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Understanding String Operations and I/O in LC-3 Assembly Language

This guide explores string handling, I/O operations, and trap service routines in LC-3 assembly. It covers concepts like ASCII representation of characters, the STRINGZ directive for storing strings in memory, and examples of how to output strings using the trap service routine. The pseudocode for displaying messages, along with register usage for managing strings and display status, is also included. This foundational understanding is key for working with strings in the LC-3 architecture.

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Understanding String Operations and I/O in LC-3 Assembly Language

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  1. Computer Science 210Computer Organization Strings, I/O, and Trap Service Routines

  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 • Can also store two characters per 16-bit word

  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. ;; 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. ;; 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. LC-3 Trap Service Routines The first four routines work with data in R0

  7. System Call • User program invokes system call. 2. Operating system code performs operation. 3. Returns control to user program. In LC-3, this is done through theTRAP mechanism.

  8. LC-3 TRAP Mechanism 1. A set of service routines. • part of operating system -- routines start at arbitrary addresses(convention is that system code is below x3000) • up to 256 routines 2. Table of starting addresses. • stored at x0000 through x00FF in memory • called System Control Block in some architectures 3. TRAP instruction. • used by program to transfer control to operating system • 8-bit trap vector names one of the 256 service routines 4. A linkage back to the user program. • want execution to resume immediately after the TRAP instruction

  9. TRAP Instruction • Trap vector • identifies which system call to invoke • 8-bit index into table of service routine addresses • in LC-3, this table is stored in memory at 0x0000 – 0x00FF • 8-bit trap vector is zero-extended into 16-bit memory address • Where to go • lookup starting address from table; place in PC • How to get back • save address of next instruction (current PC) in R7

  10. Data Path for the TRAP NOTE: PC has already been incrementedduring instruction fetch stage.

  11. RET (JMP R7) • How do we transfer control back toinstruction following the TRAP? • We saved old PC in R7. • JMP R7 gets us back to the user program at the right spot. • LC-3 assembly language lets us use RET (return)in place of “JMP R7”. • Must make sure that service routine does not change R7, or we won’t know where to return.

  12. TRAP Mechanism Operation • Lookup starting address. • Transfer to service routine. • Return (JMP R7). Will form the basis for defining our own procedures

  13. String Input • Usually terminated by a return character (ASCII 13) • Use GETC to input and OUT to echo • If not ASCII 13, store character in an array • Otherwise, quit the loop • Store a null character at the end of the characters in the array

  14. ;; Register usage: ; R0 = the input character ; R1 = the newline character ; R2 = base address of the array ; R3 = temporary working storage ; Main program code LEA R0, PROMPT ; Display the prompt PUTS LD R1, RT ; Initialize the return character LEA R2, ARRAY ; Get the base address of the array WHILE GETC ; Read and echo a character (stored in R0) OUT ADD R3, R0, R1 ; Quit if character = return BRz ENDWHILE STR R0, R2, #0 ; Store that character in the array ADD R2, R2, #1 ; Increment the address of the array cell BR WHILE ; Return to read another character ENDWHILE STR R3, R2, #0 ; Store the null character after the last input LEA R0, ARRAY ; Output the string PUTS HALT ; Main program data RT .FILL x-000D ; The return character (negated) PROMPT .STRINGZ "Enter your name: " ARRAY .BLKW 30 ; Array of 30 characters (including null) .END String input with sentinel-based loop Problem: input could overflow the array, but no data are declared below it.

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