从寄存器中指定位置取指定位数值

最新推荐文章于 2022-05-29 12:05:53 发布

原创最新推荐文章于 2022-05-29 12:05:53 发布 · 2.9k 阅读

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#exception #byte #buffer #integer #stream #output

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本文介绍了一个从字节流中提取特定位片的算法，详细解释了如何通过位移操作实现位片的精确获取，并提供了完整的代码实现。

///////////////////////////////////////////////////////////////////////////////
// GetBitSlice - Get a bit slice from a stream of bytes
// Input: pBuffer - buffer containing data stream
//          cbBuffer - size of buffer in bytes
//          dwBitOffset - bit offset from start of buffer
//          ucBitCount - number of bits (less than or equal to 32)
// Output:
//
// Return: returns a DWORD contain the bit slice shifted to fill the least significant bits
// Notes: will raise an SEH exception if integer overflow occurs
///////////////////////////////////////////////////////////////////////////////

DWORD GetBitSlice(PUCHAR pBuffer, ULONG cbBuffer, DWORD dwBitOffset, UCHAR ucBitCount)
{
UCHAR rgbShifted[4] = { 0 };
DWORD dwUsedBytes; // How many bytes have valid data.
DWORD dwLastByteIndex;
DWORD dwRemainderShift;
DWORD dwRet;

    if (ucBitCount > 32) {
        DEBUG_CHECK(FALSE, (TEXT("GetBitSlice: invalid number of bits /n")));
        return 0;
    }

// Shift the pBuffer down by dwBitOffset bits.
ShiftBytes(pBuffer, cbBuffer, dwBitOffset, rgbShifted);

    if (ucBitCount % 8 == 0) {
        // Return a byte multiple.
        dwUsedBytes = ucBitCount / 8;
    }
    else {
        // Clear the last used byte of upper bits.
        dwLastByteIndex = (ucBitCount - 1) / 8;
        dwRemainderShift = 8 - (ucBitCount % 8);
        rgbShifted[dwLastByteIndex] <<= dwRemainderShift;
        rgbShifted[dwLastByteIndex] >>= dwRemainderShift;
        dwUsedBytes = dwLastByteIndex + 1;
    }

    // Clear the unused bytes.
    if (dwUsedBytes != sizeof(rgbShifted)) {
        memset(rgbShifted + dwUsedBytes, 0, sizeof(rgbShifted) - dwUsedBytes);
    }

memcpy(&dwRet, rgbShifted, sizeof(dwRet));

return dwRet;
}

static
VOID ShiftBytes(PBYTE pbInput, ULONG cbInput, DWORD dwBitOffset, PBYTE pbOutput)
{
DWORD dwRemainderShift;
DWORD dwByteIndex;
DWORD dwEndIndex;
DWORD dwCurrOutputIndex = 0;

dwByteIndex = dwBitOffset / 8;
dwBitOffset %= 8;

dwRemainderShift = 8 - dwBitOffset;

    // Only copy 4 bytes max.
    dwEndIndex = min(dwByteIndex + sizeof(DWORD), cbInput);

    while (dwByteIndex < dwEndIndex) {
        pbOutput[dwCurrOutputIndex] = pbInput[dwByteIndex] >> dwBitOffset;

++dwByteIndex;

        if (dwByteIndex != cbInput) {
            BYTE bTemp = pbInput[dwByteIndex];
            bTemp <<= dwRemainderShift;
            pbOutput[dwCurrOutputIndex] |= bTemp;
        }

++dwCurrOutputIndex;
}
}