Department of Electrical and Computer Engineering

The University of Texas at Austin

EE 306, Fall 2015
Problem Set 5
Due: 9 November, before class
Yale N. Patt, Instructor
TAs: Esha Choukse, Ali Fakhrzadehgan, Steven Flolid, Nico Garofano, Sabee Grewal, William Hoenig,
        Adeesh Jain, Kamyar Mirzazad, Matthew Normyle, Stephen Pruett, Siavash Zangeneh, Zheng Zhao

You are encouraged to work on the problem set in groups and turn in one problem set for the entire group. Remember to put all your names on the solution sheet. Also, remember to put the name of the TA and the time for the discussion section you would like the problem set turned back to you. Show your work.

  1. Moved from Problem Set 4

    The LC-3 has just finished executing a large program. A careful examination of each clock cycle reveals that the number of executed store instructions (ST, STR, and STI) is greater than the number of executed load instructions (LD, LDR, and LDI). However, the number of memory write accesses is less than the number of memory read accesses, excluding instruction fetches. How can that be? Be sure to specify which instructions may account for the discrepancy.

  2. Moved from Problem Set 4

    (Adapted from 7.18) The following LC-3 program compares two character strings of the same length. The source strings are in the .STRINGZ form. The first string starts at memory location x4000, and the second string starts at memory location x4100. If the strings are the same, the program terminates with the value 1 in R5; otherwise the program terminates with the value 0 in R5. Insert one instruction each at (a), (b), and (c) that will complete the program. Note: The memory location immediately following each string contains x0000.

    .ORIG x3000
    LD R1, FIRST
    AND R0, R0, #0
    LOOP____________________; (a)
    LDR R4, R2, #0
    BRz NEXT
    ADD R1, R1, #1
    ADD R2, R2, #1
    ____________________; (b)
    ____________________; (c)
    ADD R3, R3, R4
    BRz LOOP
    AND R5, R5, #0
    BRnzp DONE
    NEXTAND R5, R5, #0
    ADD R5, R5, #1
    DONETRAP x25
    FIRST   .FILL x4000
    SECOND   .FILL x4100

    1. Bob Computer just bought a fancy new graphics display for his LC-3. In order to test out how fast it is, he rewrote the OUT trap handler so it would not check the DSR before outputting. Sadly he discovered that his display was not fast enough to keep up with the speed at which the LC-3 was writing to the DDR. How was he able to tell?

    2. Bob also rewrote the handler for GETC, but when he typed ABCD into the keyboard, the following values were input:


      What did Bob do wrong?

  3. Moved to Problem Set 6 (updated 11/04/15)
    (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                  
    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

    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

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

  4. (Adapted from 10.1)
    What are the defining characteristics of a stack? Give two implementations of a stack and describe their differences.

  5. (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 Exercise 10.8 on 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?

  6. (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.

  7. 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 1: OP can be ADD, SUB, MUL, or DIV for parts a and b of this problem.
    (added 11/06/15) Note 2: To perform DIV and SUB operations, the top element of the stack is considered as the second operand. i.e. If we first push "A" and then push "B" followed by a "SUB" operation, "A" and "B" will be popped from stack and "A-B" will be pushed into stack.

    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
                  PUSH Z
                  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))

  8. Moved to Problem Set 6 (updated 10/27/15) 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
            ADD  R0, R1, #0
            TRAP x21
            LD   R0, SAVER0
    A       ADD  R0, R0, #1
            BRnzp START
    DONE    RET
    SAVER0  .BLKW #1

  9. Moved to Problem Set 6 (updated 11/05/15)
    (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?

  10. Assume that you have the following table in your program:

    MASKS   .FILL x0001
            .FILL x0002
            .FILL x0004
            .FILL x0008
            .FILL x0010
            .FILL x0020
            .FILL x0040
            .FILL x0080
            .FILL x0100
            .FILL x0200
            .FILL x0400
            .FILL x0800
            .FILL x1000
            .FILL x2000
            .FILL x4000
            .FILL x8000
    1. Write a subroutine CLEAR in LC-3 assembly language that clears a bit in R0 using the table above. The index of the bit to clear is specified in R1. R0 and R1 are inputs to the subroutine.

    2. Write a similar subroutine SET that sets the specified bit instead of clearing it.

    3. Hint: You should remember to save and restore any registers your subroutine uses (the "callee save" convention). Use the RET instruction as the last instruction in your subroutine (R7 contains the address of where in the caller to return to.)

  11. Suppose we are writing an algorithm to multiply the elements of an array (unpacked, 16-bit 2's complement numbers), and we are told that a subroutine "mult_all" exists which multiplies four values, and returns the product. The mult_all subroutine assumes the source operands are in R1, R2, R3, R4, and returns the product in R0. For purposes of this assignment, let us assume that the individual values are small enough that the result will always fit in a 16-bit 2's complement register.

    Your job: Using this subroutine, write a program to multiply the set of values contained in consecutive locations starting at location x6001. The number of such values is contained in x6000. Store your result at location x7000. Assume there is at least one value in the array(i.e., M[x6000] is greater than 0).

      Hint: Feel free to include in your program

      PTR	.FILL x6001
      CNT	.FILL x6000

  12. (9.13) The following program is supposed to print the number 5 on the screen. It does not work. Why? Answer in no more than ten words, please.
    	.ORIG 	x3000
    	JSR	A
    	OUT			;TRAP  x21
    	BRnzp	DONE
    A 	AND	R0,R0,#0
    	ADD	R0,R0,#5
    	JSR	B
    ASCII	.FILL	x0030
    	ADD	R0,R0,R1

  13. Moved to Problem Set 6 (updated 11/03/15)
    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
    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
      KBDR    .FILL xFE02
    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?

  14. (8.16) What does the following LC-3 program do?
            .ORIG x3000
            LD  R0, ASCII
            LD  R1, NEG
    AGAIN   LDI  R2, DSR
            BRzp AGAIN
            STI  R0, DDR
            ADD  R0, R0, #1
            ADD  R2, R0, R1
            BRnp AGAIN
    ASCII   .FILL x0041
    NEG     .FILL xFFB6
    DSR     .FILL xFE04
    DDR     .FILL xFE06

  15. (9.5) The following LC-3 program is assembled and then executed. There are no assemble time or run-time errors. What is the output of this program? Assume all registers are initialized to 0 before the program executes.
            .ORIG x3000
            ST R0, x3007
            LEA R0, LABEL
            TRAP x22
            TRAP x25

  16. 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.
        Address         Contents
        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

    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
      SIXTEEN     .FILL #16
      WORD        .BLKW #1
      RESULT      .BLKW #1
    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?

  17. Suppose we use the unused opcode 1101 to specify a new instruction. We will require 3 states after decode (state 32) to complete the job. The control signals required to carry out the work of the new instruction are shown below. All control signals not shown below are 0.
    1. What does this new instruction do?
    2. There are two different formats for specifying the operands of this instruction. Fill them out below.