🚀 go-pugleaf

In article <21296@cbmvax.commodore.com> jesup@cbmvax.commodore.com (Randell Jesup) writes:
>In article <SPOT.91May5235753@WOOZLE.GRAPHICS.CS.CMU.EDU> spot@CS.CMU.EDU (Scott Draves) writes:
>>As far as i can tell, on a typical RISC machine, the biggest (only?)
>>advantage to 2-address ops is it saves you instruction bits.
>
>	The RPM-40 did this in order to get 16 bit instructions.
>
>>what if you made a machine with two sorts of every op: one that was
>>2-address, and one that was 3-address.  The two address ones could
>>access only part of the register file.

Why not use a mixed system? Ie Four registers in a 32-bit instruction which allows two formats eg. (Where R0 is always 0).
A
opcode dest1,src1,dest2,src2     : This allows a virtual 2 16-bit instructions
opcode dest ,src1,R0   ,src2     : Acts as a 32-bit 3 operand instruction.

This does however put a server restiction on the number and size of opcodes. However most of the common arithmetic opcodes can be covered. eg add,move,cmp.
Even with 4 operands of 5 bits each this leaves 12 bits for the opcode (and space for other formats).