1. Computer Science 210 Computer 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 LEAR2, MESSAGE; Get the base address of the string CHLOOPLDRR0, R2, #0; Get the next character from the string BRzENDMAIN; Quit when it's null (end of string) POLLLDIR1, DSR ; Poll for negative display status register BRzpPOLL; (ready bit = 1) STIR0, DDR; Display is ready, so output the character ADDR2, R2, #1; Increment the character's address BRCHLOOP ENDMAINHALT ; Main program data DSR.FILLxFE04; Address of the display status register DDR.FILLxFE06; 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 vectorsymbolroutine x20GETC read a single character (no echo) x21OUT output a character to the monitor x22PUTS write a string to the console x23IN print prompt to console, read and echo character from keyboard x25HALT halt the program The first four routines work with data in R0

7. System Call 1.User program invokes system call. 2. Operating system code performs operation. 3. Returns control to user program. In LC-3, this is done through the TRAP 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 incremented during instruction fetch stage.

11. RET (JMP R7) How do we transfer control back to instruction 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. 9-12 TRAP Mechanism Operation 1.Lookup starting address. 2.Transfer to service routine. 3.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 LEAR0, PROMPT; Display the prompt PUTS LD R1, RT ; Initialize the return character LEA R2, ARRAY; Get the base address of the array WHILEGETC; 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 ENDWHILESTR 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.