1. Computer Science 210 Computer Organization
Introduction to Strings

2. Strings
Sequences of characters, represented internally as ASCII values
Basic ASCII is 8 bits, stored in lower order byte, with higher order byte clear
A null character (ASCII 0) always terminates a string

3. ;; Author: Ken Lambert
;; This program declares the string "Assembler is fun!"
.ORIG x3000
; Program code
HALT
; Data variables
MESSAGE .STRINGZ "Assembler is fun!"
.END
The STRINGZ directive puts a string’s ASCII values in consecutive memory cells, followed by a null character (ASCII 0)

4. String output with array-based loop and polling
;; Author: Ken Lambert
;; This program outputs the string "Assembler is fun!"
;; Pseudocode design:
; for ch in "Assembler is fun!"
; loop while display status >= 0
; print ch
.ORIG x3000
;; Register usage:
; R1 = contents of display status register
; R0 = contents of display data register
; R2 = address of the next character in the string
; Main program code
LEA R2, MESSAGE ; Get the base address of the string
CHLOOP LDR R0, R2, #0 ; Get the next character from the string
BRz ENDMAIN ; Quit when it's null (end of string)
POLL LDI R1, DSR ; Poll for negative display status register
BRzp POLL ; (ready bit = 1)
STI R0, DDR ; Display is ready, so output the character
ADD R2, R2, #1 ; Increment the character's address
BR CHLOOP
ENDMAIN HALT
; Main program data
DSR .FILL xFE04 ; Address of the display status register
DDR .FILL xFE06 ; Address of the display data register
MESSAGE .STRINGZ "Assembler is fun!"
.END
String output with array-based loop and polling

5. String output with trap service routine PUTS
;; Author: Ken Lambert
;; This program outputs the string "Assembler is fun!"
;; Pseudocode design:
; print "Assembler is fun!"
;; Register usage:
; R0 = base address of the string
.ORIG x3000
; Main program code
LEA R0, MESSAGE ; Load the address of the string
PUTS ; Call the trap service routine to output it
HALT
; Main program data
MESSAGE .STRINGZ "Assembler is fun!"
.END
String output with trap service routine PUTS
Using the trap service routine reduces 8 lines of code to 2

6. Watch Your Registers!
The PUTS routine expects the address of the string in R0
Before the call, the return address (the incremented PC) is saved in R7

7. LC-3 Trap Service Routines
vector
symbol
routine
x20
GETC
read a single character (no echo)
x21
OUT
output a character to the monitor
x22
PUTS
write a string to the console
x23 IN
print prompt to console, read and echo character from keyboard
x25
HALT
halt the program
The first four routines work with data in R0

8. More Problems Find the length of a string
Convert a string to uppercase or lowercase
Convert a string of digits to an integer
Convert an integer to a string of digits

9. For Wednesday
String input