All quizzes for Spring 2017 (key)

Question 1: Suppose a program has three tasks to perform:

• Task A, which takes 10 seconds;
• Task B, which takes 5 seconds; and
• Task C, which takes 3 seconds

Initially, my program performs each of these tasks one at a time. Which of the following change will result in the fastest runtime? (Assume doing two tasks in parallel does not make either of the parallel tasks slower.)

Question 2: If the bytes 0x12, followed by 0x34, followed by 0x56, followed by 0x78 are interpreted as a 4-byte little endian integer, what value will they have?

Question 3 (0 points): If a program compiles as C with a C compiler and is strictly standards-conformant C, it will ___ compile as C++ with a standards-conformant C++ compiler.

Question 1: When we compile a C program with multiple source files, we can produce object (.o on Linux) files, which we combine into the final program. What is the process of combining these files into an executable called?

Question 4: On 64-bit x86, if a function void foo(int w, int x, int y, int z) is called using foo(1, 2, 3, 4), then where is the value 4 placed?

Question 5: On 64-bit x86, if %rax contains 0x1000 and %rdx contains 0x1, then, in AT&T syntax, what does 0x10(%rax,%rdx,4) represent?

Question 3: Given the following code, which of the following are valid variable declarations?

typedef struct bar {
    int x;
} foo;    

Select all that apply

Question 4: What does the following code output?

int x = 0;
int y = 1;

foo:
while (y < 5) {
    y += 2;
    x += 1;
    if (y == 6)
        goto bar;
}
y -= 5;
goto foo;

bar:
printf("%d\n", x);

Question 3: Figure 3.35 in the text presents a recursive implementation of factorial:

rfact:
    pushq %rbx
    movq %rdi, %rbx
    movl $1, %eax
    cmpq $1, %rdi
    jle .L35
    leaq -1(%rdi), %rdi
    call rfact
    imulq %rbx, %rax
.L35:
    popq %rbx
    ret

Calling this function with an argument of 0 requires 16 bytes of stack space, including the space for the return address used by the final ret. How much stack space does calling it with an argument of 3 require?

Question 4: Which bit sequence is the 6-bit 2's complement representation of the decimal number -16?

Question 1: Consider the following code:

unsigned int v; // input
int f;         // output
f = v && !(v & (v - 1));

What does the output f tell us about the input v?

Question 2: Consider the following code:

int x;  // input 1
int y;  // input 2
int r;  // output 
r = y ^ ((x ^ y) & -(x < y)); 

What does r represent here? Hint: -1 is represented in binary by all 1's, and a < b returns 0 or 1.

Question 3: What is the value of the address computed by (%rdx,%rcx,4), if %rdx is 0xf000 and %rcx is 0x0100?

Question 4: Code segment 1:

push   %ebx            //store ebx value to stack
subq   $10, %ebx
pop    %ebx            //restore ebx value from stack

Code segment 2:

push   %ebx            //store ebx value to stack
cmpq   $10, %ebx
pop    %ebx            //restore ebx value from stack

What is the difference between code segment 1 and 2?

Question 3: In order for a Y86-64 processor to determine the length of the currently executing instruction in memory, what information might it need?
Select all that apply

Question 4: Which of the following operations that can be done in one instruction on X86-64 can NOT be done in one instruction on Y86-64?
Select all that apply

Question 5: Consider a register (of the kind our textbook describes) which takes as input value I and a clock signal, and which outputs a value O. Which of the following statements is true?

Question 1: Each of the following statements about Y86-64 are true. Which of them is an attribute that makes Y86-64 more RISC-like?
Select all that apply

Question 2: What Y86-64 assembly is equivalent to cmovne %rax, %rbx?
Select all that apply

Question 3: What determines the length of one cycle in the single cycle microarchitecture?
Select all that apply

Question 4: After the decode stage what do we not know about the instruction?
Select all that apply

Question 1: Consider the instruction subq rA, rB. This instruction performs R[rB] <— R[rB] sub R[rA]. The instruction passes through the five stages (Fetch, Decode, Execute, Memory, Writeback). Which (if any) stages are not involved in handling this instruction?

Question 2: Consider the instruction rmmovq D(rB), rA. This instruction performs EA <— D + R[rB]; MEM[EA] <— R[rA]. The instruction passes through the five stages (Fetch, Decode, Execute, Memory, Writeback). In which stage does the hardware calculate the effective address EA (ValE in the book)?

