# 不对齐的比特赋值

> 来自FreeModbus/mbtils.c  略微修改以便移植

| 函数 | 功能 |
| ---- | ---- |
| xMBUtilSetBits | 设置比特 |
| xMBUtilGetBits | 获取比特 |

## xMBUtilSetBits
``` C
void xMBUtilSetBits( uint8_t * ucByteBuf, uint16_t usBitOffset, uint8_t ucNBits, uint8_t ucValue )
```

## xMBUtilGetBits
``` C
void xMBUtilGetBits( uint8_t * ucByteBuf, uint16_t usBitOffset, uint8_t ucNBits )
```

## Demo
``` C
eMBErrorCode
eMBRegCoilsCB( uint8_t * pucRegBuffer, uint16_t usAddress, uint16_t usNCoils,
               eMBRegisterMode eMode )
{
    eMBErrorCode    eStatus = MB_ENOERR;
    uint16_t iNCoils = usNCoils;
    uint16_t usBitOffset;
    // Default usAddress + 1
    usAddress -= 1;
    usBitOffset = usAddress - REG_RELAYS_START;
    if( ( usAddress < REG_RELAYS_START ) || ( usAddress + usNCoils > REG_RELAYS_START + REG_RELAYS_SIZE ) )
        return MB_ENOREG;
    
    switch ( eMode )
    {
    // read
    case MB_REG_READ:
        while ( iNCoils > 0 )
        {
            *pucRegBuffer++ = xMBUtilGetBits(RelaysStatusBuf, usBitOffset, iNCoils > 8 ? 8 : iNCoils);
            iNCoils -= 8;
            usBitOffset += 8;
        }
    break;
    
    // write
    case MB_REG_WRITE:
        while ( iNCoils > 0 )
        {
            xMBUtilSetBits(RelaysStatusBuf, usBitOffset, iNCoils > 8 ? 8 : iNCoils, *pucRegBuffer++);
            iNCoils -= 8;
            usBitOffset += 8;
        }
        ReverseBits(RelaysStatusBuf[1]);
        // Set Relays New Status
        M1203_1_SetAllRelaysStatus(*(u16*)RelaysStatusBuf);
    break;
    }

    return eStatus;
}

```

## Source
``` C
/*
 * FreeModbus Libary: A portable Modbus implementation for Modbus ASCII/RTU.
 * Copyright (c) 2006-2018 Christian Walter <cwalter@embedded-solutions.at>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 */

 /* ----------------------- System includes ----------------------------------*/
#include "stdint.h"

/* ----------------------- Defines ------------------------------------------*/
#define BITS_UINT8      8U

/* ----------------------- Start implementation -----------------------------*/
void
xMBUtilSetBits(uint8_t * ucByteBuf, uint16_t usBitOffset, uint8_t ucNBits,
	uint8_t ucValue)
{
	uint16_t          usWordBuf;
	uint16_t          usMask;
	uint16_t          usByteOffset;
	uint16_t          usNPreBits;
	uint16_t          usValue = ucValue;

	// assert( ucNBits <= 8 );
	// assert( ( size_t )BITS_UINT8 == sizeof( uint8_t ) * 8 );

	/* Calculate byte offset for first byte containing the bit values starting
	 * at usBitOffset. */
	usByteOffset = (uint16_t)((usBitOffset) / BITS_UINT8);

	/* How many bits precede our bits to set. */
	usNPreBits = (uint16_t)(usBitOffset - usByteOffset * BITS_UINT8);

	/* Move bit field into position over bits to set */
	usValue <<= usNPreBits;

	/* Prepare a mask for setting the new bits. */
	usMask = (uint16_t)((1 << (uint16_t)ucNBits) - 1);
	usMask <<= usBitOffset - usByteOffset * BITS_UINT8;

	/* copy bits into temporary storage. */
	usWordBuf = ucByteBuf[usByteOffset];
	usWordBuf |= ucByteBuf[usByteOffset + 1] << BITS_UINT8;

	/* Zero out bit field bits and then or value bits into them. */
	usWordBuf = (uint16_t)((usWordBuf & (~usMask)) | usValue);

	/* move bits back into storage */
	ucByteBuf[usByteOffset] = (uint8_t)(usWordBuf & 0xFF);
	ucByteBuf[usByteOffset + 1] = (uint8_t)(usWordBuf >> BITS_UINT8);
}

uint8_t
xMBUtilGetBits(uint8_t* ucByteBuf, uint16_t usBitOffset, uint8_t ucNBits)
{
	uint16_t          usWordBuf;
	uint16_t          usMask;
	uint16_t          usByteOffset;
	uint16_t          usNPreBits;

	/* Calculate byte offset for first byte containing the bit values starting
	 * at usBitOffset. */
	usByteOffset = (uint16_t)((usBitOffset) / BITS_UINT8);

	/* How many bits precede our bits to set. */
	usNPreBits = (uint16_t)(usBitOffset - usByteOffset * BITS_UINT8);

	/* Prepare a mask for setting the new bits. */
	usMask = (uint16_t)((1 << (uint16_t)ucNBits) - 1);

	/* copy bits into temporary storage. */
	usWordBuf = ucByteBuf[usByteOffset];
	usWordBuf |= ucByteBuf[usByteOffset + 1] << BITS_UINT8;

	/* throw away unneeded bits. */
	usWordBuf >>= usNPreBits;

	/* mask away bits above the requested bitfield. */
	usWordBuf &= usMask;

	return (uint8_t)usWordBuf;
}
```