🚀 go-pugleaf

In article <2725@brazos.Rice.edu> preston@titan.rice.edu (Preston Briggs) wrote:
>[stuff about how many registers you need deleted]
>...Separate load-store instructions allow lower cost of access
>to non-registered variables.  Why?  Imagine trying to add from memory
>to a register.  A CISC version might be
>	ADD	r5,0(r8)	; load from adress specisfied from r8
>				  wait for memory, doing 
>					nothing much in the meantime
>				  add the value to r5
>				  throw away the value you just waited for.
>		cost = 1 or 2 + memory acess time
>[and promotes the RISC version as:]
>	LD	r7,0(r8)	; load from adress in r8 into r7
>	.			; do some useful work while waiting for memory
>	ADD	r5,r6,r7	; r5 := r6 + r7
>		cost equals 2.  Additionally, the old value of r6 is still
>		around, and the value from memory is handy if we need it.

I don't know if I'm just stupid, but on the machine I work with, the "CISC"
example takes a total of ONE cycle, assuming that the next instruction does
not reference R5, and is not contained in the memory location referenced 
by R8.  Also, in the "RISC" example, the "useful work" that may be done
would have the same restrictions as the "CISC" version (no reference to R7,
and the instructions cannot be in the memory location referenced by R8).
Finally, the "throw away" of the value we just waited for can be avoided
by coding the "RISC" example on the "CISC" machine, with the same results:
it takes 2 cycles (if you do that "useful work" in between the two 
instructions).  

Anyway, I think the RISC/CISC battle will only be a philosophical argument (or
perhaps a religious war), and the real decision of which is best will be made
on a case by case basis by the implementer(s) of the solution(s) to a specific
problem (or set of problems).  If one actually is better than the other, the
marketplace will decide (maybe).

M. A. Pasek          Switching Software Development         NCR Comten, Inc.
(612) 638-7668          CNG--er--PU4 Port Devel.          2700 N. Snelling Ave.
pasek@c10sd3.StPaul.NCR.COM                               Roseville, MN  55113