ECE/CS 252: Introduction to Computer Engineering

Spring 2007

Homework Assignment #8

The following homework requires you to use the LC-3 simulator. Please follow instructions from HW #5 problem 8 for opening the LC-3 simulator and instructions from HW #6 for submitting screenshots of your code.

Problem 1

Which is more efficient, interrupt-driven I/O or polling? Explain.

Problem 2

Regarding "RFID Inside" on RFID implants http://www.cs.wisc.edu/~markhill/cs252/Spring2007/handouts/spectrum07_rfid_ethics.pdf

a) Give two different reasons why an employer should be able to require that employees have RFID implants.

b) Give two different reasons why an employer should NOT be able to require that employees have RFID implants.

Problem 3

The following code reads two numbers from the memory and finds if they have the same absolute value. If the absolute values are equal, 1 is stored in R3; otherwise 0 is stored in R3. The subroutine starting at the label “ABS” finds the absolute value of the argument.

.ORIG x3000 ; Instructions start at x3000

; Initialization

AND R3, R3, #0

LD R1, VAL1

LD R2, VAL2

………………….. ; Prepare argument VAL1

ST R3, SaveR3 ; Save R3 before calling subroutine

JSR ABS ; Call subroutine ABS

LD R3, SaveR3 ; Restore R3

………………….. ; Store Abs(VAL1) in R5

ADD R0, R2, #0 ; Copy R2 to R0

ST R3, SaveR3

JSR ABS ; Call subroutine ABS

LD R3, SaveR3

NOT R0, R0

ADD R0, R0, #1

ADD R5, R5, R0 ; Find R5 - R0

BRnp STOP

ADD R3, R3, #1

STOP HALT

; Subroutine for absolute value

; Argument is passed in register ______ (fill)

ABS ADD R0, R0, #0 ; Set condition code based on R0

BRzp ENDABS

NOT R3, R0

ADD R0, R3, #1

ENDABS RET ; Value is returned in register ______ (fill)

; Values

SaveR3 .FILL x0000

VAL1 .FILL x000A

VAL2 .FILL xFFF6

.END

a)Some of the lines in the code are missing. Fill in the missing lines. Also fill in the blanks in the two comment lines as indicated.

b) Execute the completed program by setting a breakpoint at the HALT instruction, and submit screenshots of your solution

Problem 4

An LC-3 program is provided below:

.ORIG X3000

LD R0, ASCII

LD R1, NEG

AGAIN LDI R2, DSR

BRzp AGAIN

STI R0, DDR

ADD R0, R0, #1

ADD R3, R0, R1

BRn AGAIN

HALT

ASCII .FILL x0041

NEG .FILL xFFB6

DSR .FILL xFE04 ; Address of DSR

DDR .FILL xFE06 ; Address of DDR

.END

a) What does this program do?

b) What is the purpose of the Display Status Register (DSR)?

c) Execute the program in the LC-3 simulator and submit a screenshots of the LC-3 simulator, as well as the LC3 Console.