1 ;; Originally from GBDK by Pascal Felber.
15 ;; Need to sign extend before going in.
51 ;; HL, DE (left, right irrelivent)
56 ;; 16-bit multiplication
63 ;; DE = less significant word of product
65 ;; Register used: AF,BC,DE,HL
68 LD HL,#0x00 ; Product = 0
69 LD A,#15 ; Count = bit length - 1
70 ;; Shift-and-add algorithm
71 ;; If MSB of multiplier is 1, add multiplicand to partial product
72 ;; Shift partial product, multiplier left 1 bit
74 SLA E ; Shift multiplier left 1 bit
76 jp NC,.mlp1 ; Jump if MSB of multiplier = 0
77 ADD HL,BC ; Add multiplicand to partial product
79 ADD HL,HL ; Shift partial product left
81 jp NZ,.mlp ; Continue until count = 0
82 ;; Add multiplicand one last time if MSB of multiplier is 1
83 BIT 7,D ; Get MSB of multiplier
84 JR Z,.mend ; Exit if MSB of multiplier is 0
85 ADD HL,BC ; Add multiplicand to product