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;
; Ullrich von Bassewitz, 2009-08-17
;
; CC65 runtime: multiplication for ints
;
.export tosumulax, tosmulax
.import mul8x16, mul8x16a ; in mul8.s
.import popptr1
.importzp tmp1, ptr1, ptr4
;---------------------------------------------------------------------------
; 16x16 multiplication routine
tosmulax:
tosumulax:
sta ptr4
txa ; High byte zero
beq @L3 ; Do 8x16 multiplication if high byte zero
stx ptr4+1 ; Save right operand
jsr popptr1 ; Get left operand (Y=0, X untouched by popptr1)
; Do ptr4:ptr4+1 * ptr1:ptr1+1 --> AX
tya ; A = 0
ldy ptr1+1 ; check if lhs is 8 bit only
beq @L4 ; -> we can do 8x16 after swap
sta tmp1
ldy #16 ; Number of bits
lsr ptr4+1
ror ptr4 ; Get first bit into carry
@L0: bcc @L1
clc
adc ptr1
tax
lda ptr1+1 ; Hi byte of left op
adc tmp1
sta tmp1
txa
@L1: ror tmp1
ror a
ror ptr4+1
ror ptr4
dey
bne @L0
lda ptr4 ; Load the result
ldx ptr4+1
rts ; Done
; High byte of rhs is zero, jump to the 8x16 routine instead
@L3: jmp mul8x16
; If the high byte of lhs is zero, swap the operands in ptr1/4 and
; use the 8x16 routine. On entry, A and Y are zero and X has the value
; of ptr4+1
@L4: stx ptr1+1 ; Store hi-byte from ptr4
ldy ptr1 ; Save right operand (8 bit)
ldx ptr4 ; Copy left 16 bit operand to right
stx ptr1
sty ptr4 ; Copy low 8 bit of right op to left
ldy #8
jmp mul8x16a ; There, ptr4+1 will be also cleared
|
