base:8bit_multiplication_16bit_product_fast_no_tables
8bit multiplication with 16bit product
This code aims to be fast, without using tables.
; mul 8x8 16 bit result for when you can't afford big tables ; by djmips ; ; inputs are mul1 and X. mul1 and mul2 should be zp locations ; A should be zero entering but if you want it will factor in as 1/2 A added to the result. ; ; output is 16 bit in A : mul1 (A is high byte) ; ; length = 65 bytes ; total cycles worst case = 113 ; total cycles best case = 97 ; avg = 105 ; inner loop credits Damon Slye CALL APPLE, JUNE 1983, P45-48. MUL: cpx #$00 beq zro dex ; decrement mul2 because we will be adding with carry set for speed (an extra one) stx mul2 ror mul1 bcc b1 adc mul2 b1: ror ror mul1 bcc b2 adc mul2 b2: ror ror mul1 bcc b3 adc mul2 b3: ror ror mul1 bcc b4 adc mul2 b4: ror ror mul1 bcc b5 adc mul2 b5: ror ror mul1 bcc b6 adc mul2 b6: ror ror mul1 bcc b7 adc mul2 b7: ror ror mul1 bcc b8 adc mul2 b8: ror ror mul1 inx ; Optional - this preserves X across the call - could also do inc mul2 or leave out rts zro: stx mul1 txa rts
base/8bit_multiplication_16bit_product_fast_no_tables.txt · Last modified: 2023-03-15 03:25 by djmips