Moving values around

Two numbers go into registers, the program works out the perimeter of the rectangle they describe, and the answer stays in d2. 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 M68K" lecture and the "Data and address registers" lecture. What is new here is the size on the last instruction, .b writes one byte of a register and leaves the three bytes above it exactly as they were.

The # in move.l #30, d0 means the number 30 itself; without it the instruction reads four bytes from address 30 instead. The copy into d2 and the two adds after it are one C line, perimeter = 2 * (width + height), written one operation at a time, because an M68K instruction has two operands and the one on the right is the one that gets written. add.l d2, d2 adds d2 to itself, which is how you double a number without a multiplication.

d2 comes out at 00000054, which is 84, and d3 at FFFFFF54. The 54 is the perimeter and the FFFFFF is what d3 was already holding: the size on the instruction is the only thing that decided how much of the register changed.

Try changing move.b d2, d3 to move.l d2, d3. d3 comes out at 00000054 with the FFFFFF gone, because a long is the whole register.