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  1. #include <stdio.h>
  2.  
  3. typedef unsigned char uint8;
  4. typedef unsigned short uint16;
  5.  
  6. uint16 Mul8x8(uint8 a, uint8 b)
  7. {
  8.   int cnt;
  9.   uint16 prod = 0;
  10.  
  11.   for (cnt = 8; cnt > 0; cnt--)
  12.   {
  13.     prod += prod;
  14.  
  15.     if (a & 0x80)
  16.       prod += b;
  17.  
  18.     a += a;
  19.   }
  20.  
  21.   return prod;
  22. }
  23.  
  24. const uint8 Multipliers[][2] =
  25. {
  26.   { 0x00, 0x01 },
  27.   { 0x01, 0x00 },
  28.   { 0x33, 0x10 },
  29.   { 0x11, 0x0C },
  30.   { 0x0F, 0x0F },
  31.   { 0x80, 0x80 },
  32.   { 0xFF, 0xFF },
  33. };
  34.  
  35. int main(void)
  36. {
  37.   int i;
  38.  
  39.   for (i = 0; i < sizeof(Multipliers) / sizeof(Multipliers[0]); i++)
  40.   {
  41.     uint8 a = Multipliers[i][0];
  42.     uint8 b = Multipliers[i][1];
  43.  
  44.     uint16 p = a * b;
  45.     uint16 p2 = Mul8x8(a, b);
  46.  
  47.     printf("0x%02X * 0x%02X = 0x%04X %c= 0x%04X\n",
  48.            a, b, p, "!="[p == p2], p2);
  49.   }
  50.  
  51.   return 0;
  52. }
  53.  
Success #stdin #stdout 0s 1788KB
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stdout
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0x00 * 0x01 = 0x0000 == 0x0000
0x01 * 0x00 = 0x0000 == 0x0000
0x33 * 0x10 = 0x0330 == 0x0330
0x11 * 0x0C = 0x00CC == 0x00CC
0x0F * 0x0F = 0x00E1 == 0x00E1
0x80 * 0x80 = 0x4000 == 0x4000
0xFF * 0xFF = 0xFE01 == 0xFE01
https://ideone.com/NwsykN
language:
C99 (gcc 8.3)
created:
11 years ago
visibility:
public

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