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[/] [forwardcom/] [libraries/] [multiply_int_light.as] - Blame information for rev 168

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1 95 Agner
/****************************  multiply_int_light.as **************************
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* Author:        Agner Fog
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* date created:  2021-05-26
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* Last modified: 2021-05-26
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* Version:       1.11
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* Project:       ForwardCom library libc_light.li
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* Description:   multiply_int: multiply two 32-bit signed integers
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* This function is for small CPUs with limited capabilities and no multiplication instruction
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*
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* Copyright 2021 GNU General Public License http://www.gnu.org/licenses
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*****************************************************************************/
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code section execute
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_multiply_int function public reguse=1,0
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  // multiplies two 32 bit signed integers. no overflow check
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  // input r0, r1: factors
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  // save r1, r2, r3
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  int64 sp -= 3*8
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  int64 [sp] = r1
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  int64 [sp+8] = r2
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  int64 [sp+0x10] = r3
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  // get signs
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  int32 r2 = r0 < 0                    // sign of r0
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  int32 r0 = -r0, mask = r2            // abs(r0)
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  int32 r3 = r1 < 0                    // sign of r1
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  int32 r1 = -r1, mask = r3            // abs(r1)
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  int r3 ^= r2                         // sign of product
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  // get the smallest factor in r1
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  if (uint32 r1 > r0) {
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    int32 r2 = r1
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    int32 r1 = r0
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    int32 r0 = r2
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  }
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  int32 r2 = 0                         // summation of r0 multiplied by each bit in r1
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  while (int32 r1 != 0) {              // multiplication loop
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    uint32 r2 += r0, mask = r1         // add r0 if bit 0 of r1
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    uint32 r1 >>= 1
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    uint32 r0 <<= 1
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  }
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  int32 r0 = r3 ? -r2 : r2             // apply sign
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  // restore r2, r3
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  int64 r1 = [sp]
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  int64 r2 = [sp+8]
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  int64 r3 = [sp+0x10]
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  int64 sp += 3*8
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  return
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code end

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