Department of Electrical and Computer Engineering

The University of Texas at Austin

EE 306, Fall 2015
Problem Set 4
Due: 26 October, 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. (Adapted from 5.31)
    The following diagram shows a snapshot of the 8 registers of the LC-3 before and after the instruction at location x1000 is executed. Fill in the bits of the instruction at location x1000.
    R0 x0000 x0000
    R1 x1111 x1111
    R2 x2222 x2222
    R3 x3333 x3333
    R4 x4444 x4444
    R5 x5555 xFFF8
    R6 x6666 x6666
    R7 x7777 x7777
    Memory LocationValue
    x1000 0001 101 000 1 11000

  2. The memory locations x3000 to x3007 contain the values as shown in the table below. Assume the memory contents below are loaded into the simulator and the PC has been set to point to location x3000. Assume that a break point has been placed to the left of the HALT instruction (ie at location x3006 which contains 1111 0000 0010 0101). Assume that before the program is run, each of the 8 registers has the value x0000 and the NZP bits are 010.
    Memory LocationValue
    X3000 0101000000100000
    X3001 0001000000100101
    X3002 0010001000000100
    X3003 0001000000000000
    X3004 0001001001111111
    X3005 0000001111111101
    X3006 1111000000100101
    X3007 0000000000000100
    1. In no more than 15 words, summarize what this program will do when the “Run” button is pushed in the simulator. Hint: What relationship is there between the value loaded from memory and the final value in R0 after the program has completed?
    2. 5 is put in R0 and shifted left the value at location x3007 times
    3. What are the contents of the PC, the 8 general purpose registers (R0-R7), and the N, Z, and P condition code registers after the program completes?
    4. PCx3006
    5. What is the total number of CPU clock cycles that this program will take to execute until it reaches the breakpoint? Note: You should refer to the state machine (pg 568) to determine how many cycles an instruction takes. Assume each state that access memory takes 5 cycles to complete and every other state takes 1 cycle to execute.
    Memory LocationValueInstructionCycles takes to exectue oncenumber of times executedTotal Cycles for instruction
    X3000 0101000000100000AND919
    X3001 0001000000100101ADD919
    X3002 0010001000000100LD15115
    X3003 0001000000000000ADD9436
    X3004 0001001001111111ADD9436
    X3005 0000001111111101Branch9 if not taken 10 if taken3 times taken 1 time not taken39
    Total Cycles 9+9+15+36+36+39 = 144
  3. What does the following program do (in 15 words or fewer)? The PC is initially at x3000.
    Memory LocationValue
    x3000 0101 000 000 1 00000
    x3001 0010 001 011111110
    x3002 0000 010 000000100
    x3003 0000 011 000000001
    x3004 0001 000 000 1 00001
    x3005 0001 001 001 000 001
    x3006 0000 111 111111011
    x3007 1111 0000 0010 0101

    Counts the number of bits that are set to 1 in the word at x3100

  4. Prior to executing the following program, memory locations x3100 through x4000 are initialized to random values, exactly one of which is negative. The following program finds the address of the negative value, and stores that address into memory location x3050. Two instructions are missing. Fill in the missing instructions to complete the program. The PC is initially at x3000.
    Memory LocationValue
    x3000 1110 000 011111111
    x3001 0110 001 000 000000
    x3002 0000 100 000000010
    x3003 0001 000 000 1 00001
    x3004 0000 111 111111100
    x3005 0011 000 001001010
    x3006 1111 0000 0010 0101

  5. 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.

    A large number of LDI instructions (two read accesses) and STI instructions (one read access and one write access) could account for this discrepancy.

  6. (7.2) An LC-3 assembly language program contains the instruction:

      ASCII       LD R1, ASCII

    The symbol table entry for ASCII is x4F08. If this instruction is executed during the running of the program, what will be contained in R1 immediately after the instruction is executed?

    R1 <-- M[ASCII]
    R1 = 0010 001 1 1111 1111
              LD   R1,     #-1

  7. (7.10) The following program fragment has an error in it. Identify the error and explain how to fix it.

      ADD R3, R3, #30    The immediate value is too large.
      ST R3, A
    A   .BLKW 1

    Will this error be detected when this code is assembled or when this code is run on the LC-3?

    The error will be detected by the assembler since it will not be able to form the 16 bits of the instruction which performs the addition.
    One possible solution is to seperate the addition to two add instruction with immediate of #15.
      ADD R3, R3, #15
      ADD R3, R3, #15
      ST R3, A
    A   .BLKW 1

  8. (Adapted from 6.14) Consider the following machine language program:

      AND R2, R2, #0  R2 <- 0
    LOOP   ADD R1, R1, #-3  R1 <- R1-3
      BRn END  End when R1 is negative
      ADD R2, R2, #1  R2 <- R2+1
      BRnzp LOOP
    END   HALT

    What are the possible initial values of R1 that cause the final value in R2 to be 3?

