QUOTE The 8/A cannot have an EAE UNQUOTE Don't think this is correct. Because, such a machine would have no serious (integer) arithmetic capability. Also, the PDP 8/A Minicomputer Handbook 1976/77 has a lengthy section [pp9-56 .. 9-65] on the KE8-E (Extended Arithmetic Element : EAE). However, the "PDP-8 Summary of Models and Options" (posted by Doug Jones) indicates that the EAE can be fitted iff the PDP-8/E CPU boardset is fitted. That is PDP-8/A models 600/620 were fitted with the KK8E CPU and optionally the 8/E EAE. And, I imagine "field specials" were doubtless built. Another wrinkle; correction invited. Martin -----Original Message----- From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]On Behalf Of Johnny Billquist Sent: 21 October 2005 12:00 To: cctalk at classiccmp.org Subject: Re: Wondering which OS to use (PDP-8/A) << snip >> Yes, as far as software is concerned, most everything written for an 8/I or 8/E will also run on an 8/A. << snip >> However, the 8/E and 8/A are *very* similar. In fact, some 8/A systems had the KK8E CPU in them. Things that are incompatible, as far as I can remember offhand right now: BSW instruction exists only on 8/E and 8/A. MQ register exists always on 8/E and 8/A, but only exists on 8/I if you have EAE. <<<<NB>>>> The 8/A cannot have an EAE. <<<<NB>>>> The EAE of the 8/E can run in two modes, where one is compatible with the EAE of the 8/I. If you execute a RAR RAL, you'll get different results depending on CPU. I don't remember exactly what the 8/I does, but the 8/E will load the AC with the PC for the high five bits, while the low seven will be fixed (can't remember the exact value). The 8/A will load the AC with the PC. << snip >>