Instructions:
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. (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?



2. (7.10) The following program fragment has an error in it. Identify the error and explain how to fix it. Note: R3 is the only register which should change its value after the program runs.

   	ADD R3, R3, #30
   	ST R3, A
   	HALT
   A	.BLKW 1

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



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

   	AND R2, R2, #0
   LOOP	ADD R1, R1, #-3
   	BRn END
   	ADD 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?



4. Updated 10/27/2009. (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
      	.END

   2. What does the above program do?



5. 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
   END	HALT

   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?)



6. (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
   	LD R2, SECOND
   	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
   NEXT	AND R5, R5, #0
   	ADD R5, R5, #1
   DONE	TRAP x25
   FIRST	.FILL x4000
   SECOND	.FILL x4100
   	.END




7. Updated 10/29/2009. 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 don't generate electricity and stores the result at x3501. However, the program contains a mistake which prevents it from correctly counting the number of operating power plants. 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
   	LD R1, NUMBITS
   	LDI R2, VECTOR
   	ADD R3, R0, #1
   CHECK	AND R4, R2, R3
   	BRz NOTOPER
   	ADD R0, R0, #1
   NOTOPER	ADD R3, R3, R3
   	ADD R1, R1, #-1
   	BRzp CHECK BRp CHECK
   	STR R0, R2, #1
   	TRAP x25
   NUMBITS	.FILL #16
   VECTOR	.FILL x3500
   	.END




8. Postponed to Problem Set 5 10/30/2009 (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.




9. 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:

      
      AAAAAAAAAAAAAAAAABBBBBBBBBBBBBBBBBBBBCCCCCCCCCCCCCCCCCCCDDDDDDDDDDDDDDDDDDDD
      

      What did Bob do wrong?





10. 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
            HALT
    Addr1   .FILL x300B
    Addr2   .FILL x000A
    Addr3   .BLKW 1
    Addr4   .FILL x300D
    Addr5   .FILL x300C
            .END
    
    

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





11. 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.