CS/ECE 252 Introduction to Computer Engineering Spring 2011 All Sections Instructor: Andy Phelps TAs: Newsha Ardalani, Peter Ohmann, Jai Menon URL: http://www.cs.wisc.edu/~aephelps/courses/cs252/Spring2011/

Homework 5 // Due at the BEGINNING of lecture Mon, Mar 28

Primary contact for this homework: Peter Ohmann [ohmann at cs dot wisc dot edu]

You may do this homework with one other person from your section. You must put both names on the assignment. Please staple multiple pages together.

Problem 1 (2 points)

A memory has 512 distinct memory locations and an addressability of 4 bits. Can we determine the size of the MAR and/or the MDR (i.e. the number of bits in each) based on this? If not, what other information would we need? If so, what is the size of each?

The MAR size is determined by the total memory locations in the memory. 512 = 2⁹, so the MAR is 9 bits.
The MDR size is determined by the addressability. The MDR is 4 bits.

Problem 2 (3 points)

What does the following LC-3 instruction do? (Be specific; that is, give specific register numbers or memory locations.)

Address	Binary	Hex
0x450A	0011 0110 0000 0111	0x3607

The instruction is: ST R3 PC+7. So, specifically, the value currently in R3 would be stored at address 0x450B + 7 = 0x4512.

Problem 3 (3 points)

We wish to execute a single instruction which subtracts the decimal number 30 from R1 and stores the result in R2. Can we do this? If yes, give the instruction. If not, explain why not.

No, we cannot perform this operation in a single instruction. The ADD instruction only has an immediate field of 5 bits. Therefore, the most negative number we can represent is -16.

Problem 4 (6 points)

1. Give a single LC-3 instruction which moves the value in R1 into R5.
   

   Instruction	Binary	Hex
   ADD R5 R1 0x0	0001 1010 0110 0000	0x1A60

   Instruction	Binary	Hex
   AND R5 R1 0x01F	0101 1010 0111 1111	0x5A7F

   Instruction	Binary	Hex
   AND R5 R1 R1	0101 1010 0100 0001	0x5A41

2. Give a single LC-3 instruction for:
   R1 ← 0
   

   Instruction	Binary	Hex
   AND R1 R1 0x0	0101 001X XX10 0000	~0x5260

3. The LC-3 has no subtract instruction, but we can perform subtraction on the LC-3, nevertheless. Give a set of three LC-3 instructions to compute:
   R3 ← R2 - R1
   

   Instruction	Binary	Hex
   NOT R1 R1	1001 0010 0111 1111	0x927F
   ADD R1 R1 0x1	0001 0010 0100 0001	0x1241
   ADD R3 R2 R1	0001 0110 1000 0001	0x1681

   Instruction	Binary	Hex
   NOT RX R1	1001 001X XX11 1111	~0x927F
   ADD RY RX 0x1	0001 YYYX XX00 0001	~0x1241
   ADD R3 R2 RY	0001 0110 1000 0YYY	~0x1681

4. The LC-3 has no OR instruction, but we can perform the or operation on the LC-3, nevertheless. Give a set of four LC-3 instructions to compute:
   R3 ← R2 | R1
   

   Instruction	Binary	Hex
   NOT R2 R2	1001 0100 1011 1111	0x94BF
   NOT R1 R1	1001 0010 0111 1111	0x927F
   AND R3 R2 R1	0101 0110 1000 0001	0x5681
   NOT R3 R3	1001 0110 1111 1111	0x96FF

Problem 5 (5 points)

Write an LC-3 program to perform the following operation:
R3 ← |R1 + R2|
That is, the program should add R1 to R2, take the absolute value of the sum, and then halt.

Note: Your program should start at memory address 0x3000. The instructions for your answer may be in either binary or hex, but please also include your set of instructions in the book's assembly-style format (e.g. R1 ← R2 + R3).

Of course, there are a number of ways to do this, and the following is just one example:

Address	Binary	Hex	Instruction
0x3000	0001 0110 0100 0010	0x1642	R3 ← R1 + R2
0x3001	0000 0110 0000 0010	0x0602	BRzp 0x3004
0x3002	1001 0110 1111 1111	0x96FF	R3 ← NOT (R3)
0x3003	0001 0110 1110 0001	0x16E1	R3 ← R3 + 1
0x3004	1111 0000 0010 0101	0xF025	HALT

Problem 6 (5 points)

Given the program below:

Address	Binary	Hex	Instruction
0x3000	0010 0010 0000 0111	0x2207	R1 ← M[0x3008]
0x3001	0010 0100 0000 0111	0x2407	R2 ← M[0x3009]
0x3002	0101 0110 1110 0000	0x56E0	R3 ← R3 & 0x0
0x3003	0001 0110 1100 0001	0x16C1	R3 ← R3 + R1
0x3004	0001 0100 1011 1111	0x14BF	R2 ← R2 - 0x1
0x3005	0000 0011 1111 1101	0x03FD	BRP 0x3003
0x3006	1111 0000 0010 0101	0xF025	HALT
0x3007	1100 0000 1100 0000	0xC0C0	RET (N/A)
0x3008	0000 0000 0000 0101	0x0005	DATA (5)
0x3009	0000 0000 0000 0100	0x0004	DATA (4)
0x300A	1100 0000 1100 0000	0xC0C0	RET (N/A)

1. Write out each instruction in the book's assembly-style format (e.g. R1 ← R2 + R3). Label each line in your answer by its address in memory (as above).
   (see above)
2. What are the values in R1, R2, and R3 when the program halts? If the program does not halt, state this.
   

   Register	Value
   R1	5
   R2	0
   R3	20

3. What operation does this program implement?
   Multiply
4. This program is not particularly useful (despite the fact that it does perform some operation). What new instruction should be inserted just prior to address 0x3006 so that we could actually use the result in the future?
   Any sort of ST operation so that the value is retained in memory somewhere.

Problem 7 (3 points)

The purpose of this problem is to get you set up with the LC-3 PennSim simulator, which will be important for subsequent homework. You can get the LC-3 simulator from the PennSim link in the sidebar.

If you need additional help getting set up with the PennSim simulator, try this page.

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

1. The names of the LC-3 registers that can be seen in the simulator window, and their corresponding values.
   

   Register	Value
   R0	x0000
   R1	x0000
   R2	x0000
   R3	x0000
   R4	x0000
   R5	x0000
   R6	x0000
   R7	x0000
   PC	x0200
   MPR	x0000
   PSR	x8002

2. The starting memory address, as shown by the highlighted row.
   0x0200
3. The "CC" value by the registers is clearly not a register. What is it?
   NZP