Warm-up Quiz Questions

02 Command line and C compilation

If your e-ID was "person2ab", what would be the best command to log into stu via SSH in a terminal?

  1. ssh stu.cs.jmu.edu
  2. ssh person2ab@stu.cs.jmu.edu
  3. ssh <person2ab>@stu.cs.jmu.edu
  4. ssh person2ab
  5. ssh <person2ab@stu.cs.jmu.edu>

03 C intro

Consider the following C program: 

#include <stdio.h>

void do_stuff (int a, int *b)
{
    int triple = a * 3;
    b = &triple;
}

int main ()
{
    int n = 0;
    do_stuff(5, &n);
    printf("%d\n", n);
    return 0;
}

What will be printed if we run this program?

  1. 0
  2. 3
  3. 5
  4. 15
  5. None of these are correct

04 C arrays and strings

Which of the following expressions will evaluate to true if and only if a single command-line parameter is passed to "./intro" for P0?

  1. argc == 0
  2. argc == 1
  3. argc == 2
  4. argc != NULL
  5. There is not enough information to know.

Which of the following expressions will evaluate to true if and only if the first command-line parameter passed to "./intro" for P0 starts with "-g"?

  1. strncmp(argv[0], "-g", 2) != NULL
  2. strncmp(argv[0], "-g", 2) == 0
  3. strncmp(argv[0], "-g", 1) == 0
  4. strncmp(argv[1], "-g", 2) != NULL
  5. strncmp(argv[1], "-g", 2) == 0
  6. strncmp(argv[1], "-g", 1) == 0
  7. None of these will work.

05 C structs and I/O

Consider the following C code: 

typedef union {
    short tiny;
    int medium;
    long big;
} thing_t;

Assume that the 'short' type must be at least two bytes, the 'int' type must be at least four bytes, and the 'long' type must be at least eight bytes. What is the minimum amount of space required to store a 'thing_t' variable?

  1. 8 bytes
  2. 14 bytes
  3. 16 bytes
  4. 24 bytes
  5. There is not enough information to know.

06 C Miscellaneous

What's wrong with the following C code, assuming that we compile with --std=c99 and the 'nodelist_t' struct type has an integer element called 'size'? 

bool nodelist_is_empty(nodelist_t* list)
{
    if (list->size == 0) {
        return true;
    } else {
        return false;
    }
}
  1. It won't compile due to a syntax error
  2. It's not valid according to the C99 standard
  3. It invokes undefined behavior
  4. It's silly!
  5. Nothing -- this code is totally fine

07 Binary Information

Which of the following C expressions perform exactly two pointer dereferences, assuming p is a pointer variable? (Select all that are correct.)

  1. *p
  2. p[0]
  3. *p[0]
  4. p[0][0]
  5. **p[0]
  6. None of these

08 Integers

What is the decimal value of an int8_t variable in C if the actual bits being stored are equivalent to 0x80 in hex?

  1. -128
  2. 16
  3. 64
  4. 128
  5. Not enough information to determine

09 Binary Arithmetic

Consider the following binary subtraction problem. Assuming 4-bit representations, is this an overflow in unsigned, two's complement, both, or neither? (note the borrow into the most-significant bit!) 

  0011
- 1001
======
  1010
  1. Overflow in unsigned ONLY
  2. Overflow in two's complement ONLY
  3. Overflow in BOTH unsigned and two's complement
  4. Overflow in NEITHER unsigned nor two's complement
  5. Not enough information to determine

BONUS: What about 0011 - 0101?

Notes: 

    2
    0011    3    3
  - 1001 -  9 - -7
  ====== ==== ====
    1010   10   -6
         !!!! !!!!

    12
    0011    3    3
  - 0101 -  5 -  5
  ====== ==== ====
    1110   14   -2
         !!!! (ok)

10 Floating Point

What is the decimal value encoded as 0x44 with a four-bit exponent and a three-bit significand? (i.e., 0 1000 100 in binary)

HINT: the bias for a four-bit exponent is 7.

  1. 0.2
  2. 1.1
  3. 3.0
  4. 8.0
  5. Not enough information to determine

BONUS: Encode the decimal value 0.125 in the same format.

11 Assembly Intro

What is the return value (i.e., the value of the RAX register) at the end of the following x86-64 assembly program? 

