Moving values around

Two numbers go into registers, the program works out the perimeter of the rectangle they describe, and the answer stays in a. Nothing is read from memory and nothing branches: every value is in a register from the first instruction to the last, which is what the registers panel next to the program shows you.

This is the first program of the ladder, and the ones after it are built out of the same three moves: a number into a register, a register into another register, and an add.

You need to know: the "Getting started with the Z80" lecture and the "Registers, pairs and the shadow set" lecture. What is new here is the last two instructions, hl is two registers at once and writing l leaves h exactly as it was.

ld a, 30 writes the number 30 into a, and ld b, 12 does the same with 12 and b. Put the 30 in parentheses and ld a, (30) reads the byte at address 30 instead, which is a different instruction and a different program.

The two adds are one C line, perimeter = 2 * (width + height), written one operation at a time. Every 8 bit addition on this machine writes its answer into a and reads one of its two operands from there, so add a, b is the only shape an addition has, and add a, a adds the accumulator to itself, which is how you double a number without a multiplication.

a comes out at 54, which is 84, and hl at FF54. The 54 is the perimeter and the FF is what the high half of hl was already holding: l and h are one byte each, and naming one of them is what decided how much of the pair changed.

The M68K writes a size on the instruction, move.b or move.l, to say the same thing. Here there are no sizes at all, because the registers you name say it for you: ld l, a moves one byte and ld hl, 0xFF00 moves two.

Try changing ld l, a to ld h, a. hl comes out at 5400, with the perimeter in the high half and the FF gone, because h is the other one of the two registers the pair is made of.