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!', 01
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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 the decimal number at its start, and leaves the result in d1.l. The number stops at the first character that is not a digit, so 12abc reads as 12, and a line with no number at all reads as 0. 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 #151
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In an interactive run, task 4 waits for you to type a number in the console, after its output, 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.

Task 15 prints an unsigned number in the base selected by d2.b. Hexadecimal digits are uppercase: 255 in base 16 prints FF. Text stored with dc.b uses Windows-1252, so € occupies one byte, $80, and printing that byte produces € in the console. The screen's fixed font has only ASCII glyphs, so that character occupies a blank cell there.

    move.l #currency, a1
    move.b #14, d0
    trap #15
    move.l #255, d1
    move.b #16, d2
    move.b #15, d0
    trap #15
    move.b #9, d0
    trap #15
currency: dc.b '€ ', 01
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Read a key and control the echo

Task 5 reads one key without waiting for a whole line. It writes the character's byte to d1.b; Enter is $0D (13). Tasks 2, 4 and 18 instead read a line ended with Enter. Task 2 stores its first 79 characters at a1, followed by zero, and returns their count in the whole of d1.l.

Task 12 controls whether typed input is shown: d1.b = 0 turns echo off, and another value turns it on. Task 16 hides the waiting input prompt with 0, shows it with 1, turns the line feed after a key read of Enter off with 2, and on with 3. These settings start on for each Build, and Undo restores a setting changed by the program. With echo off, a line read still starts a new line when Enter finishes it. Scripted Testcase answers are never echoed.

    move.b #0, d1
    move.b #12, d0      ; hide the key's echo
    trap #15
    move.b #5, d0       ; read one key
    trap #15
    andi.l #$FF, d1    ; keep only the character byte
    move.b #3, d0       ; print its decimal code
    trap #15
    move.b #9, d0
    trap #151
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Build and Run, then press Enter in the console. The program prints 13. If a program first uses the screen, keyboard or mouse, its later reads take input from the focused screen instead; click that panel before typing.

Files and unsupported tasks

Tasks 50 to 59 access the Project's Files. File numbers start at 0, with at most eight open at once. Task 51 opens an existing File, 52 creates or empties one, 53 reads bytes, 54 writes them, 55 changes the position, and 56 closes it. Task 50 closes all of them, 57 deletes a File, and 59 checks whether it exists. Task 58 asks for a Project path in a modal dialog, which can be cancelled. Read each task's registers and result codes in the trap Documentation: d0.w reports success (0), end of file (1), failure (2), or read-only access (3). Undo restores file changes and positions with the instruction that made them.

The simulator does not provide every device or hardware feature that a program might request. Sound tasks 70 to 77 stop with an Audio Peripheral message, and cycle-counter tasks 30 and 31 need a 68000 timing model. Serial and network tasks also stop with a reason; the unsupported task list names each one. An invalid argument can stop a supported task too, such as base 37 for task 15.

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 ', 01
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    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 ', 01
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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 here1

    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 #151
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