Extract and combine bits from different bytes c c++

Question

I have declared an array of bytes:

uint8_t memory[123];

which i have filled with:

memory[0]=0xFF;
memory[1]=0x00;
memory[2]=0x         


        

Answer 1

I think this should do it.

int start_bit = 10, end_bit = 35; // input

int start_byte = start_bit / CHAR_BIT;
int shift = start_bit % CHAR_BIT;
int response_size = (end_bit - start_bit + (CHAR_BIT - 1)) / CHAR_BIT;
int zero_padding = response_size * CHAR_BIT - (end_bit - start_bit + 1);

for (int i = 0; i < response_size; ++i) {
  response[i] =
      static_cast<uint8_t>((memory[start_byte + i] >> shift) |
                           (memory[start_byte + i + 1] << (CHAR_BIT - shift)));
}
response[response_size - 1] &= static_cast<uint8_t>(~0) >> zero_padding;

---

If the input is a starting bit and a number of bits instead of a starting bit and an (inclusive) end bit, then you can use exactly the same code, but compute the above end_bit using:

int start_bit = 10, count = 9;  // input
int end_bit = start_bit + count - 1;