CS/ECE 252 Introduction to Computer Engineering

Spring 2009 All Sections
Instructor David A. Wood
TAs Mengmeng Chen, Maheswaran Venkatachalam & Daniel Wong
URL: http://www.cs.wisc.edu/~david/courses/cs252/Spring2009/

Homework 7// Due at lecture Apr 17, Mon

Primary contact for this homework: Mengmeng Chen [mchen at cs dot wisc dot edu]

You may choose to do this homework in a group of TWO or THREE students or all by yourself. If you do it in a group, each of the group member should hand in ONE copy of the homework that lists the common section number and names and UW ID numbers of all the other students.  Please staple multiple pages together.

Problem 1.

Consider the following two instructions in a LC-3 assembly language program:

ASCII      LD R0, ASCII

L             ST R0, L

The symbol table entry for ASCII is x4F08.

(a)    What will be contained in R0 immediately after the first instruction is executed.

R0  <- mem[ASCII] = x21FF, note that x21FF is  the binary code for instruction  LD R0, #-1

(b)    What will be stored at memory location x4F09 after the second instruction is executed. 

Mem[x4F09] = R0 = x21FF. Essentially the first instruction will be copied to x4F09 and overwrite the second instruction.

Problem 2.

Refer to Problem 7.15 on ItCS. Answer the following questions:

(a)    What does the program do? (Explain it in less than 25 words.)

The program will scan through the sequence and multiply all the non-negative numbers by 2 and update the value in its memory location.

(b)    Write out the symbol table of the program.

Symbol

Address

LOOP

x3002

L1

x3007

NEXT

x3009

DONE

x300B

NUMBERS

x300C

MASK

x300D

 

Problem 3.

You are given a string “introductiontocomputersystem” and you are going to write an assembly language program to find the letter that occurs at least three times in the string. Make sure you put your result in R0 and halt your program in the end.

Hint: Use .BLKW to allocate memory to keep track of each letters’ occurrences. Your program should work with arbitrary strings that contain only lowercase letters.

(a)    Draw a flowchart of your program.

(b)    Submit a screenshot of your commented code and what is stored in R0 after you run your program on LC-3.

One way to solve the problem.

Problem 4.

The following program adds the values stored in the memory location A, B, and C, and stores the result into memory. There are two errors in the code. For each, describe the error and indicate whether it will be detected at assembly time or at run time.
Line No.

1

 

.ORIG  x3000

2

ONE

LD     R0,  A

3

 

ADD  R1,  R1, R0

4

TWO

LD     R0,  B

5

 

ADD  R1,  R1,  R0

6

THREE

LD     R0,  C

7

 

ADD  R1,  R1,  R0

8

 

ST      R1M  SUM

9

 

TRAP  x25

10

A

.FILL  x0001

11

B

.FILL  x0002

12

C

.FILL  x0003

13

D

.FILL  x0004

14

 

.END

 

     Bug1.  (Line 3) R1 is not initialized before being used. This bug can only be detected by debugging at run time.

     Bug2. (Line 8) Actually there are two bugs here. ‘R1M’ is not a valid register (It’s a typo). Symbol ‘SUM’ is not defined. These two bugs will be detected at assembly time.

Problem 5.

You are given two pieces of program. They have been written by different programmers to store x0015 into memory location x4000.

a.       

 

.ORIG  x5000

 

AND   R0, R0, #0

 

ADD   R0, R0, #15

 

ADD   R0, R0, #6

 

STI      R0, PTR

 

<rest of the program…..>

 

HALT

PTR

.FILL   x4000

 

.END

 

b.       

 

.ORIG    x4000

 

.FILL     x0015

 

.END

 

 

(1)    Why there are two ‘ADD’ instructions in module (a)?

The range of the immediate value in ADD instruction is only from +15 to -16. +15 is the maximum number we can add at a time. Therefore the addition needs two steps.    

(2)    Explain the fundamental differences in their approaches?

The first approach uses instructions to dynamically initialize the value stored in some memory location. The initialization is done at run time. The second approach uses the pseudo-op provided by the assembler to statically initialize the memory content. The initialization is done at assembly time.

(3)    Give one example where module a ‘s approach is preferred.

The advantage of the second approach is simplicity. However, it requires that the programmer knows what the value should be before running the program. Sometimes, the initial value can only be calculated by running part of the program. Furthermore, sometimes the necessity of initializing some memory location is also determined at run time.

In both scenarios, you can use the first approach to come up with a subroutine and reuse it when it is necessary. (you will have to modify it a little bit).