Re: NetBSD without MMU ?

On Fri, Apr 12, 2002 at 11:35:39AM -0700, Byan, Stephen wrote:
> 
> BTW, the PDP-11/20 and PDP-11/40 didn't have memory mappers; that was an
> option for the PDP-11/45.

	If I recall correctly, the 11/20 did not have a memory
mapper, nor segment registers either; it was a flat-address-space
machine.  I might be wrong about the segment registers, because
I never worked with one, but this is what I recall (the 11/20 was
the first PDP-11).

	The 11/40 certainly did have full-up memory management
and protection via segment registers.  What it lacked, which was
introduced with the 11/45 and carried forward to the 11/70, was
separated instruction and data spaces.  So, an 11/45 or 11/70
program could be twice as large as an 11/40 program, if you filled
up both instruction and data space.  I'm on fairly solid ground
here; I used all three of these machines extensively.

	The 11/45 also introduced a full-up floating-point unit.
The 11/40 had four (4) single-operand floating-point instructions
(FADD, FSUB, FMUL and FDIV) which (I believe) used the regular
registers in pairs.  This didn't stop Arthur David Olsen (I think)
from writing a floating-point C compiler for the 11/40 that used
these instructions.

Mike O'Brien