Fastest possible multiplication routine?

By doing shifts on DE (or BC) and accumulate on HL, you'll be faster in some cases

MUL073 for example can be implemented like this (input args may need to be modified to fit your code):

MUL073:
        ld      h,d
        ld      l,e
        sla     e
        rl      d
        sla     e
        rl      d
        add     hl,de
        sla     e
        rl      d
        sla     e
        rl      d
        add     hl,de 
        ld      a,h
        ret

Saves 26 cycles if I did the math correct.

When I do the maths i find that this is a MUL021 routine.
My suggestion:

MUL073:
      ld e,l
      ld d,h
      add hl,hl
      add hl,hl
      add hl,hl
      ex de,hl
      add hl,de
      ex de,hl
      add hl,hl
      add hl,hl
      add hl,hl
      add hl,de
      ld a,h
      ret

The instructions

sla e

and

rl d

are relatively expensive two byte instructions.
I count 118+14 cycles for my code above. Prodatron's routine has the same cycle count.
The code of dvik costs 138+32 cycles after correction.

Using

   jp (hl)

instead of

   push bc
   ret[

is possible, but then you need to put the

   ex de,hl

in each of the individual multiplication routines, making the fall-through pattern unusable, so you get severe code-bloat.

Yeah the MUL73 is not correct. I think I remembered wrong. The shifts are quite nice in some cases even though this was a bad example.

The jp (hl) does indeed make the fallthroughs not work but it saves like 10-15% cpu time. With other changes I think the average execution time could be decreased by at least 20%.

The cost with one optimized for speed is of course bigger code size. This implementation is rather big as well but in apps with a lot of multiplications the extra code size may be worth it.

I'll send you the version I did earlier. Its not complete but I think I can make it complete based on your version. I started doing a similar routine but didn't finish all cases. I tested numbers 0

But if you want really fast and have a lot of memory you can do a single table lookup. I was actually considering that for the app I was writing.

Would be interesting to know what multiplication routine (if any) alone coder used in his speccy port of wolfenstein.

Have a look at the MAP

how much faster the R800 does with its inbuilt MUL-commands?
MULUB takes 14 cycles
MULUW takes 36 cycles

More information: http://www.msxtop.msxall.com/Docs/Z80R800GuideBrochure.pdf