1. Computer Science 210 Computer Organization
Trap Service Routines and String Input

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 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

3. 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 the TRAP mechanism.

4. 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

5. TRAP Instruction Trap vector Where to go How to get back
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

6. Data Path for the TRAP
NOTE: PC has already been incremented during instruction fetch stage.

7. 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.

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

9. 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

10. String input with sentinel-based loop
;; 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
HALT
; Main program data
RT .FILL x-000D ; The return character (negated)
PROMPT .STRINGZ "Enter your name: "
ARRAY .BLKW ; 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.

11. Defining Subroutines Chapter 9
For Friday
Defining Subroutines
Chapter 9