You can verify this actually changes things by doing a RUN AT 2 and give the OUT/IN sequence again. As you will see from the list that follows, not every NextREG is dedicated solely to one function; in this case the only bits that concerned us were Bits 0 and 1 and that's why we used a 2-bit bitmask with the bitwise AND operator &. For the same example using just the REG command, our line would have been as simple as:

REG 7,3

In our example in Chapter 17 where we read NextREG 5h we also used bit shifting which is a great way to get the value of a single bit in a register. In our case we only needed bit 2 of the register so after getting the specific bit by bitwise AND (&) the register value with a 3-bit bitmask, we shifted it two places (bits) to the right by using the right bit-shifting operator (>>). That way we were able to get the value of the single register bit.

Generally speaking, you will often want to modify individual bits in a NextREG without changing the remaining ones.

You can do this by first reading the NextREG and then masking off the bits you want to leave unchanged by using bitwise AND (&) and finally write that value with the new bits added in.

We'll list all of NextREGs below in numerical order. Not every register is accessible, so pay attention to the key at the start of the list to understand whether a register can be read, written or both.