Print a string

The first program of the ladder that anybody outside the editor could see the result of. It prints a line, then prints a second line with a number at the end of it, and ends itself. The answer is in the console panel under the program instead of in a register.

Everything up to here left its result in the registers or in memory, because printing is a request to the environment and the M68K has one instruction for making requests. This is that instruction.

You need to know: the "trap #15 and its tasks" lecture. What is new here is the whole shape of a request, the task number in d0.b says what you want, the other registers carry the arguments, and trap #15 hands it over.

dc.b 'Hello, world!', 0 writes fourteen bytes: thirteen character codes and the zero you wrote yourself. Task 13 and task 14 both find the end of a string by looking for that zero, and both stop with an error after 16 kilobytes, so a string you forgot to terminate ends the run instead of printing forever.

The task number goes in d0.b and not d0.l, which is why move.b is what every one of these lines uses: the task is a byte and the rest of d0 is left alone. Some tasks answer in d0, so it is not a register you can leave a task number sitting in and expect to find it later.

Task 17 is task 14 and then task 3 in one request: it prints the string at a1 and then the signed number in d1, which is how a line with a value in it is written without printing the two halves separately. move.b #9, d0 and trap #15 is task 9, terminate, and it is how you stop before your data rather than falling off the end of the program.

Try changing the first move.b #13, d0 to move.b #14, d0. Task 14 prints without the new line, so the console reads Hello, world!The answer is 42 all on one line.