c言語 16ビット乗算器の作成

Question

c言語で「8ビット乗算器を用いて16ビット乗算器の作成」という課題が与えられました

#include <stdio.h>
int main()
{
unsigned long a, b, c, d;
unsigned long a1, a0, b1, b0, c11, c10, c01, c00;

       for (b = 0x00;  b < 0x100;  ++b)
       { b1 = (0x0F0 & b) >> 4;
         b0 = 0x0F  &  b;
         for  (a = 0x00; a < 0x100;  ++a)
         {    d  = a * b;
              a1 = (0x0F0 & a) >> 4;
              a0 =  0x0F &  a;
             c00 = a0 * b0;
             c01 = a0 * b1;
             c10 = a1 * b0;
             c11 = a1 * b1;
             c  = (c11 << 8) + (c10 << 4) + (c01 << 4) + c00;
             if (c != d)
                 printf("%lu * %lu = %lu != %lu¥n", a, b, d, c);
         }
       }

return 0;
}

こんな感じ？

Comments

[cateye]

で、質問は何でしょう？

[ikadzuchi]

nビット乗算器とは、nビット×nビット=2nビットか、nビット×nビット=nビットか、どちらを指すのでしょうか。

Answer 1

uint16通しで掛け算させればいいだけでは

Answer 2

こんな感じじゃなくて？

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

uint16_t mul8(uint8_t a, uint8_t b) {
    return a * b;
}

uint32_t mul16(uint16_t a, uint16_t b) {
    uint16_t c = mul8(a, b);
    uint16_t d = mul8(a >> 8, b);
    uint16_t e = mul8(a, b >> 8);
    uint16_t f = mul8(a >> 8, b >> 8);
    return c + ((d + e) << 8) + (f << 16);
}

int main()
{
    uint16_t a = 12345;
    uint16_t b = 10000;
    uint32_t c = mul16(a, b);
    printf("%u * %u = %u\n", a, b, c);
    printf("%#04x * %#04x = %#08x\n", a, b, c);
    return 0;
}