trap #15 and its tasks

So far, a program's answer has stayed in a register or in memory. To print text or read a number, it needs help from the environment running it. On the 68000, trap #n is a real instruction that enters an exception handler: code that takes over when the instruction runs. The processor provides trap #0 through trap #15. It does not define printing tasks.

This editor supplies its own service agreement for trap #15. A number in d0.b selects a task, and each task specifies which registers carry its arguments and answer. The editor implements only trap #15, so its assembler rejects the other trap numbers as unsupported here. They are still valid 68000 instructions elsewhere.

For each request:

1. Put the task number in d0.b (the low byte of d0).
2. Put any arguments in the registers that task uses.
3. Run trap #15, then read any answer from the register the task specifies.

Print a string

Task 14 prints the string whose address is in a1. It reads bytes until the first zero byte and prints no newline. Task 13 uses the same argument and adds a newline after the string. The zero byte ends the text being printed; it does not end the program.

The strings below use dc.b to place text in memory. For these ASCII characters, the quoted text emits one byte per character, including each space and punctuation mark. The , 0 after the quote appends the zero byte that ends the string. A label names its first byte, and move.l #greeting, a1 puts that address in a1.

    move.l #greeting, a1
    move.b #14, d0      ; print without a newline
    trap #15

    move.l #ending, a1
    move.b #13, d0      ; print, then add a newline
    trap #15

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

    org $2000
greeting: dc.b 'Hello, ', 0
ending:   dc.b 'world!', 0

The console shows Hello, world! on one line. Task 14 leaves the cursor after the space, so task 13 continues there and then moves to the next line. Both strings have a zero terminator. Task 9 ends the program before execution can run into the bytes after org $2000.

Print and read numbers

Task 3 prints the signed decimal number in d1.l. Task 4 waits for a line of input, reads it as a decimal number, and leaves the result in d1.l. The task number in d0.b changes between requests; the value in d1 is the input or answer for the selected task.

    move.b #4, d0       ; read a decimal number into d1.l
    trap #15
    add.l d1, d1        ; double it
    move.b #3, d0       ; print the signed number in d1.l
    trap #15
    move.b #9, d0
    trap #15

In an interactive run, task 4 waits for you to type a number in the input box under the console and press Enter. In this example, the supplied input line is 21, so the program receives it without waiting for typing and prints 42.

Task 17 is a handy combination: it prints a zero-terminated string from a1, then prints the signed decimal number in d1.l, with no newline added. It performs the same output as task 14 followed by task 3.

Your turn

Print The answer is 42 and end the program. The string at $2000 stops before the number; task 17 can print both parts in one request.

; your code here

    org $2000
message: dc.b 'The answer is ', 0

    move.l #message, a1
    move.l #42, d1
    move.b #17, d0
    trap #15
    move.b #9, d0
    trap #15

    org $2000
message: dc.b 'The answer is ', 0

Now read a small nonnegative number, print its square, and print nothing else. With the supplied input 9, the result should be 81.

; your code here

    move.b #4, d0       ; read a number into d1
    trap #15
    move.l d1, d2
    mulu.w d2, d1       ; d1 = n * n
    move.b #3, d0       ; print the result
    trap #15
    move.b #9, d0
    trap #15