Question 3: Consider the instruction mrmovq D(rB), rA. This instruction performs EA <— D + R[rB]; R[rA] <— MEM[EA]. The instruction passes through the five stages (Fetch, Decode, Execute, Memory, Writeback). In which stage does the hardware retrieve the value stored in register rB?

Question 4 (0 points): What is the result of the following HCL (or HCL2D) expression if x is 10 and y is 5?

[
    y in {4,6} : x + y,
    y <= 5 : x - y,
    1 : 0,
]

Question 1: In the single-cycle processor design we discussed in class, one option was for the memory address input to the data memory to come directly from the single ALU. For which of the following memory-accessing instructions was this not done because the ALU was used for something else?
Select all that apply

Question 2: In the single-cycle processor design we discussed in class, some instructions do not use the memory stage. What is true about these instructions?
Select all that apply

Question 3: In the single-cycle processor design we discussed in class, from which sources could the input of the program counter register come?
Select all that apply

Question 4: Which of the following are illegal in HCL2D?
Select all that apply

Question 1: Adding pipelining to a previously unpipelined processor ______ the latency of instructions.

Question 2: Adding pipelining to a previously unpipelined processor ______ the throughput of instructions

Question 3: In the pipelined version of the Y86-64 processor, which values are passed in pipeline registers between the fetch and decode stage?
Select all that apply

Question 1: Consider the pipelined Y86-64 implementation we discussed in class. Suppose the register delay is 10 ps and the length of the critical path through memories and combinatorial logic in each stage is:

• 100 ps for fetch;
• 75 ps for decode;
• 80 ps for execute
• 100 ps for memory
• 60 ps for writeback

What is the minimum clock cycle time this processor could have and still operate correctly?

Question 2: Consider the following Y86-64 assembly snippet:

mrmovq 4(%r11), %r10
addq %rax, %rbx
subq %rcx, %rdx
andq %rsi, %rdi 
xorq %rsp, %rbp
irmovq $10, %r9

In our pipelined implementation, when the subq instruction is in its decode stage, what stage is the mrmovq instruction in?

Question 3: For the pipelined Y86-64 design we discussed in class, for which of the following instruction sequences is there a data hazard?
Select all that apply

Question 4: For the pipelined Y86-64 design we discussed in class, for which of the following instruction sequences is there a control hazard?
Select all that apply

Question 1: Using our textbook's notion of "stall" and "bubble" signals, if a pipeline register's input is receiving a stall signal of 1 and a bubble signal of 0, then its output after the next rising edge of the clock will be:

Question 2: Our textbook talks about both data dependencies and data hazards. Which are differences between these?
Select all that apply

Question 3: We can perform forwarding of %rax to implement

irmovq $15, %rax
addq %rbx, %rax

without any stalling. Suppose the addq instruction receives the forwarded value for %rax from irmovq in its decode stage. Where can it retrieve (i.e. forward) this value from?

Question 4: Figure 5.12 indicates that the integer multiplication functional units on the Intel "Haswell" microarchitecture have an issue time of 1 cycle and a latency of 3 cycles. How long does it take one of these functional units to perform two integer multiplications from when the first multiplcation is started until the last finishes?

Question 3: Consider a pipelined Y86-64 processor with six stages, resulting from splitting the memory stage into two parts. That is, the processor has the following stages:

• Fetch
• Decode
• Execute
• Memory Part 1
• Memory Part 2
• Writeback

Suppose the results of any data memory load or store is not available until near the end of the second memory stage. On this processor, even after implementing all possible forwarding, which of the following instruction sequences would require a pipeline stall to resolve hazards:
Select all that apply

Question 4: IBM 360/91 had out-of-order instruction execution and completion but no support for precise exceptions. Suppose you were writing a floating point program in this machine. Which statement is true?
Select all that apply

Question 1: Which component in the memory hierarchy is the fastest?

Question 2: Which component in the memory hierarchy is non-volatile (can retain data without power)?

Question 3: Suppose the memory access pattern of your program looks like this: a[1], a[2], a[3], a[7], a[8], a[9], a[13], a[14], a[15]. What kind of locality correctly describes this access pattern?

Question 4: Suppose the memory access pattern of your program looks like this: a[1], b[1], c[1], a[1], b[1], c[1]. What kind of locality correctly describes this access pattern?

Question 1: Which of the following are true about programs in the data-flow model?
Select all that apply

Question 2: On an out-of-order processor, which of the following can happen while the processor is completing a very long load (mrmovq) instruction?
Select all that apply

Question 3: Suppose a level-1 cache has an access time of 2 nanoseconds and a 95% hit rate, and the corresponding level-2 cache has a preceived access time of 10 nanoseconds. What is the preceived access time of the level 1 cache?

