The bigger of two numbers

Two signed numbers are in d0 and d1. This program puts the larger number in d2 and their absolute difference in d3: the distance between them, always zero or positive. Try it with small signed numbers (for example, from -1000 to 1000), so the subtraction and its negation fit in a signed long.

The first decision uses cmp.l d1, d0. M68K writes the source first, so this compares d0 with d1 by calculating d0 - d1 for the flags. It leaves both registers unchanged. The signed branch bge then asks whether d0 is greater than or equal to d1.

    move.l #37, d0      ; a = 37
    move.l #64, d1      ; b = 64

    cmp.l d1, d0        ; set flags from a - b; keep a and b
    bge a_is_bigger     ; if a >= b, use a
    move.l d1, d2       ; otherwise bigger = b
    bra done            ; skip the other assignment
a_is_bigger:
    move.l d0, d2       ; bigger = a
done:

    move.l d0, d3       ; start with a
    sub.l d1, d3        ; d3 = a - b
    bpl positive        ; if d3 is non-negative, keep it
    neg.l d3            ; otherwise make it positive
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Step through the choices

Select Build, then select Step once per instruction. After cmp.l d1, d0, the difference used for the flags is 37 - 64 = -27. The flags panel shows N=1, Z=0, and V=0; bge is not taken. The next move puts 64 in d2, and bra done skips a_is_bigger.

Keep stepping. sub.l d1, d3 stores -27 in d3 and sets N=1. bpl tests that flag, so it is not taken; neg.l d3 changes -27 to 27. At the end, the registers panel shows d2=00000040 (64) and d3=0000001B (27). The panel displays these values in hexadecimal.

The two decisions prepare their flags differently: cmp calculates a difference only for the flags, while sub stores the difference and sets flags at the same time. Here, bpl can read the N flag left by sub directly. The bra done is needed because the first choice has two assignments and only one may run; the second choice only needs to skip neg.l when the difference is already non-negative.

Try it

Swap the starting values: put 64 in d0 and 37 in d1. Before selecting Build and stepping again, predict which branches are taken and what d2 and d3 will show.

Show answer

bge and bpl are both taken. d2 is 00000040 (64), and d3 is 0000001B (27).