## Department of Electrical and Computer Engineering

### The University of Texas at Austin

EE 306, Fall 2013
Problem Set 6
Due: Not to be turned in
Yale N. Patt, Instructor
TAs: Ben Lin, Mochamad Asri, Ameya Chaudhari, Nikhil Garg, Lauren Guckert,
Jack Koenig, Saijel Mokashi, Sruti Nuthalapati, Sparsh Singhai, Jiajun Wang
Note: This problem set is unusually long, and is not to be turned in. We have put it together and handed it out to give you some challenging examples to help you prepare for the final exam.
1. Moved from problem set 5 (Adapted from 6.16) Shown below are the partial contents of memory locations x3000 to x3006.

 15 0 x3000 0 0 1 0 0 0 0 x3001 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 1 x3002 1 0 1 1 0 0 0 x3003 x3004 1 1 1 1 0 0 0 0 0 0 1 0 0 1 0 1 x3005 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 x3006

The PC contains the value x3000, and the RUN button is pushed.

As the program executes, we keep track of all values loaded into the MAR. Such a record is often referred to as an address trace. It is shown below.

MAR Trace
x3000
x3005
x3001
x3002
x3006
x4001
x3003
x0021

Your job: Fill in the missing bits in memory locations x3000 to x3006.

2. Moved from problem set 5

Jane Computer (Bob's adoring wife), not to be outdone by her husband, decided to rewrite the `TRAP x22` handler at a different place in memory. Consider her implementation below. If a user writes a program that uses this `TRAP` handler to output an array of characters, how many times is the `ADD` instruction at the location with label `A` executed? Assume that the user only calls this "new" `TRAP x22` once. What is wrong with this `TRAP` handler? Now add the necessary instructions so the `TRAP` handler executes properly.

Hint: `RET` uses `R7` as linkage back to the caller (`RET` is equivalent to `JMP R7`).

``````
; TRAP handler
; Outputs ASCII characters stored in consecutive memory locations.
; R0 points to the first ASCII character before the new TRAP x22 is called.
; The null character (x00) provides a sentinel that terminates the output sequence.

.ORIG x020F
START   LDR R1, R0, #0
BRz DONE
ST R0, SAVER0
TRAP x21
LD R0, SAVER0
BRnzp START
DONE    RET

SAVER0  .BLKW #1
.END
``````

3. Moved from problem set 5 (Adapted from 9.2)
1. How many `TRAP` service routines can be implemented in the LC-3? Why?

2. Why must a `RET` instruction be used to return from a `TRAP` routine? Why won't a `BRnzp` (unconditional `BR`) instruction work instead?

3. How many accesses to memory are made during the processing of a `TRAP` instruction?

4. Moved from problem set 5 (Adapted from 8.15)
1. What does the following LC-3 program do?
``````
.ORIG  x3000
LD R3 , A
STI R3, KBSR
AGAIN   LD R0,B
TRAP X21
BRnzp AGAIN
A       .FILL X4000
B       .FILL X0032
KBSR    .FILL XFE00
.END
``````
2. If someone strikes a key, the program will be interrupted and the keyboard interrupt service routine will be executed as shown below. What does the keyboard interrupt service routine do?
``````
.ORIG X1000
LDI R0,KBDR
TRAP X21
TRAP X21
RTI
KBDR    .FILL XFE02
.END
``````
3. Finally, suppose the program of part a started executing, and someone sitting at the keyboard struck a key. What would you see on the screen?

What are the defining characteristics of a stack? Give two implementations of a stack and describe their differences.

6. A zero-address machine is a stack-based machine where all operations are done by using values stored on the operand stack. For this problem, you may assume that the ISA allows the following operations:

PUSH M - pushes the value stored at memory location M onto the operand stack.

POP M - pops the operand stack and stores the value into memory location M.

OP - Pops two values off the operand stack and performs the binary operation OP on the two values. The result is pushed back onto the operand stack.

Note: OP can be ADD, SUB, MUL, or DIV for parts a and b of this problem.
Note: See the stack machine supplemental handout for help on this problem.