Question 4: Assume that you are designing a cache for low-power sensor nodes. It measures the environment temperature every minute and sends the average temperature from the last 24 hours to the base station. This sensor requires the caches to be extremely power-efficient. Which design decision would you make?

Question 4: Which of the following techniques are likely to reduce the number of conflict cache misses programs experience, assuming the cache size remains fixed?
Select all that apply

Question 5: Switching a cache from a write-through to write-back policy is likely to have which of the following effects on the cache?
Select all that apply

Question 1: Consider the following two functions:

void sumArray1(int *pSum, int *array, int n) {
    for (int i = 0; i < n; ++i)
        *pSum += array[i];
}

void sumArray2(int *pSum, int *array, int n) {
    int temp = 0;
    for (int i = 0; i < n; ++i)
        temp += array[i];
    *pSum += temp;
}

Suppose an optimizing compiler generates much slower code for sumArray1 than sumArray2. What is a likely cause of this?
Select all that apply

Question 2: Which of the following optimizations the textbook discusses are likely to substantially increase machine code size?
Select all that apply

Question 3 (0 points): (Question dropped.) The textbook discusses how a loop like:

for (int i = 0; i < N; i += 2) {
    acc = acc * (a[i] * b[i]);
    acc = acc * (a[i+1] * b[i+1]);
}

is usually faster than one like:

for (int i = 0; i < N; i += 2) {
    acc = (acc * a[i]) * b[i];
    acc = (acc * a[i+1]) * b[i+1];
}

(Assume the compiler obeys the explicitly specified order of operations.) Which of the following are different about how a modern out-of-order processor will execute the two loops?

The first answer was meant to say acc * (a[i] * b[i]) and acc * (a[i+1] + b[i+1]) to make the question unambiguous. This is why the question was dropped. However, it's still the case the processor can execute a[i+1] * b[i+1] from the next iteration of the loop in parallel with acc * (a[i+1] * b[i+1]) from the previous.
Select all that apply

Question 4: Which of the following transformation our textbook discusses likely to decrease the number of instructions a program executes? (This should have been a select-all, but since it wasn't, either of the correct answers gets full credit.)

Question 1: Some of the optimizations we talked about compilers do not automatically perform aggressively because they can make code much slower when done excessively. Which are examples of these?
Select all that apply

Question 2: Which of these are a way to eliminate aliasing so a compiler can perform more optimizations?

Question 3: Using mulitple accumulators improves performance by

Question 4: In the example in lecture, trying to use too many accumulators resulted in lower performance than using fewer (with the same amount of loop performance). This was most likely because of

Question 1: Suppose that your program reads a file from the disk using the fread function. The library you are using implements this fread function using a system call referred to as read. How would you characterize the exception caused by read?

Question 2: Which of the following events will initiate an interrupt?
Select all that apply

Question 3 (0 points): Which of the following events will initiate an abort?
Select all that apply

Question 4: Suppose that your program writes to a read-only page. Which of the following exceptions will occur in this case?

Question 1: Each of the following is either synchronous or asynchronous. Which are synchronous?
Select all that apply

Question 2: We have seen this code in our lecture.

void fork5()
{
        printf("L0\t”);
        if (fork() == 0) {
            printf("L1\t”);
            if (fork() == 0) {
                printf("L2\t”);
        }
        }
        printf("Bye\t”);
}

Which of the following are infeasible?
Select all that apply

Question 3: Which of the following are true about signal handlers?
Select all that apply

Question 4: Which of the following are true about signals?
Select all that apply

Question 1: Which of the following statements are false?
Select all that apply

Question 2: Virtual memory can be thought of as a mechanism that caches virtual pages in the physical memory. A virtual page can be stored at any physical page frame. Which of the following statement is true?

Question 3: If we need 32 bits to represent a virtual address and our page size is 4KB, how many page table entries (PTE) do we need?

Question 4: A page table is a map of <vpn, ppn>, where vpn is a virtual page number and ppn is a physical page number. The process of generating the appropriate ppn for a vpn is referred to as:

Question 1: The PTBR contains the base address of the page table. Which of the following statements are false?
Select all that apply

Question 2: A single-level page table has only one table, where a multi-level page table has multiple tables organized in a hierarchical manner. Which of the following statements are true?
Select all that apply

Question 3: Which of the following can have a virtually-indexed-physically-tagged L1 cache?
Select all that apply

Question 4: Which of the following statements are true about the translation lookaside buffer (TLB)?
Select all that apply