Help: PDP-11 instruction classification

Johnny Billquist bqt at Update.UU.SE
Wed May 10 17:11:24 AEST 2000

On Tue, 9 May 2000, Steven M. Schultz wrote:

Hi, all.

> > From: lars brinkhoff <lars at>
> >   CIS: commercial instruction set (opcodes 0x7d00..0x7eff).
> 	Not a popular option at all.  At the time DEC was trying to make the
> 	11 more of a COBOL machine but the CIS option was too little, too late
> 	and expensive (plus it wouldn't fit as I recall on a 11/70 - just the
> 	11/44 and newer).

Actually, not even those. It's the 11/44 and the KDF-11 based systems
only, which means 11/23 and 11/24.

> > FIS and CIS isn't imlemented in Supnik's simulator, and I haven't
> > found any documentation.  Does anyone know more about those?  Why
> > is there both an FADD and an ADDF instruction?
> 	Two different machines.  The FADD was part of the FIS option for the 
> 	11/40 only (I don't recall ever hearing of someone adding the FIS
> 	to a 11/45 or 70).  The 11/45, 70 and later all had the FPU 
> 	as an option or standard (the KDJ-11{A,B} had the instructions standard
> 	but you could buy (~$600 at the time) an accelerator chip to speed
> 	them up).   I don't think there were any models after the 11/70
> 	that used the FIS.

FIS is only for 11/35, 11/40 and the LSI-11. And it's a real brain-damaged
thing too. Lucky us FPU came along.

> 	There was also the CSM (Call Supervisor Mode) instruction which was
> 	on the 11/44 only (or did it make it into the later KDJ-11 line as
> 	well - I forget).

Yes, CSM is in the J-11. There is also TSTSET in J11, but I wonder if
anyone uses it.

Speaking of CSM, by the way. Slighty off-topic perhaps, but in
RSX-11M-PLUS they used to have a rather complex way of getting to
supervisor mode in user programs, since the 11/70 didn't have CSM. Then
came the J-11, and DEC changed things around on systems having CSM to use
this instruction instead. It turned out that this improved code quite a
lot, so they implemented the CSM instruction by emulation on the 11/70 as


Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt at           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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