Casting Pointers -- fast *portab - (nf)

rpw3 at fortune.UUCP rpw3 at fortune.UUCP
Wed Feb 8 22:03:53 AEST 1984

fortune!rpw3    Feb  8 02:25:00 1984

And of course (?) everyone knows by now (?) that you can get even better
with a 68000 by using the move-multiple-long (register load/store)
instructions to eat and spew big gulps.

	1. Save a few regs
	2. While bunches left to do
	   a. gulp into the regs
	   b. spew out to memory
	   c. adjust indices
	3. copy the odd few words.

Now, that strategy doesn't compare well against loop-unrolled move-long,
since the move long takes care of the indices (movl a1 at +,a2 at +) and
the moveml doesn't, but the moveml's can be loop-unrolled too! In that
case, each load/store pair has a higher address offset word in the
instruction ("moveml <d0-d6,a0-a4>,a5(offset1)"), and you fix up the
whole loop with two adds at the end. In the limiting case (which you
can get close to attaining while doing buffer-block moves), you only
fetch 8 bytes of instructions for each 40 bytes of data copied (note
that's 80 bytes touched), or just over 10% overhead.

(See the code for "blt" that comes with the the MIT "C" compiler.)

Rob Warnock

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