The bigger of two numbers

Two unsigned numbers are in b and c. This program leaves the larger one in d and the distance between them in e. A subtraction that needs to borrow tells the program which route to take.

Open the program in the editor, choose Build, then use Step. Watch whether each jr nc jumps, which value reaches d, and whether neg runs.

    .org 0x8000
    ld b, 37        ; x = 37
    ld c, 64        ; y = 64

    ld a, b
    cp c
    jr nc, x_is_at_least_y
    ld d, c         ; x was smaller, so y is the larger value
    jr done
x_is_at_least_y:
    ld d, b         ; x was larger, or the two values were equal
done:

    ld a, b
    sub c
    jr nc, positive_distance
    neg
positive_distance:
    ld e, a
    halt

cp c subtracts c from a to set the flags, then keeps the value already in a. Put b into a first, so the comparison is x - y. A borrow is needed when the first unsigned byte is smaller than the second. In that case the C flag is set. jr nc means “jump if there was no borrow,” so it jumps to x_is_at_least_y when x is at least y.

With the values shown, b is 25 and c is 40 in the registers panel. cp c needs a borrow, so the first jr nc falls through to ld d, c; d becomes 40. The following jr done skips the labelled x route. Without that jump, ld d, b would run next and replace the answer.

The distance calculation performs a real subtraction with sub c. It also starts with a = 25. Subtracting 40 gives E5, the one-byte wrapped form of 37 minus 64, and sets C because it borrowed. The second jr nc falls through to neg. neg calculates 0 - a, changing E5 to 1B, which is 27. Finally ld e, a copies that distance into e. The panel therefore ends with d = 40, e = 1B, and de = 401B.

When x is greater than or equal to y, each subtraction has no borrow. The first jump goes to the labelled x route, and the second jump goes straight to positive_distance, leaving neg skipped. Equal values use that same route and leave a distance of zero.

Try these changes one at a time. Predict the final d and e, then build and run to check.

• Set b to 70 and c to 20. Both jr nc instructions jump, and neg is skipped.
• Set both b and c to 42. Both jr nc instructions jump, and neg is skipped.

Check your answers

With 70 and 20, d = 46 and e = 32. With 42 in both registers, d = 2A and e = 00.