Faster multiply?

[boriel]

Okay, today I've managed to get a little time for 8 bit mutiplication. This is the current (new) code:

Code:

ld b, 8 ld l, a xor a __MUL8LOOP: add a, a ; a *= 2 sla l jp nc, __MUL8B add a, h __MUL8B: djnz __MUL8LOOP ret

And this is yours, replacing JR with JP (2 T-states faster, each). I've also commented it a lot, because some instructions has been removed (unneeded).

Code:

LD E, H ; H is the 2nd factor LD HL,0 LD D, L ; DE => H ;LD A, (NUM1) ;; Not needed, already done LOOP: ;; RR A ; (Divide A by 2 - copying the 1's column bit into the carry flag.) RRA ; 1 byte, 4 T-states; RR A => 2 bytes, 8T-States and are equivalent! ; NOTE: JR is 3 T-States faster than JP when the condition is not met ; In this case, it's most likely numbers will be little ones (containing more 0s than 1s), so JP JP NC, JP1; (Jump over the add if we have to) ; ADD HL,DE ; 11 T-states JP1: RET Z ; (Leave when we finish - A has gone to zero) ; 5 T-States if condition not met, 10 if met SLA E ; 8 T-states ; RL D ;; Multiply DE*2 ; Needless in 8 bit, only E is needed! JP LOOP ; 2 T-States faster

Note, your routine returns result in L not in A. Perhaps this routine could be rearranged?

Here's the benchmark I've used to time MUL8:

Code:

DIM a as Ubyte = 8 DIM t as Uinteger AT 23672 ' REM t = Frames DIM q as UByte DIM tmp as UInteger POKE t, 0 : ' Sets the clock to 0 in a single instruction FOR tmp = 0 to 65534 q = a * 165 NEXT Print CAST(Fixed, t) / 50 END ' End the program OK instead of an STOP error (STOP is an "error") PRINT q ' Avoid -O3 variable removal

I haven't timed your routine. To do so, edit mul8.asm in library-asm replacing mul8 code with yours.
Also remember you have the old mul8, not the new one. With the new one, this benchmark gives 8.11 segs.