.globl main
main:
    mov $2, %rax
    mov $3, %rbx
    mov $7, %rcx
    imul %rbx, %rcx
    imul %rcx, %rax
    ret
  1. 6
  2. 7
  3. 14
  4. 21
  5. 42
  6. None of the above -- the program will crash

12 Assembly Data Movement

The following is a hex dump of a program's memory starting at address 0x400 (in little endian ordering). 

0400 00 00 00 00 00 00 00 00  01 02 03 04 05 06 07 08
0410 15 23 ae 7c 01 02 59 68  aa bb cc dd 34 56 78 90

What is the decimal value of the integer number stored at address 0x408? (You may need to use a calculator.)

  1. 0
  2. 1
  3. 513
  4. 67,305,985
  5. 578,437,695,752,307,201
  6. Not enough information to determine

13 Assembly Control Flow

What is the appropriate C-expression for CONDITION if when given the following C code: 

void cond(long a, long *p)
{
    if (CONDITION) {
        *p = a;
    }
}

GCC generates the following assembly code: 

    # void cond(long a, long *p)
    # a in %rdi, p in %rsi
cond: 
    testq    %rsi, %rsi
    jz       .L1
    cmpq     %rdi, (%rsi)
    jge      .L1
    movq     %rdi, (%rsi)
.L1:
    ret
  1. p || a < *p
  2. p || a > *p
  3. p && a > *p
  4. p && a >= *p
  5. p && a < *p
Textbook solution

Textbook solution

14 Assembly Procedures

What value will be on the top of the stack after the following callq instruction executes? 

0x4004f7 callq 4004db
0x4004fc halt
  1. 0x4004db
  2. 0x4004f7
  3. 0x4004fc
  4. Some other value.

15 Assembly Miscellaneous

Assume an array of 32-bit integers is stored in memory starting at address 0x600240. What is the address of the first byte of the fourth element of the array?

Hint: draw a diagram!

  1. 0x600243
  2. 0x60024c
  3. 0x600250
  4. 0x600252
  5. 0x600256

16 Y86 Intro

What is the instruction code (icode) and the instruction function (ifun) of the following Y86 instruction? 

0x114: 60 10 | addq %rcx, %rax

Hint from the textbook: "Every instruction has an initial byte identifying the instruction type. This byte is split into two 4-bit parts: the high-order, or code part, and the low-order, or function part."

  1. icode: 0, ifun: 6
  2. icode: 1, ifun: 0
  3. icode: 6, ifun: 0
  4. icode: 6, ifun: 1
  5. icode: 60, ifun: 10

17 Combinational Circuits

What is the result of x NAND x?

  1. 0
  2. 1
  3. x
  4. !x
  5. Not enough information to answer

18 Sequential Circuits

The following is an AND/OR SR latch and its truth table. What will happen when the "S" input is turned on, assuming both the "R" input and the previous value for "Q" are both off?

AND/OR SR Latch
R S Q (before) Q (after)
0 0 0 0
0 0 1 1
0 1 0 1
0 1 1 1
1 0 0 0
1 0 1 0
1 1 0 0
1 1 1 0
  1. Q will turn on
  2. Q will stay off
  3. R will turn on
  4. S will turn off
  5. The circuit will explode
  6. Not enough information to answer

19 CPU Architecture

How does the clock period (cycle length) of a CPU typically change when converted from a sequential CPU design to an equivalent but pipelined CPU design?

HINT: Consider this slide, added at the end of today's slide deck after the original CS 261 videos were recorded:

CPU Pipelining Diagram
  1. The clock period will typically increase
  2. The clock period will typically stay the same
  3. The clock period will typically decrease
  4. The result depends on the number of instructions being executed
  5. Not enough information to answer

20 Y86 Semantics

What is the minimal number of bits required to provide enough unique register IDs for 32 registers?

HINT: How many bits are required in Y86?

  1. 4
  2. 8
  3. 16
  4. 32
  5. 64
  6. None of these are correct

21 Memory

Which of the following numbers is not equal to the others?

  1. 42
  2. 42.0
  3. 0x42
  4. 0x2A
  5. None -- they are all equal.

22 Caching

Assume the following cache for an 8-bit address: s=3 bits for the set index (S=8 sets), E=1 (direct- mapped), and b=2 bits for the byte offset within a line (B=4 bytes). This leaves t=8-3-2=3 bits for the tag.

What is the set index for address 226 (0xE2)?

  1. 0
  2. 2
  3. 3
  4. 7
  5. 14
  6. None of these are correct