Help: PDP-11 instruction classification

Steven M. Schultz sms at
Wed May 10 13:32:53 AEST 2000

Hi -

> From: lars brinkhoff <lars at>
> Does this mean that an unexpanded 11/40 has no EIS instructions,

	Quite correct. 

> but with the EIS option, it has more instructions than an 11/45?

	Adding the EIS brings an 11/40 to the same level as an 11/45 without
	floating point.

> GCC seems to think that all PDP-11 models has ASH, but this seems
> wrong.  It's only in EIS, right?

	Yes - that's part of the EIS.  Standard on the 11/45, 11/70 (and the
	later KDJ-11 systems such as the 11/53, 73, 83,etc)

>   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).

>        MODF, ADDF, SUBF, and DIVF (opcodes 0xf000..0xffff).
> Would this be correct?

	Looks right to me.

> 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.

	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).

	Steven Schultz
	sms at

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