CISP317 Practice Final Exam

Prof. Tak Auyeung

Instructions: You may bring any material that is handwritten or printed prior to the examination to help you. You can also bring a calculator if you think it may help you. However, you can only use the calculator for numerical computations only. You cannot let your calculator compile a program, or to communicate with others.

You, as an individual, are expected to do your own work. This means you cannot seek, receive or otherwise acquire any assistance except clarifications from the professor during an examination. Any communication involving the contents of the subject matter or the examination is considered cheating. Do not initiate or accept such communication, or the result of your examination is automatically voided.

New rules, read this! As of 2003.09.22, I no longer deduct points for wrong answers. Each correct answer is worth one point, each wrong answer is worth zero point, and each unanswered question is also worth zero point. This means you should guess and leave no question unanswered.

As a result, I also need to adjust the letter grade assignment break points. For your individual examinuation, ``A'' means at least 94%, ``B'' means at least 74%, ``C'' means at least 54%, ``D'' means at least 34% and ``F'' means below 34%. The break points for the final grade are now 26.83%, 48.5%, 70.17% and 91.83% as minimums for ``D'', ``C'', ``B'' and ``A'', respectively.

Please note that this change does not affect your letter grade at all, it is just a number game to make some people feel better about guessing.

Make sure you write down you name on the upper right corner first, otherwise I cannot give points to anonymous students!

The baseline is XX, there are YY questions.

1. Choose values of r0 and r1 such that the following instruction results in the N flag being set.

   add r0,r1
   

   1. r0=0x20, r1=0x30
   2. r0=0x40, r1=0x30
   3. r0=0x20, r1=0xf0
   4. r0=0xa0, r1=0x30
   5. r0=0xe0, r1=0x30
2. Assuming unsigned 8-bit variables `a' and `b' are already loaded into registers (two registers in r16 to r31), choose the correct implementation of the following pseudocode:

   
   
   

   1. L0:
          cp a,b
          brcc L1
          cpi b,50
          brcc L2
      L1:
          dec b
          inc a
          rjmp L0
      L2:
      

   2. L0:
          cp a,b
          brcs L1
          cpi b,50
          breq L2
          brcs L2
      L1:
          dec b
          inc a
          rjmp L0
      L2:
      

   3. L0:
          cp a,b
          brcs L1
          cpi b,50
          brcc L2
      L1:
          dec b
          inc a
          rjmp L0
      L2:
      

   4. L0:
          cp a,b
          brcs L1
          cpi b,50
          breq L2
          brcs L2
      L1:
          dec b
          inc a
          rjmp L0
      L2:
      

   5. L0:
          cp a,b
          brcc L1
          cpi b,50
          brne L1
          brcc L2
      L1:
          dec b
          inc a
          rjmp L0
      L2:
      

3. Assuming the following program starts from the beginning and ends at label eop. What should be the value of register r0 at label eop?

        ; ... code to set up the stack pointer
        sub r0,r0
        rcall sub1
        rcall sub2
   eop: nop
       
   sub1:
        rcall sub2
        rcall sub2
        ret
   
   sub2:
        inc r0
        ret
   

   1. 0
   2. 2
   3. 3
   4. 4
   5. 6
4. Referring to the program in the previous question, what is the number of bytes required for the stack?
   1. 0
   2. 1
   3. 2
   4. 4
   5. 6
5. Which instruction can replace the following one without change of behavior?

   lds r16,1
   

   1. ldi r16,1
   2. sts 16,r1
   3. sts 1,r16
   4. mov r1,r16
   5. ldi r1,16
6. Using the 4-bit signed representation, the value -3 is represented as the bit pattern $1101_2$. Sign extenstion is a process to widen the binary representation of numbers. If we want to sign extend the representation of -3 to use 8 bits, what will it look like?
   1. $00001101_2$
   2. $11010000_2$
   3. $11111101_2$
   4. $11011111_2$
   5. $11110011_2$
7. It is important to check that the result of an operation is within the range of numbers that can be represented. When two unsigned values, stored in 8-bit representation are added, how do we check the result is valid? Assume the values are stored in r0 and r1, and we use the following instruction to perform the addition. Choose a conditional branch to invalid_result if and only if the result is out of the range of r0.

   add r0,r1
   

   1. brmi invalid_result ; N = 1
   2. brcs invalid_result ; C = 1
   3. brlt invalid_result ; V = 1
   4. breq invalid_result ; Z = 1
   5. brne invalid_result ; Z = 0
8. What do we know at the nop instruction in the following code? Assume the state of pin 2 of port A remains the same for at least the amount of time taken to execute 10 instructions. Assume pin 2 of port A is configured as input.

        ldi   r16,5
   L0:
   L1:  sbic  PINA,2
        rjmp  L1
   L2:  sbis  PINA,2
        rjmp  L2
        dec   r16
        brne  L0
        nop
   

   1. the code gets to nop without any transitions at pin 2 of port A
   2. the code arrives nop after a 0-1 transition at pin 2 of port A
   3. the code arrives nop after a 0-1-0-1-0-1-0-1-0-1 transition pattern at pin 2 of port A
   4. this code never gets to nop because it is an infinite loop, regardless of the state of pin 2 of port A
   5. the code arrives nop after a 1-0 transition at pin 2 of port A
9. Choose the pseudocode that matches the following assembly code.

   L1:  cp   V1, V2
        breq  L2
        inc   V1
        dec   V2
        rjmp  L1
   L2:  nop
   

   1. 
      
      
   2. 
      
      
   3. 
      
      
   4. 
      
      
   5. 
      
      
10. What is loaded into r16 after the following code?

         ldi   r26,low(L1)
         ldi   r27,high(L1)
         ld    r16,X
         subi  r16,6
    

    1. the address of label L1
    2. -6
    3. the value at address L1
    4. the value at address L1 minus 6
    5. the least significant byte of address L1