1. Draw a picture of the stack after each of the instructions below are executed. What is the minimum number of memory locations that have to be used on the stack for the purposes of this program? Also write an arithmetic equation expressing u in terms of v, w, x, y, and z. The values u, v, w, x, y, and z are stored in memory locations U, V, W, X, Y, and Z.
```            PUSH V
PUSH W
PUSH X
PUSH Y
MUL
PUSH Z
SUB
DIV
POP U
```
2. Write the assembly language code for a zero-address machine (using the same type of instructions from part a) for calculating the expression below. The values a, b, c, d, and e are stored in memory locations A, B, C, D, and E.

e = ((a * ((b - c) + d))/(a + c))

7. The memory locations given below store students' exam scores in form of a linked list. Each node of the linked list uses three memory locations to store
1) Address of the next node
2) Starting address of the memory locations where name of the student is stored
3) Starting address of the memory locations where the his/her exam score is stored
in the given order. The first node is stored in locations x4000 ~ x4002 . The ASCII code x0000 is used as a sentinel to indicate the end of the string. Both the name and exam score are stored as strings.
Write down the student's name and score in the order that it appears in the list
``` ```

x4000           x4016
x4001           x4003
x4002           x4008
x4003           x004D
x4004           x0061
x4005           x0072
x4006           x0063
x4007           x0000
x4008           x0039
x4009           x0030
x400A           x0000
x400B           x0000
x400C           x4019
x400D           x401E
x400E           x004A
x400F           x0061
x4010           x0063
x4011           x006B
x4012           x0000
x4013           x0031
x4014           x0038
x4015           x0000
x4016           x400B
x4017           x400E
X4018           x4013
x4019           x004D
x401A           x0069
x401B           x006B
x401C           x0065
x401D           x0000
x401E           x0037
x401F           x0036
x4020           x0000
```
```
8. (Adapted from 10.9) The input stream of a stack is a list of all the elements we pushed onto the stack, in the order that we pushed them. The input stream from Excercise 10.8 from page 284 of the book for example is ABCDEFGHIJKLM
The output stream is a list of all the elements that are popped off the stack in the order that they are popped off.
a. If the input stream is ZYXWVUTSR, create a sequence of pushes and pops such that the output stream is YXVUWZSRT.
b. If the input stream is ZYXW, how many different output streams can be created.

9. Do Problem 6.16 on page 175 in the textbook.

10. (Adapted from 10.6) Rewrite the PUSH and POP routines such that the stack on which they operate holds elements that take up two memory locations each. Assume we are writing a program to simulate a stack machine that manipulates 32-bit integers with the LC-3. We would need PUSH and POP routines that operate with a stack that holds elements which take up two memory locations each. Rewrite the PUSH and POP routines for this to be possible.
11. Consider the following LC-3 assembly language program. Assumming that the memory locations DATA get filled before the program executes, what is the relationship between the final values at DATA and the initial values at DATA?
```	.ORIG   x3000
LEA     R0, DATA
AND     R1, R1, #0
LOOP2   JSR     SUB1
BRzp    LABEL
JSR     SUB2
BRp     LOOP2
BRp     LOOP1
HALT
DATA    .BLKW   #10
SUB1    LDR     R5, R2, #0
NOT     R5, R5
LDR     R6, R2, #1
RET
SUB2    LDR     R4, R2, #0
LDR     R5, R2, #1
STR     R4, R2, #1
STR     R5, R2, #0
RET
.END
```
12. During the initiation of the interrupt service routine, the N, Z, and P condition codes are saved on the stack. By means of a simple example show how incorrect results would be generated if the condition codes were not saved. Also, clearly describe the steps required for properly handling an interrupt.

1. The program below counts the number of zeros in a 16-bit word. Fill in the missing blanks below to make it work.
```            .ORIG x3000
AND   R0, R0, #0
LD    R1, SIXTEEN
LD    R2, WORD
A           BRn   B
________________
B           ________________
BRz   C
________________
BR    A	; note: BR = BRnzp
C           ST    R0, RESULT
HALT

SIXTEEN     .FILL #16
WORD        .BLKW #1
RESULT      .BLKW #1
.END```
2. After you have the correct answer above, what one instruction can you change (without adding any instructions) that will make the program count the number of ones instead?

13. Fill in the missing blanks so that the subroutine below implements a stack multiply. That is it pops the top two elements off the stack, multiplies them, and pushes the result back on the stack. You can assume that the two numbers will be non-negative integers (greater than or equal to zero) and that their product will not produce an overflow. Also assume that the stack has been properly initialized, the PUSH and POP subroutines have been written for you and work just as described in class, and that the stack will not overflow or underflow.

