Count the 1 Bits in a Byte
Rotate a byte through the carry flag one bit at a time and tally the ones. The first program that looks inside a byte instead of at it.
A byte sits at 2000H. Count how many of its eight bits are 1 and store
that count at 2001H.
Every program so far has treated a byte as a number. This one treats it as eight separate facts, and the 8085 gives you no way to ask about bit 5 directly. What it gives you instead is the rotate instructions, which shove the whole byte sideways and catch the bit that falls off the end in the carry flag.
That is the trick worth taking away: the carry flag is not only an overflow indicator. It is the machine's one-bit window onto the edge of the accumulator, and rotating is how you move each bit into that window in turn.
There are four rotate instructions and choosing between them is most of the lesson, so the first step uses one on a single bit before the second step loops over all eight.
Steps
- 1 Look at One Bit
- 2 Make It Run Over All Eight Bits