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