From mboxrd@z Thu Jan  1 00:00:00 1970
From: IVIE@cc.usu.edu (Roger Ivie)
Date: Wed, 21 Aug 2002 01:01:07 -0700 (MST)
Subject: [pups] Re: PDP-9
Message-ID: <01KLJHDIATV69D4L6T@cc.usu.edu>

Dennis Ritchie said:
> The 7, 9, 15 were very compatible.  I think the -15
> had some scheme for using an index register, which
> the earlier ones didn't have, but it was otherwise
> pretty much identical in IS architecture.

According to Gordon Bell's "Computer Engineering", the primary differences
from the -4 to the -7 were switching from 6-bit to ASCII I/O devices and
the addition of a trap mechanism. The -9 primarily changed the memory
system, going to 2-1/2D core; it was also microcoded. The -15 went to
TTL ICs and added index registers and memory relocation. He says
"The PDP-9 instruction compatibility was acheived with three minor
exceptions about which no complaints were received", although I don't
see an explanation of the exceptions.

http://research.microsoft.com/users/GBell/Computer_Engineering/

-- 
Roger Ivie
ivie at cc.usu.edu