Diff between 11/20 and 11/45

[johnh at psych.usyd.edu.au]

There is huge difference between the machines, but not backwards!

The 11/20 doesn't have :-

		EIS instructions like div, mul, ash etc
		FPU instructions like fmul ...
		MMU no memory management of any sort, 56Kb memory, 8Kb I/O page
		    and hence no user modes, 16 bit addressing

So a program written for a 11/20 should work untouched on an 11/45 except for
some very minor (and ugly) instruction sequences involving using the same
register for both source and destination eg mov r2,-(r2), or jmp (r2)+.
The behaviour of the trace trap and T bit is also different.

There is a list of differences some some of the PDP/11 handbooks (perhaps the
latter architecture book).


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