Version 6

Version 6

Version 6 is like version 5 except the expanded ALU is not used. Here are the op codes:

I wrote up an 8 bit multiplication routine, first in C:

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

unsigned short mul(unsigned char A,unsigned char B)
{
  // Returns:
  //   A = A * B
  unsigned short res=0;
  unsigned char i=8;  // 8 bit

  LOOP:
    res=res+res;
    if (A>=0x80) {
      res=res+B;
    }
    A=A+A;
    i=i-1;
  if (i>0) goto LOOP;
  return res;
}

int main(void)
{
  unsigned char A,B;
  unsigned short M;
  int i,j;


  for (i=0;i<=255;i++) {
    for (j=0;j<=255;j++) {
      A=(unsigned char)i;
      B=(unsigned char)j;
      M=mul(A,B);
      if (i*j!=M) printf("%6d %6d\n",i*j,M);
    }
  }

  return 0;
}

I tested all cases so I know it works. I then dumbed it down to 4 bits:

unsigned short mult(unsigned char A,unsigned char B)
{
  // Returns:
  //   C = A * B
  unsigned short C=0;
  unsigned char D=4;  // 4 bit

  LOOP:
    C=(C+C)&0X0F;     // 4 bit adjustment
    if (A>=8) {       // MSB of 4 bit
      C=(C+B)&0X0F;   // 4 bit adjustment
    }
    A=(A+A)&0X0F;     // 4 bit adjustment
    D=(D-1)&0X0F;     // 4 bit adjustment 
  if (D>0) goto LOOP; // 4 bit adjustment
  return C;
}

int main(void)
{
  unsigned char A,B;
  unsigned short C;

  A=(unsigned char)5;
  B=(unsigned char)3;
  C=mult(A,B);
  printf("C=5*3 %d\n",C);
  return 0;
}

Although the code handles overflow into the high order bit of the result variable (C), in this implementation, I have not considered overflow of C. Therefore 3x5 is big as the algorithm can handle: