CPU Models and Limitations:

   Various improvements were made to the PDP-8 instruction set over time.
   These interact with design compromises, so that the PDP-8/S, while newer
   than the straight-8, is the least capable model after the PDP-5.

   Model        Limitations
   5            No IAC rotate, no CMA rotate, no EAE, 4K maximum, 15x slower
                PC at 0000, Interrupts at 0001, no DMA (data break)
   8/S          No IAC rotate, no CMA rotate, no EAE, 8K maximum, 15x slower
   (straight) 8 No IAC rotate, IOT 0 is special, no SWP, no SCL
                Nonexistent memory reference is special
   LINC-8	Like straight-8, but with LINC & LINC peripherals, 4K
   8/L          group 3 CLA is NOP, no EAE, 8K maximum, protect switch
                CDF/CIF to non-existant field is a NOP
                RAL RAR and RTL RTR, perform both, AND results
   8/I          RAL RAR and RTL RTR, perform both, AND results
   PDP-12       Same as 8/I, but with LINC & LINC peripherals
                IOT 0 is special, DTLA conflicts with PUSHJ
   8/E/F/M	CAF, BSW, Omnibus I/O, MQL, MQA, SWP
                Odd semantics for RAL RAR, RTL RTR
                EAE option has mode B
   8/A		CAF, BSW, Omnibus I/O, PUSH/POP, 128K support
                Yet different semantics for RAL RAR
                non-standard LPT
   VT78		CAF, BSW, Omnibus I/O, PUSH/POP
		RAL RAR and RTL RTR are NOPs
		Autoindex suppressed for current page accesses
		No restart from HLT, No DMA (data break)
   DECmates	CAF, BSW, R3L, Omnibus I/O, PUSH/POP
		RAL RAR is R3L, RTL RTR is NOP
                No EAE, EAE operations hang
                Incompatible second serial port (also model dependent)
		KSF, TSF, PSF implementation botched
		No restart from HLT in DECmate I
		Color graphics options
		Optional Z80 and 8086 coprocessor

   Most of these differences have to do with the treatment of IOT and OPR
   instructions.  There is a difference in the treatment of non-existant
   memory, and of current page indirection where the current page is page
   zero.