accessing bits from byte

"it is not working Logic error is there in this program..."

Here's an obvious first error:

startByte=startByte/8;

Anyway, your code seems too complex. Here's a very generic example that uses bit masks to count off when to do source and destination buffer reads and writes:

unsigned char src_buf[8];
unsigned char dst_buf[8];

void bitcpy(unsigned char *dst,
            unsigned char *src,
            unsigned char start_bit,
            unsigned char n_bits)
{
    unsigned char src_byte, src_mask;
    unsigned char dst_byte, dst_mask;

    src += start_bit / 8;
    src_mask = 0x80 >> (start_bit % 8);
    src_byte = *src;

    dst_mask = 0x80;
    dst_byte = 0;

    while (n_bits--) {
        if (src_byte & src_mask) {
            dst_byte |= dst_mask;
        }
        if ((src_mask >>= 1) == 0) {
            src_mask = 0x80;
            src_byte = *++src;
        }
        if ((dst_mask >>= 1) == 0) {
            dst_mask = 0x80;
            *dst++ = dst_byte;
            dst_byte = 0;
        }
    }

    if (dst_mask != 0) {
        *dst = dst_byte;
    }
}

void main(void)
{
    src_buf[0] = 0xFA;
    src_buf[1] = 0x0A;
    src_buf[2] = 0x5B;

    bitcpy(dst_buf, src_buf, 4, 20);

    for (;;);
}

"it is not working Logic error is there in this program..."

Here's an obvious first error:

startByte=startByte/8;

Anyway, your code seems too complex. Here's a very generic example that uses bit masks to count off when to do source and destination buffer reads and writes:

unsigned char src_buf[8];
unsigned char dst_buf[8];

void bitcpy(unsigned char *dst,
            unsigned char *src,
            unsigned char start_bit,
            unsigned char n_bits)
{
    unsigned char src_byte, src_mask;
    unsigned char dst_byte, dst_mask;

    src += start_bit / 8;
    src_mask = 0x80 >> (start_bit % 8);
    src_byte = *src;

    dst_mask = 0x80;
    dst_byte = 0;

    while (n_bits--) {
        if (src_byte & src_mask) {
            dst_byte |= dst_mask;
        }
        if ((src_mask >>= 1) == 0) {
            src_mask = 0x80;
            src_byte = *++src;
        }
        if ((dst_mask >>= 1) == 0) {
            dst_mask = 0x80;
            *dst++ = dst_byte;
            dst_byte = 0;
        }
    }

    if (dst_mask != 0) {
        *dst = dst_byte;
    }
}

void main(void)
{
    src_buf[0] = 0xFA;
    src_buf[1] = 0x0A;
    src_buf[2] = 0x5B;

    bitcpy(dst_buf, src_buf, 4, 20);

    for (;;);
}

    if (dst_mask != 0) {
        *dst = dst_byte;
    }

To avoid an extra store when the number if bits is a multiple of 8, that should have been:

    if (dst_mask != 0x80) {
        *dst = dst_byte;
    }

Sorry, my bad. Where are all you code reviewers anyway?