    For R2 to contain the value 3, the BRn must not have intiated a branch for 3 consecutive times. Therefore, R1 wasn't negative after the instruction ADD R1, R1, #-3 was exectued 3 times and was negative after the instruction was executed 4 times. That is, R1-3x3 = R1-9 >= 0 and R1-3x4 = R1-12 < 0. Solving the inequalities yields, 9 <= R1 < 12. Since a register contains integers, R1 could have been 9, 10, or 11.

  9. (Adapted from 7.16) Assume a sequence of nonnegative integers is stored in consecutive memory locations, one integer per memory location, starting at location x4000. Each integer has a value between 0 and 30,000 (decimal). The sequence terminates with the value -1 (i.e., xFFFF).
    1. Create the symbol table entries generated by the assembler when translating the following routine into machine code:

        .ORIG x3000
        AND R4, R4, #0
        AND R3, R3, #0
        LD R0, NUMBERS
      LOOP   LDR R1, R0, #0
        NOT R2, R1
        BRz DONE
        AND R2, R1, #1
        BRz L1
        ADD R4, R4, #1
        BRnzp NEXT
      L1   ADD R3, R3, #1
      NEXT   ADD R0, R0, #1
        BRnzp LOOP
      DONE   TRAP x25
      NUMBERS   .FILL x4000

      Symbol Table

      Label Memory Address
      LOOP x3003
      L1 x300A
      NEXT x300B
      DONE x300D
      NUMBERS x300E

    2. What does the above program do?

      The instruction AND R2, R1, #1 performs a bit mask (x0001) to decide whether the least significant bit of the value is 0 or 1. The LSB of a number is used to determine whether the integer was even or odd. For example, numbers with a zero LSB are: 0000 (#0), 0010 (#2), 0100 (#4), 0110 (#6), which are all even. Hence, R3 counts the amount of even numbers in the list and R4 counts the amount of odd numbers.

  10. Below is a segment of LC-3 machine language program.

      ADD R2, R1, #0
    HERE   ADD R3, R2, #-1
    AND R3, R3, R2
    BRz END
    ADD R2, R2, #1
    BRnzp HERE

    If the data in R1 is an unsigned integer larger than 1, what does the program do? (Hint: what is the relationship between the resulting integer in R2 and the original integer in R1?)

    The program finds out the smallest power of 2 which is larger than or equal to the unsigned integer in R1.

  11. (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
    LOOPLDR R3, R1, #0; (a)
    LDR R4, R2, #0
    BRz NEXT
    ADD R1, R1, #1
    ADD R2, R2, #1
    NOT R4, R4; (b)
    ADD R4, R4, #1; (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

  12. The data at memory address x3500 is a bit vector with each bit representing whether a certain power plant in the area is generating electricity (bit = 1) or not (bit = 0). The program counts the number of power plants that generate electricity and stores the result at x3501. However, the program contains a mistake which prevents it from correctly counting the number of electricity generating (operational) power plants. Identify it and explain how to fix it.

    .ORIG x3000
    AND R0, R0, #0
    ADD R3, R0, #1
    CHECK   AND R4, R2, R3
    ADD R0, R0, #1
    NOTOPER  ADD R3, R3, R3
    ADD R1, R1, #-1
    LD R2, VECTOR  <- missing instruction
    STR R0, R2, #1
    TRAP x25
    NUMBITS   .FILL #16
    VECTOR   .FILL x3500

    R2 contains the bit vector, and not the address at which the bit vector is contained. The instruction LDI R2, VECTOR loaded the value at x3500 into R2 since the value at the memory address labeled as VECTOR was used as the address from which to load. The store instruction STR R0, R2, #1 uses the value of R2 to evaluate an address. However, R2 must be modified to contain an address for an STR instruction to work. Thus, LD R2, VECTOR is an additional required instruction.

  13. The following program does not do anything useful. However, being an "electronic idiot," the LC-3 will still execute it.
            .ORIG x3000
            LD R0, Addr1
            LEA R1, Addr1
            LDI R2, Addr1
            LDR R3, R0, #-6
            LDR R4, R1, #0
            ADD R1, R1, #3
            ST R2, #5
            STR R1, R0, #3
            STI R4, Addr4
    Addr1   .FILL x300B
    Addr2   .FILL x000A
    Addr3   .BLKW 1
    Addr4   .FILL x300D
    Addr5   .FILL x300C

    Without using the simulator, answer the following questions:

    1. What will the values of registers R0 through R4 be after the LC-3 finishes executing the ADD instruction?

    2. R0 - x300B
      R1 - x300D
      R2 - x000A
      R3 - x1263
      R4 - x300B

    3. What will the values of memory locations Addr1 through Addr5 be after the LC-3 finishes executing the HALT instruction?

    4. Addr1 - x300B
      Addr2 - x000A
      Addr3 - x000A
      Addr4 - x300B
      Addr5 - x300D