I/O & TRAP

TRAP Routines & I/O

System calls for input and output.

TRAP Instructions

TRAP instructions invoke operating system service routines. They're the LC-3's way of doing I/O and system operations.

Trap

Alias

Description

TRAP x20	GETC	Read one character into R0 (no echo)
TRAP x21	OUT	Print character in R0
TRAP x22	PUTS	Print string starting at address in R0
TRAP x23	IN	Read character into R0 (with echo)
TRAP x25	HALT	Stop the program

You can use either form: TRAP x21 or OUT — they're identical.

Output: Printing Characters

OUT prints the ASCII character in the low 8 bits of R0:

.ORIG x3000
; Print 'A' (ASCII 65)
AND R0, R0, #0
ADD R0, R0, #15   ; 15
ADD R0, R0, #15   ; 30
ADD R0, R0, #15   ; 45
ADD R0, R0, #15   ; 60
ADD R0, R0, #5    ; 65 = 'A'
OUT

; Easier: print a whole string
LEA R0, MSG
PUTS

HALT
MSG .STRINGZ " Hello!"
.END

Input: Reading Characters

GETC reads a single character from the keyboard into R0. IN does the same but also echoes the character to the screen.

.ORIG x3000
LEA R0, PROMPT
PUTS            ; Print prompt
GETC            ; Read character into R0
OUT             ; Echo it back
LEA R0, NL
PUTS            ; Print newline
HALT
PROMPT .STRINGZ "Type a character: "
NL .STRINGZ "\n"
.END

Exercise

Write an "echo" program: read a character from the user (use GETC), then print it 3 times using OUT.

.ORIG x3000
; Read a character
 
 
; Print it 3 times
 
 
HALT
.END

.ORIG x3000
; Read a character


; Print it 3 times


HALT
.END

x00000

PCx3000

NZP

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