Question

int id = ((reply[0] & 0x3));
id = (id<<7) | (reply[1] & 0x7F);

Suppose I have a reply array storing 2 byte reply[0] and reply[1].
I wanna read the last two bits in the first byte and all the bits in the second byte, then adding them together

why the code above is going to work, can anyone explain?

Answer 1

Convert to binary and you will get your answer.
binary of 3 is 11. if you use logical & with 11 you will end up only with the last two bits. That’s the answer to your first part.

For the second part, I don’t think it will do what you intended it too. You can use logical AND (for carry) and logical XOR to add individual bits.

Try to figure it out ^_^