trap #15 and its tasks

Every program so far has left its answer in a register or in memory. To print a line, read what you typed or draw a circle, an M68K program asks the environment, and the instruction it asks with is trap.

trap #15, and no other trap

A real 68000 has sixteen trap vectors, trap #0 to trap #15, and each one jumps somewhere the operating system filled in. This editor imitates EASy68K, whose whole input and output lives behind one of them, so trap #15 is the only one it assembles. Write trap #14 and the build fails with "Only implemented TRAP is 15 for IO, received 14".

The request is always three steps:

1. put the task number in d0.b, which says which service you want,
2. put the arguments in the registers that task reads, usually d1, d2 or a1,
3. run trap #15.

Anything the task answers with comes back in d1, or in d0 for two of them. The task number is a byte, so move.b #14, d0 is the usual way to set it, and the rest of d0 is left alone.

Printing a string

The console panel below the editor shows Hello, world!Hello, world! and then a new line. Task 14 prints and stops; task 13 prints and adds a carriage return, which is the difference between the two all the way through this table: the even and odd pairs are the same job with and without a new line.

Both of them find the end of the string by looking for the zero byte, and both stop after 16 kilobytes with an error, so a string you forgot to terminate ends the run instead of printing forever. Tasks 0 and 1 are the counted forms, which take the length in d1.w and never look for a terminator.

move.b #9, d0 and trap #15 is task 9, terminate. On this simulator a program also ends when there is no next instruction, so you have been ending programs without it; task 9 is how you end one from the middle, and how you stop before your data.

Printing numbers

The console shows -42 11111111 7. Task 3 reads d1 as a signed long, task 15 reads it as an unsigned one and takes the base in d2.b, anything from 2 to 36, so #2 prints binary and #16 prints hexadecimal. Task 20 is task 3 padded on the left to d2.b columns, which is how you line numbers up in a table.

Try changing move.b #2, d2 to move.b #16, d2 and running again: the same d1 prints as FF.

Reading

Press Run and the program stops at the trap #15 with the prompt in the console and waits: type a number in the box under it and press Enter, and the run carries on inside that one instruction. The reading tasks are:

• task 4 reads a line and parses it as a decimal number into d1.l.
• task 5 reads one character into d1.b, without waiting for Enter.
• task 2 reads a whole line into the buffer at a1, terminates it with a zero and puts its length in d1.w. The buffer is yours, and ds.b is how you reserve it.
• task 7 answers d1.b = 1 when a character is waiting and 0 when none is, and takes nothing. It is how a program checks without stopping, since tasks 5 and 2 wait.
• task 18 is task 14 and then task 4, a prompt and a number in one request, and task 17 is task 14 and then task 3.

Asking about the environment

Both answers come out at 0: nobody has typed anything, and the program is fast. Task 8 counts hundredths of a second since the run started, which is where this editor differs from EASy68K, where the same task counts from midnight. Programs measure how long something took by subtracting two reads of it, and that works the same either way.

Task 23 is the clock's other operation. It lets d1.l hundredths of a second of program time pass before the next instruction runs. The editor stays responsive while it waits, so Stop still answers, and it is what paces an animation. Both of them are on a virtual clock inside a testcase, where a delay finishes at once.

The 10 at the front of elapsed is a newline written as its ASCII code, which is how you put one inside a string that a task prints in the middle.

The full table

Every task this editor answers, with what it reads and what it leaves behind. The drawing ones and the ones that read the keyboard and the mouse are the next lecture.

task	what it does	reads	answers
0	print a counted string and a new line	a1 = string, d1.w = length
1	print a counted string	a1 = string, d1.w = length
2	read a line into a buffer	a1 = buffer	the string at (a1), d1.w = its length
3	print a signed number	d1.l
4	read a number		d1.l
5	read one character		d1.b
6	print one character	d1.b
7	is a character waiting		d1.b = 1 or 0
8	hundredths of a second since the run started		d1.l
9	end the program
11	move the text cursor, or clear the screen	d1.w	d1.w when asked
13	print a string and a new line	a1 = string
14	print a string	a1 = string
15	print an unsigned number in a base	d1.l, d2.b = base, 2 to 36
17	print a string, then a signed number	a1 = string, d1.l
18	print a string, then read a number	a1 = string	d1.l
19	read the state of up to four keys	d1.l = four key codes	d1.l
20	print a signed number in a field	d1.l, d2.b = width
23	let that many hundredths of a second pass	d1.l
24	turn the simulator's shortcut keys on or off	d1.l	accepted and ignored
33	set or read the screen size	d1.l	d1.l when asked
61	read the mouse	d1.b = which state	d0.b = buttons, d1.l = position
80-96	drawing	see the next lecture

The same table, with a paragraph on each task, is on the trap tasks documentation page.

The tasks that are refused

EASy68K has more tasks than this, and eleven of them stop the run here with a message saying which one and why. Refusing them out loud is deliberate: a program that asks for one finds out, instead of running to the end having quietly done nothing.

task	what it was for	why it is refused
10	print to the printer	the editor has no printer
12	turn keyboard echo off	typed input is always echoed, the way a terminal does it
16	display properties	the editor's input prompt is not a program setting
21	font properties	the screen draws text in one fixed cell font
22	read a character off the text screen	the screen holds pixels, not a grid of characters
25	scroll a rectangle of text	the screen holds pixels, not a grid of characters
30, 31	clear and read the cycle counter	no cycle counting is emulated
32	hardware and simulator control	there is no hardware window and no automatic IRQ
60	turn the mouse interrupt on	mouse input is polled with task 61
62	turn the keyboard interrupt on	keyboard input is polled with tasks 7 and 19

The last two come back in the interrupts lecture, which is where the reason they cannot work is explained.

Your turn

Print The answer is 42 and end the program. The string is written for you at $2000, and the number is not part of it: print the string and the number 42 with one task.

Show solution

The second one reads a number and prints its square, with nothing else in the output. The test types 9, so the console reads 81.

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