The following two pieces of code do the same thing.

a = 7;
b = 23;
c = b - a;

Suppose r18 is for storing a, r19 is for storing b, and r20 is for storing c. What is the missing line?

ldi r18, 7 ; r18 = 7 (a)
ldi r19, 23 ; r19 = 23 (b)
mov r20, r19 ; r20 = r19
[MISSING LINE] ; This line does c = b - a

Question

The following two pieces of code do the same thing.

a = 7;
b = 23;
c = b - a;

Suppose r18 is for storing a, r19 is for storing b, and r20 is for storing c. What is the missing line?

ldi r18, 7      ; r18 = 7   (a)
ldi r19, 23     ; r19 = 23  (b)
mov r20, r19    ; r20 = r19
[MISSING LINE]  ; This line does c = b - a

BlackTom AI · Accepted Answer

The AVR subtraction instruction sub Rd, Rr subtracts the register Rr from Rd and stores the result back into Rd, so to compute c = b - a we must have Rd initially contain b and then subtract a.

Relevant concepts:
- ldi Rx, imm loads an 8-bit immediate into register Rx.
- mov Rd, Rr copies the contents of Rr into Rd.
- sub Rd, Rr performs Rd := Rd - Rr (with flags updated).

Step-by-step reasoning and intermediate values:
1. ldi r18, 7 places the value 7 into r18 (a = 7).
2. ldi r19, 23 places the value 23 into r19 (b = 23).
3. mov r20, r19 copies b into r20 so r20 now holds 23; we want the result in r20 (c).
4. To compute c = b - a, subtract a (r18 = 7) from the register that currently holds b (r20 = 23). Using sub r20, r18 executes r20 := 23 - 7.

Full arithmetic shown:
- Before sub: r20 = 23 (0x17), r18 = 7 (0x07).
- sub r20, r18: r20 = 23 - 7 = 16 (0x10).
- After sub: r20 = 16 (c = 16), r19 remains 23, r18 remains 7.

Why this is correct:
- The sub instruction’s operand order is critical: the destination (Rd) is reduced by the source (Rr). We needed Rd to contain b, hence mov r20, r19 first, then sub r20, r18.

Why other possible variants would be wrong:
- sub r20, r19 would compute r20 := r20 - r19 = 23 - 23 = 0, not b - a.
- sub r19, r18 would change r19 (b) instead of storing the result in r20.
- sub r18, r20 would compute a - c (or 7 - 23) producing an incorrect result (and likely a wrapped value in 8-bit arithmetic).

Therefore the missing line is:
sub r20, r18

This matches the provided correct answer.

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