Print a string

So far, you have read a program's result from a register or memory panel. This program puts its result in the console: a greeting on one line and a number on the next. The editor's M68K simulator provides printing through trap #15. Before each trap #15, the program puts a task number in the low byte of d0 and any text or number that task needs in other registers.

    lea greeting, a1    ; a1 gets the address of the first character
    move.b #13, d0      ; task 13: print the string and start a new line
    trap #15

    lea question, a1    ; address of the second string
    move.l #42, d1      ; the number to print after it
    move.b #17, d0      ; task 17: print the string, then the number
    trap #15

    move.b #9, d0       ; task 9: end the program
    trap #15

    org $2000
greeting: dc.b 'Hello, world!', 0
question: dc.b 'The answer is ', 01
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Select Build, then Run. The console shows:

Hello, world!
The answer is 42
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The labels greeting and question name the first byte of each string in memory. lea greeting, a1 puts the address of greeting in a1, so task 13 knows where to start reading. Each dc.b line stores the characters followed by a zero byte. The zero marks the end of that string; it is not printed. Keep it when you change the text.

Task 13 prints from the address in a1 through the character before the zero, then starts a new line. The second call uses task 17: it prints the string at a1, followed immediately by the signed decimal value in d1.l. The space after is is part of the string, which is why the output reads is 42. Task 17 adds no newline. Task 9 then ends the program before the simulator tries to run the string bytes as instructions.

move.b #13, d0 writes only d0's low byte, called d0.b; the upper bytes of d0 stay as they were. This simulator reads that low byte to choose the task. A different task can use the same trap #15 instruction because its task number changes.

Select Build again to reset, then use Step. At the first trap #15, check that a1 points to $2000 and d0.b is 13 (hexadecimal 0D); after the step, the greeting appears. The next trap #15 adds the second line. The last one ends the run.

Try a different number

Change #42 to #-7 in the move.l instruction. Predict the second line, then select Build and Run to check it.

Show answer

The second line becomes The answer is -7. Task 17 prints the value in d1.l as a signed decimal number. The string and its zero byte are unchanged.