33 lines
789 B
NASM
33 lines
789 B
NASM
.cpu cortex-m4
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.thumb
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.syntax unified
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.globl multiply
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.text
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.thumb_func
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// R0 = a, R1 = b, R2 = result (m), R3 = odd/even mask/result
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multiply:
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MOVS.N R2, #0 // set result to 0
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CMP.N R1, #0
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BEQ.N exit // goto exit if b == 0
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multiply2:
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CMP.N R1, #1
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BNE.N check_even // goto check_even if b == 1
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ADDS.N R2, R0 // move a to result
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B.N exit // goto exit
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check_even:
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MOVS.N R3, #1 // set R3 to 1 as bit mask
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ANDS.N R3, R1 // and R3 (bit mask) with b
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BNE.N odd // goto exit if b == 0
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even:
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LSLS.N R0, R0, #1 // a = a << 1
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LSRS.N R1, R1, #1 // b = b >> 1
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B.N multiply2 // recurse
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odd:
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ADDS.N R2, R0 // add a to result
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LSLS.N R0, R0, #1 // a = a << 1
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LSRS.N R1, R1, #1 // b = b >> 1
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B.N multiply2 // recurse
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exit:
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MOVS.N R0, R2 // move result to R0
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BX.N LR
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