🚀 go-pugleaf

In article <18505@amdcad.AMD.COM> tim@amdcad.UUCP (Tim Olson) writes:
/For example, the transputer is a stack machine.  To perform the sequence
/
/	a = b+c;
/
/(assuming a,b, and c are register variables) requires 4 instructions:
/
/	push b
/	push c
/	add
/	pop a
/
/while on the 68000 it requires 2:
/
/	mov b, a
/	add c, a
/
/and on many RISC machines it requires only 1:
/
/	add	a, b, c
/

That last example is a good example of a VAX-class CISC machine, not
of any RISC machine I know of.  RISC code would probably be identical
to the 68000 example (which is more of a RISC machine than it is given
credit for--just wait until the 78000 is announced and you'll see what
I mean).

Also, assuming that the values (a, b, c) are already in registers
obscures the difference between RISC and 68000.  Assuming memory values
gives:

   RISC              680x0               VAX

   LOAD a(FP), R0    MOVE.W a(A6), D0    ADD a(R1), b(R1), c(R1)
   LOAD b(FP), R1    ADD.W b(A6), D0 
   ADD R0, R1        MOVE.W D0, c(A6)
   STORE R1, c(FP)
  
The RISC code will run in 4 cycles; on a 68030 with no-wait-state memory
the 680x0 code will take at least 3 cycles but could take as many as 10 cycles 
and will probably average 5 or 6 cycles, although that will depend on the 
state of the instruction pipeline and on what's currently in the code and data 
caches; I don't have enough information about the VAX instruction set to give 
cycles--it probably is different for each VAX model anyway.

--alan@pdn