2.4.7 Instruction Types: Input/Output

Inputting data from the keyboard and outputting results to the screen are handled by Monitor Systems Calls written in by the programmer. They are made by executing a "trap #11" instruction and a .byte word indicating the call. The full list of system calls can be found in the Flight-68k Users Manual but some of the most useful ones will be looked at in this section.

The call:

		
		trap	#11
		.byte	0,0

has the effect of ending the users program and returns control to the monitor. It is usually at the end of all assembly programs.

There are a number of calls that concern output to the screen:

```
		trap	#11
		.byte	0,8
```
prints out the lowest four bits of d0 to the screen as one hex character.
```
		trap	#11
		.byte	0,9
```
prints out the lowest eight bits (the byte) of d0 to the screen as two hex characters.
```
		trap	#11
		.byte	0,10
```
prints out the lowest sixteen bits (the word) of d0 to the screen as four hex characters.
```
		trap	#11
		.byte	0,11
```
prints out the whole (longword) of d0 to the screen as eight hex character.
```
		trap	#11
		.byte	0,5
```
prints out a string of characters addressed at a6. After using this code, d0.b is cleared so its previous contents will be lost. The text is defined in the form:
```
text1:	.text "write to screen",00
```
where text1 is a label identifier and write to screen is displayed. To load the text into a6, use the instruction:
```
	lea	text1,a6
```
which loads the effective address of text1 into a6. Output examples can be seen in the program add3.

To input a value, the code used is:

		trap	#11
		.byte	0,19

which reads a string of hex characters into d1.l, inserting any leading zeroes in necessary.The string is stopped after a non-hex character is input or eight characters have been input. The register d2.l holds the number of hex characters input and d0.b holds the non- hex terminating character so information previously held in these registers is lost.

A useful system call is:

		trap	#11
		.byte	0,12

which starts a new line on the screen.