Computer Science 210Computer Organization

Strings, I/O, and Trap Service Routines

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

;; Author: Ken Lambert

;; This program declares the string "Assembler is fun!"

.ORIG x3000

; Program codeHALT

; Data variablesMESSAGE .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)

;; 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 codeLEA R2, MESSAGE ; Get the base address of the string

CHLOOP LDR R0, R2, #0 ; Get the next character from the stringBRz 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 characterADD R2, R2, #1 ; Increment the character's addressBR CHLOOP

ENDMAIN HALT

; Main program dataDSR .FILL xFE04 ; Address of the display status registerDDR .FILL xFE06 ; Address of the display data registerMESSAGE .STRINGZ "Assembler is fun!"

.END

String output with array-based loop and polling

;; 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 codeLEA R0, MESSAGE ; Load the address of the stringPUTS ; Call the trap service routine to output itHALT

; Main program dataMESSAGE .STRINGZ "Assembler is fun!"

.END

String output with trap service routine PUTS

Using the trap service routine reduces 8 lines of code to 2

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 INprint prompt to console,read and echo character from keyboard

x25 HALT halt the program

The first four routines work with data in R0

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.

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

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

Data Path for the TRAP

NOTE: PC has already been incrementedduring instruction fetch stage.

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.

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

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

;; Register usage:; R0 = the input character; R1 = the newline character; R2 = base address of the array; R3 = temporary working storage

; Main program codeLEA R0, PROMPT ; Display the promptPUTSLD R1, RT ; Initialize the return characterLEA R2, ARRAY ; Get the base address of the array

WHILE GETC ; Read and echo a character (stored in R0)OUTADD R3, R0, R1 ; Quit if character = returnBRz ENDWHILESTR R0, R2, #0 ; Store that character in the arrayADD R2, R2, #1 ; Increment the address of the array cellBR 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 dataRT .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.