Read two numbers and print their sum

The program asks for two numbers, waits while you type them, and prints their sum. Press Run and the console stops at the first prompt: type a number into the box under it, press Enter, and the run carries on inside that one trap #15.

Printing only talks. Reading listens, which means the program stops dead in the middle of an instruction until somebody answers, and what comes back is a number already in a register rather than text you have to make sense of yourself.

    lea first, a1
    move.b #18, d0      ; task 18: print the prompt, then read a number
    trap #15
    move.l d1, d2       ; a = what was typed

    lea second, a1
    move.b #18, d0
    trap #15
    add.l d1, d2        ; a = a + b

    lea answer, a1
    move.l d2, d1       ; task 17 prints the number in d1
    move.b #17, d0      ; task 17: the string, then the number
    trap #15

    move.b #9, d0
    trap #15

    org $2000
first:  dc.b 'First number: ', 0
second: dc.b 10, 'Second number: ', 0
answer: dc.b 10, 'The sum is ', 0

Task 18 is two tasks in one request: it prints the string at a1 the way task 14 does and then reads a line and parses it as a decimal number, the way task 4 does. The answer is in d1, which is also the register task 17 prints from, so the first thing the program does after each read is get the number out of d1 before the next task overwrites it.

The 10 at the front of second and answer is a newline written as its ASCII code, which is how you put one inside a string that a task prints. dc.b 10, 'Second number: ', 0 is one string of seventeen bytes and the first of them is the line break.

Task 4 reads a decimal number and nothing else. Type anything that is not one and the run ends with Expected a number, got "NaN" under the editor, so a program that has to survive whatever people type reads the line with task 2 and picks it apart itself.

Change add.l d1, d2 to sub.l d1, d2 and type 17 and 25 again. The console reads The sum is -8. The same bits, FFFFFFF8, would have printed as 4294967288 through task 15, which reads them as unsigned. The register did not change; the task that printed it did.