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/****************************  multiply_int_light.as **************************

* Author:        Agner Fog

* date created:  2021-05-26

* Last modified: 2021-05-26

* Version:       1.11

* Project:       ForwardCom library libc_light.li

* Description:   multiply_int: multiply two 32-bit signed integers

* This function is for small CPUs with limited capabilities and no multiplication instruction

*

* Copyright 2021 GNU General Public License http://www.gnu.org/licenses

*****************************************************************************/

code section execute

_multiply_int function public reguse=1,0

  // multiplies two 32 bit signed integers. no overflow check

  // input r0, r1: factors

  // save r1, r2, r3

  int64 sp -= 3*8

  int64 [sp] = r1

  int64 [sp+8] = r2

  int64 [sp+0x10] = r3

  // get signs

  int32 r2 = r0 < 0                    // sign of r0

  int32 r0 = -r0, mask = r2            // abs(r0)

  int32 r3 = r1 < 0                    // sign of r1

  int32 r1 = -r1, mask = r3            // abs(r1)

  int r3 ^= r2                         // sign of product

  // get the smallest factor in r1

  if (uint32 r1 > r0) {

    int32 r2 = r1

    int32 r1 = r0

    int32 r0 = r2

  }

  int32 r2 = 0                         // summation of r0 multiplied by each bit in r1

  while (int32 r1 != 0) {              // multiplication loop

    uint32 r2 += r0, mask = r1         // add r0 if bit 0 of r1

    uint32 r1 >>= 1

    uint32 r0 <<= 1

  }

  int32 r0 = r3 ? -r2 : r2             // apply sign

  // restore r2, r3

  int64 r1 = [sp]

  int64 r2 = [sp+8]

  int64 r3 = [sp+0x10]

  int64 sp += 3*8

  return

code end