Computer Science 210 Computer Organization Introduction to Strings

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

;; 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)

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

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

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

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

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

For Wednesday String input