CS/ECE 252 Introduction to Computer Engineering

Load the following program in PennSim and answer the following questions.

        .ORIG x3000
        LEA R1, STRZ
        AND R2, R2, #0 
        LD  R4, CHAR

LOOP    LDR R3, R1, #0	
        BRz DONE	Branch if null
        ADD R3, R3, R4
        BRnp SKIP	Branch if not ' '
        ADD R2, R2, #1
SKIP    ADD R1, R1, #1
        BR  LOOP	Always branch

DONE    ST  R2, COUNT
        HALT

CHAR    .FILL xFFE0
COUNT   .FILL x0000
STRZ    .STRINGZ "Hello World as always!"
        .END

1. (1 point) How many times is the instruction at label "LOOP" executed?

   It will branch to LOOP until it loads a null character in R3. The length of the string is 22 characters + 1 null character, so 23 times.

2. (1 point) How many times is the instruction at label "SKIP" executed?

   Skip is executed for each character except null, so 22

3. (2 points) Describe what this program does in 1-2 sentences.

   It counts the number of space character in the string at STRZ and stores that value in COUNT

Problem 2 (4 Points)

Show the symbol table created by the assembler for the following program.

        .ORIG x3000
        LD  R0, LENGTH
        AND R1, R1, #0 
        LEA R2, ARRAY

L00P    LDR R3, R2, #0
        ADD R1, R1, R0
        ADD R2, R2, #1
        ADD R0, R0, #-1
        BRp L00P

        ADD R3, R3, R0
        BRz DONE
        ST  R2, SUM
DONE    HALT
LENGTH  .FILL x05
ARRAY   .BLKW 5
PRSTR   .STRINGZ "Done!"
SUM     .FILL x0000
        .END

Problem 3 (2 points)

The following program is supposed to store the result 4 * R0 at memory address 0x9000. There are two errors in the code. Identify which instructions have an error, and the corresponding type of error.

        .ORIG 0x3000
FOURX   AND R1, R1, #0
        AND R3, R3, #0
        ADD R3, R3, #4
LOOP    ADD R1, R1, R0        
        ADD R3, R3, #-1
        BRzp LOOP	; Logical error: Loop 5 times instead of 4
        ST  R1, x9000	; Syntax error: x9000 is out of range
        HALT
        .END

Problem 4 (4 Points)

Recall the state machine from homework 4:

The subroutine "FSM" implements the above state machine. The input is stored at the label "INPUT" and the resulting output is stored at the label "OUTPUT". The input bit sequence is read from the most significant bit (MSB) to the least significant bit (LSB). For example, if the content of INPUT was "1011 0011 0110 1000", then the first input bit is 1 and the last input bit is 0. The first output bit is stored as the MSB of OUTPUT, and the last output bit is stored as the LSB. The labels S0, S1, S2, S3 corresponds to states 00, 01, 10, and 11, respectively. Fill in the missing instructions to complete the subroutine.

        .ORIG  x3000
        LEA    R0, PROMPT
        PUTS
        JSR    FSM
        HALT

FSM     ST    R0, TMP_R0
        ST    R1, TMP_R1
        ST    R2, TMP_R2
        ST    R3, TMP_R3
        LD    R0, INPUT
        AND    R1, R1, 0        ; R1 = output
        LEA    R2, S0           ; R2 = address of current state
        ADD    R3, R1, 15
        ADD    R3, R3, 1        ; R3 = 16
        ; Loop over all bits
LOOP    JMP	R2    ; Determines which state to jump into

S0      ; Output = 0
        ADD    R0, R0, 0        ; For checking next state
        BRzp    ST_END        ; if MSB(R0) == 0, stay in S0, else S1
        LEA    R2, S1        
        BRnzp    ST_END

S1      ; Output = 1
        ADD	R1, R1, 1
        ADD    R0, R0, 0        ; For checking next state
        BRzp    S1_S3            ; if MSB(R0) == 0, go to S3, else S0
        LEA    R2, S0        
        BRnzp    ST_END
S1_S3    LEA    R2, S3        
        BRnzp    ST_END

S2        ; Output = 0
        ADD    R0, R0, 0        ; For checking next state
        BRzp    S2_S0            ; if MSB(R0) == 0, go to S0, else S1
        LEA    R2, S1        
        BRnzp    ST_END
S2_S0   LEA	R2, S0
        BRnzp    ST_END

S3      ; Output = 1
        ADD	R1, R1, 1
        ADD    R0, R0, 0        ; For checking next state
        BRzp    S3_S2            ; if MSB(R0) == 0, go to S2, else S0
        LEA    R2, S0        
        BRnzp    ST_END
S3_S2   LEA    R2, S2        

ST_END
        ADD    R1, R1, R1        ; Left shift R1, the temporary result
        ADD    R0, R0, R0        ; Left shift R0
        ADD    R3, R3, -1        
        BRp LOOP
        ST	R1, OUTPUT
        LD    R3, TMP_R3
        LD    R2, TMP_R2
        LD    R1, TMP_R1
        LD    R0, TMP_R0
        RET

INPUT     .FILL    0
OUTPUT    .FILL    0

TMP_R0    .FILL 0
TMP_R1    .FILL 0
TMP_R2    .FILL 0
TMP_R3    .FILL 0

PROMPT    .STRINGZ "Modify the value at label INPUT to test the FSM.\n"

        .END

Problem 5 (4 points)

Solution: hw7_p5_sol.txt

Grading:

Correct result for all test cases: 4 points

Otherwise:

• 1 point for loop that terminates
• 1 point for correct call to RSH
• 1 point for correct implementation of if statement

Problem 6 (12 points)

Solution: hw7_p6_sol.txt

Grading:

CHECKINPUTVALID

• Correct output for all test cases: 2 points
• Otherwise 1 point per test case
  • Case 1: correct output on valid input
  • Case 2: correct output on invalid input

CHECKCOLUMNVALID

• Correct output for all test cases: 2 points
• Otherwise 1 point per test case
  • Case 1: correct output on valid input
  • Case 2: correct output on invalid input

CHECKEND

• Correct output for all test cases: 8 points
• Otherwise 2 points per test case
  • Case 1: correct output on continuing game (game hasn't ended)
  • Case 2: correct output on non-full board win
  • Case 3: correct output on full board win
  • Case 4: correct output on tie