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C > ビット演算 > sizedCopy_shifted() > MSBの位置を指定してコピー

Last updated at Posted at 2015-11-04

http://qiita.com/7of9/items/4659da725d849eaf6b44
の続き

以下の指定をして変数から変数へ数値をコピーしたい

  • コピーする数値の桁数指定 (例: 3桁)
  • コピーする数値のMSB位置指定
    • コピー先のビットサイズ指定

#include <stdio.h>

void sizedCopy_shifted(int srcSize, int srcVal, int dstMSB, int dstSize, unsigned long *dstPtr)
{
    // dstMSB : [0..] index of dst MSB

    unsigned long mask = (1u << srcSize) - 1;
    int lsbPos = dstMSB + srcSize - 1; // LSB position of the srcVal
    int shift = dstSize - lsbPos - 1;
    unsigned long workVal;

    mask = (mask << shift);
    workVal = (srcVal << shift);
    *dstPtr &= ~mask;
    *dstPtr |= (workVal & mask);
}

int main(void) {
    unsigned long ulval = 0;

    //            srcSize, srcVal, dstMSB, dstSize, *dstPtr
    sizedCopy_shifted(3, 0x5, /* dstMSB=*/6, 10, &ulval); // to 0x0A
    sizedCopy_shifted(3, 0x4, /* dstMSB=*/3, 10, &ulval); // to 0x4A
    sizedCopy_shifted(3, 0x3, /* dstMSB=*/0, 10, &ulval); // to 0x1CA

    printf("0x%X", ulval);
    return 0;
}

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