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 5 // Due at lecture Mar 30, 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.

Program 1.

a. An addressing mode is a mechanism for specifying where the operand is located.

b. Memory, register, or part of the instruction

c. PC-relative mode(memory), register mode(register), indirect mode(memory), immediate mode(instruction), Base+offset mode(memory)

d. Register mode

Program 2.

If both R0[15] and R1[15] are 0. (Otherwise an overflow will happen.)

Program 3.

For LDR, two memory accesses; STI, three memory accesses; TRAP, two memory accesses.

Note that, this includes one memory access that is needed to load the instruction into IR.

Problem 4.

Suppose the following LC-3 program is loaded into memory starting at location x30FF:

X30FF 1110 0010 0000 0001 R[1] <- x3100 + 1

X3100 0110 0100 0100 0010 R[2] <- MEM[X3101+2] = x1482

X3101 1111 0000 0010 0101 HALT

X3102 0001 0100 0100 0001

X3103 0001 0100 1000 0010

If the program is executed, what is the value in R2 at the end of execution?

Therefore, R2 will be x1482 at the end of the execution.

Problem 5.

Write an LC-3 program that compares two numbers in R2 and R3 and puts the larger number in R1. If the numbers are equal, then R1 is set equal to 0. For this question, you need to hand in your answer electronically. The handin directories have been created for everyone. You should put your code into a ASCII file with the name <yourstudentid>.asm. To hand in your file, follow the next two steps:

1. add the following line to your .cshrc.local file:

set path = ($path /s/handin/bin)

2. Then run the following command:

handin -c cs252-1 -a homework5 -d DIRECTORY

 DIRECTORY is the path to the directory where your file for this assignment is located. handin will go through the specified directory and hand in all the necessary files. To check that your files have been copied successfully, look inside the directory ~cs252-1/handin/{yourID}/{ASSIGNMENT_NAME}, and make sure that your file has been copied.

Problem 6.

An LDR instruction, located at x3200, uses R4 as its base register. The value currently in R4 is x4011.

(a) What is the largest address that this instruction can load from? x4011+0x001F = 0x4030

(b) Suppose we redefine the LDR offset to be zero-extended unsigned number, rather than sign-extended 2’s complement number. Then what would be the largest address that this instruction could load from? 0x4050 (when offset6 = 111111)

(c) With the new definition, what would be the smallest address that this instruction could load from? 0x4011(when offset = 000000)

Problem 7.

The instructions are:

R[1] <- not(R[1])

R[2] <- R[0]+R[1]

R[2] <- NOT(R[2])

Branch to 0x3100 if result is zero.

Since the control branch to x3100, we know NOT(R[2]) is zero and therefore R[2] is 0x1111. And not(R[1])+R[0] is 0x1111 only if R0=R1

Problem 8.

Problem 5.26 on page 149 of ItCS. Do not answer the question (c) and (d). Instead, answer the following two questions:

(a) 16 bit. Since 64K = 2^16.

(b) The largest immediate value is 1. We need to use one more bit for Opcode, one more for DR and one more for SR1. So imm5 became imm2.

(c) Given the constraints, which instructions can’t be implemented correctly?

ADD(register mode), AND(register mode)

(For LDI and other indirect mode instruction, it is actually doable if you always load two bytes together into the registers. )

(d) How far forward and backward can the branch instruction go with this new format?

-128 ~ +127(now is PCoffset8)

Problem 9.

The purpose of this problem is to get you setup with the LC-3 simulator, which will be important for subsequent homework. You can get the LC-3 simulator in one of the following ways:

1. Unsupported PC Use: You can download and extract the LC-3 Simulator on your own personal computer. However, we cannot support any problems encountered and you are on your own. Nevertheless, many students in previous semesters preferred this option. You can find it at http://highered.mcgraw-hill.com/sites/0072467509/student_view0/lc-3_simulator.html

2. Supported CAE Account: You can access the LC-3 simulator in CAE machines if you have a CAE account. To add the LC-3 simulator to your CAE applications, go to START->'CAE Applications'->'Add Applications to Start Menu'-> 'Available Applications'->'By Alphabetical'->G-L and select LC-3 Editor and LC-3 Simulator. Then you can open LC-3 simulator from 'START'->'CAE Applications'->'Engineering'->'LC-3 Simulator'.
3. Supported CS Account: If you are officially enrolled in 252 with a CS professor (Wood), a CS account has been created for you, and you can log on to any CS Instructional Windows machine in room 1368 in Computer Science and Statistics (CSS). Please check Computer Science Lab (CSL) Getting Started for information about activating your CS account. If you don't have an account please go to CSL on the 2nd floor. After logging in you can open the LC-3 Simulator from 'START'->'All Programs'->'LC3'->'Simulate'.

After opening the LC-3 simulator, please report the following:

1. The names of the LC-3 registers that can be seen in the simulator window.
2. The starting memory address, as shown by the blue arrow.