Note: All blanks must be filled for the program to operate correctly.
```MUL        _______________
ST  R0, SAVER0
ST  R1, SAVER1
ST  R2, SAVER2
ST  R5, SAVER5
AND R2, R2, #0
JSR POP
JSR POP
_______________

_______________
BRp AGAIN
JSR PUSH
_______________
LD R0, SAVER0
LD R1, SAVER1
LD R2, SAVER2
LD R5, SAVER5
RET
```
14. The program below calculates the closest integer greater than or equal to the square root of the number stored in NUM, and prints it to the screen. That is, if the number stored in NUM is 25, "5" will be printed to the screen. If the number stored in NUM is 26, "6" will be printed to the screen. Fill in the blanks below to make the program work.

Note: Assume that the value stored at NUM will be between 0 an 81.
```         .ORIG x3000
AND R2, R2, #0
LD R3, NUM
BRz OUTPUT
NOT R3, R3
_______________
AND R1, R1, #0
BRp INLOOP
_______________
BRn OUTLOOP
OUTPUT   LD R0, ZERO
_______________
TRAP x21
HALT
NUM      .BLKW 1
ZERO     .FILL x30
.END
```
15. The figure below shows the part of the LC-3 data path that deals with memory and I/O. Note the signals labeled A through F. A is the memory enable signal, if it is 1 memory is enabled, if it is 0, memory is disabled. B, C, and D are the load enable signals for the Device Registers. If the load enable signal is 1, the register is loaded with a value, otherwise it is not. E is the 16-bit output of INMUX, and F is the 2-bit select line for INMUX.

The initial values of some of the processor registers and the I/O registers, and some memory locations are as follows:

 R0 = x0000 PC = x3000 KBSR = x8000 KBDR = x0061 DSR = x8000 DDR = x0031 M[x3009] = xFE00 M[x300A] = xFE02 M[x300B] = xFE04 M[x300C] = xFE06

During the entire instruction cycle, memory is accessed between one and three times (why?). The following table lists two consecutive instructions to be executed on the LC-3. Complete the table with the values that each signal or register takes right after each of the memory accesses performed by the instruction. If an instruction does not require three memory accesses, draw a line accross the unused accesses. To help you get started, we have filled some of the values for you.

 PC Instruction Access MAR A B C D E[15:0] F[1] F[0] MDR x3000 LD R0, x9 1 x3000 x2009 2 3 x3001 LDR R0, R0, #0 1 2 3

16. Note: This problem is NOT easy. In fact, it took me a while to solve it, and I am supposed to be an expert on 306 material. So, if you are struggling to pass this course, I suggest you ignore it. On the other hand, if you are a hot shot and think no problem is beyond you, then by all means go for it. We put it on the problem set to keep some of the hot shots out of mischief. We would not put it on the final, because we think it is too difficult to put on the exam.

A programmer wrote this program to do something useful. He, however, forgot to comment his code, and now can't remember what the program is supposed to do. Your job is to save him the trouble and figure it out for him. In 20 words or fewer tell us what valuable information the program below provides about the value stored in memory location INPUT. Assume that there is a non-zero value at location INPUT before the program is executed.

HINT: When testing different values of INPUT pay attention to their bit patterns. How does the bit pattern correspond to the RESULT?
```              .ORIG x3000
LD R0, INPUT
AND R3, R3, #0
LD R1, COUNT
LOOP          LDR R2, R6, #0
AND R5, R0, R2
BRz SKIP
BRp LOOP
ST R3, RESULT
HALT
COUNT         .FILL #4
.FILL 0xF0F0
.FILL 0xCCCC
.FILL 0xAAAA
INPUT         .BLKW 1
RESULT        .BLKW 1
.END
```
17. Figure out what the following program does.
```
.ORIG X3000
LEA R2, C
LDR R1, R2, #0
LDI R6, C
LDR R5, R1, #-3
ST R5, C
LDR R5, R1, #-4
LDR R0, R2, #1
JSRR R5
AND R3, R3, #0
LEA R4, B
A	STR R4, R1, #0
BRP A
HALT
LDR R0, R2, #0
JSRR R5
TRAP X29