78 lines
1.0 KiB
ArmAsm
78 lines
1.0 KiB
ArmAsm
// #LAYOUT# STD * #TAKE
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// #LAYOUT# * BASIC_0 #TAKE
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// #LAYOUT# * * #IGNORE
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//
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// Helper routine, based on code by djmips from:
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// - https://codebase64.org/doku.php?id=base:8bit_multiplication_16bit_product_fast_no_tables
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//
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// input: .A (and Zero flag set according to .A), INDEX+3
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// output: .X (high byte), .A (low byte), carry always clear
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//
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mul_FAC2_FAC1_8x8:
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// Special case (optimization) - handle 0 in INDEX+2
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beq mul_FAC2_FAC1_8x8_zero
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sta INDEX+2
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lda #$00
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dec INDEX+3 // decrement because we will be adding with carry set for speed (an extra one)
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ror INDEX+2
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bcc !+
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adc INDEX+3
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!:
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ror
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ror INDEX+2
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bcc !+
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adc INDEX+3
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!:
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ror
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ror INDEX+2
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bcc !+
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adc INDEX+3
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!:
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ror
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ror INDEX+2
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bcc !+
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adc INDEX+3
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!:
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ror
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ror INDEX+2
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bcc !+
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adc INDEX+3
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!:
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ror
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ror INDEX+2
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bcc !+
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adc INDEX+3
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!:
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ror
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ror INDEX+2
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bcc !+
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adc INDEX+3
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!:
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ror
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ror INDEX+2
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bcc !+
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adc INDEX+3
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!:
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ror
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ror INDEX+2
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inc INDEX+3
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tax
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lda INDEX+2
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clc
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rts
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mul_FAC2_FAC1_8x8_zero:
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lda #$00
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tax
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clc
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rts
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