace NIL pointer
> >   dereferences)
> > - there will be memory protection
> 
> My thoughts exactly.
Ok,
but I have to remember that there is a very nice chip called 68070.
This chip is a sort of 68010 with serial I/O, Timer, two DMA-Channels and
a segmented mmu.
With this mmu you can split the address-space of every process in 8 segments
of 2mb each. A segment is specified by a base-address (1k-aligned) and a
length (also in 1k-units). You can make segments where the address specifies
the upper bound of the segment (for the stack) and segments where the address
specifies the lower bound of the segment. If you wish to run a normal
minix-68k binary on a system where such a mmu is in use, you have to
specify the textsegment as a downward-growing segment and the data segment
as an upward growing segment. Thus on the 68070 not the BASE-address of the
textsegment but the END-address of the textsegment is fix.
The mmu provides 8 on-chip segmentregisters.
	Matthias Pfaller (leo @ marco.de)

PS: There is at least one comercial computer with this CPU; see comp.os.os9.