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70 lines
1.8 KiB
ArmAsm
70 lines
1.8 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 popsreg
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.importzp sreg, tmp1, 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 popsreg ; Get left operand
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; Do ptr4:ptr4+1 * sreg:sreg+1 --> AX
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lda #0
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ldx sreg+1 ; Get high byte into register for speed
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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 sreg
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pha
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txa ; hi byte of left op
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adc tmp1
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sta tmp1
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pla
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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 rhs is zero, swap the operands and use the 8x16
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; routine. On entry, A and X are zero
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@L4: ldy sreg ; Save right operand (8 bit)
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ldx ptr4 ; Copy left 16 bit operand to right
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stx sreg
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ldx ptr4+1 ; Don't store, this is done later
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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
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