The FastMult routine using logs can be optimised somewhat from it's original by removing the slow SET instructions and extra direct addressing operation:
FastMult: ld l,c ld h,&82 ld d,(hl) ; d = 32 * log_2(c) ld l,b ld a,(hl) ; a = 32 * log_2(b) add a,d ld l,a ld a,0 adc a,0 ld h,a ; hl = d + a add hl,hl set 2,h ; hl = hl + $0400 set 7,h ; hl = hl + &8000 ld e,(hl) inc hl ld d,(hl) ; de = 2^((hl)/32) ret
to
FastMult: ld l,c ld h,&82 ld d,(hl) ; d = 32 * log_2(c) ld l,b ld a,(hl) ; a = 32 * log_2(b) add a,d ld l,a adc &42 sub l ld h,a ; hl = d + a add hl,hl ld e,(hl) inc l ld d,(hl) ; de = 2^((hl)/32) ret
The add hl,hl can also be removed if the tables are split with low-byte in one page and high-byte in the other and changing the &42 to &84. Since there are 512 entries in the anti-log table, you'd need two INC L's.
- So the more optimal way would look like this:
FastMult: ld l,c ld h,logtab/256 ld a,(hl) ; a = 32 * log_2(c) ld l,b add (hl) ; cf,a = 32 * log_2(c) + 32 * log_2(b) ld l,a ld a,h adc 1 ld h,a ld e,(hl) inc h inc h ld d,(hl) ; de = 2^((hl)/32) ret
The two low and highbyte antilogtabs have to follow directly after the logtab. -- Prodatron 11:51, 7 May 2007 (CEST)
This is all well and good, but the routine is actually quite innacurate. Executioner 03:41, 8 May 2007 (CEST)