Moving values around

This program calculates the perimeter of a rectangle whose width is 30 and height is 12. It keeps every value in data registers: d0 holds the width, d1 the height, and d2 the answer.

The calculation is 30 + 12 = 42, then 42 + 42 = 84.

    move.l #30, d0          ; width = 30
    move.l #12, d1          ; height = 12
    move.l d0, d2           ; start d2 with the width
    add.l d1, d2            ; d2 = width + height = 42
    add.l d2, d2            ; d2 = 42 + 42 = 84

    move.l #$FFFFFF00, d3  ; give d3 a visible starting value
    move.b d2, d3          ; copy just d2's low byte into d31
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Run it and inspect the registers

In the playground, select Build, then Run. When the program has finished, look at the registers panel:

register	value to check	why
d0	0000001E	30, the width
d1	0000000C	12, the height
d2	00000054	84, the perimeter
d3	FFFFFF54	its low byte was replaced with 54

The panel displays registers in hexadecimal, without 0x. Decimal 84 is hexadecimal $54.

To watch one instruction at a time, select Build again to reset the program, then select Step. After the third instruction, d2 matches d0; after the fourth it is 42 (0000002A); after the fifth it is 84.

#30 and #12 mean the numbers themselves. In these instructions, the source is on the left and the destination is on the right. So add.l d1, d2 adds the value in d1 to d2 and stores the sum in d2. add.l d2, d2 uses d2 as both source and destination, doubling its value.

The final move.b writes one byte. The low byte of d2 is 54, so it replaces only the final two hexadecimal digits of d3. The upper three bytes, FFFFFF, stay as they were.

Try it

Change the two starting numbers so the rectangle is 20 by 5. Before running, work out what d2 and d3 should show. Select Build, then Run, and check your prediction in the registers panel.