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Computer Science 61 and E61
Systems Programming and Machine Organization

Final Sample Questions

The final will be cumulative, though it will be weighted more towards the second half of the class. So why not check out:

This bank of questions is taken from prior midterms and finals. The course does change from year to year, so some of the questions may refer to concepts we did not emphasize this year, and some concepts we did emphasize this year may not be represented here.

The final will 3 hours long. It will be open-note, open-book, open-computer, semiopen-network, using rules very similar to those in the midterm.

1. Computer arithmetic

Bitwise operators and computer arithmetic can represent vectors of bits, which in turn are useful for representing sets. For example, say we have a function bit that maps elements to distinct bits; thus, bit(X) == (1 << u) for some u. Then a set {X0, X1, X2, …, Xn} can be represented as bit(X0) | bit(X1) | bit(X2) | … | bit(Xn). Element Xi is in the set with representation n if and only if (bit(Xi) & n) != 0.

QUESTION 1A. What is the maximum number of set elements that can be represented in a single unsigned variable on an x86 machine?

QUESTION 1B. Match each set operation with the C operator(s) that could implement that operation. (Complement is a unary operation.)

intersection ==
equality ~
complement &
union ^
toggle membership
(flip whether an element is in the set)

QUESTION 1C. Complete this function, which should return the set difference between the sets with representations a and b. This is the set containing exactly those elements of set a that are not in set b.

unsigned set_difference(unsigned a, unsigned b) {

QUESTION 1D. Below we’ve given a number of C expressions, some of their values, and some of their set representations for a set of elements. For example, the first row says that the integer value of expression 0 is just 0, which corresponds to an empty set. Fill in the blanks. This will require figuring out which bits correspond to the set elements A, B, C, and D, and the values for the 32-bit int variables a, x, and s. No arithmetic operation overflows; abs(x) returns the absolute value of x (that is, x < 0 ? -x : x).

Expression e Integer value Represented set
0 0 {}
a == a ______________ {A}
(unsigned) ~a < (unsigned) a ______________ {A}
a < 0 ______________ ______________
(1 << (s/2)) - 1 ______________ {A,B,C,D}
a * a ______________ {C}
abs(a) ______________ ______________
x & (x - 1) ______________ {}
x - 1 ______________ {A,D}
x ______________ ______________
s ______________ ______________

2. Data structure assembly

Here are four assembly functions, f1 through f4.

	movl	4(%esp), %eax
	movl	8(%esp), %ecx
	testl	%ecx, %ecx
	jle	.L2
	xorl	%edx, %edx
	movl	4(%eax), %eax
	incl	%edx
	cmpl	%ecx, %edx
	jne	.L3
	movl	(%edx), %eax
	movl	8(%esp), %edx
	leal	0(,%edx,4), %ecx
	movl	4(%esp), %eax
	movl	(%eax,%ecx), %eax
	addl	%ecx, %eax
	movl	(%eax), %eax
	pushl	%esi
	pushl	%ebx
	movl	12(%esp), %ecx
	movl	16(%esp), %esi
	movl	20(%esp), %eax
	testl	%esi, %esi
	jle	.L9
	xorl	%edx, %edx
	movl	%eax, %ebx
	andl	$1, %ebx
	movl	4(%ecx,%ebx,4), %ecx
	incl	%edx
	sarl	%eax
	cmpl	%esi, %edx
	jne	.L10
	movl	(%ecx), %eax
	popl	%ebx
	popl	%esi
	movl	8(%esp), %edx
	movl	4(%esp), %eax
	movl	(%eax,%edx,4), %eax

QUESTION 2A. Each function returns a value loaded from some data structure. Which function uses which data structure?

  1. Array
  2. Array of pointers to arrays
  3. Linked list
  4. Binary tree

QUESTION 2B. The array data structure is an array of type T. Considering the code for the function that manipulates the array, which of the following types are likely possibilities for T? Circle all that apply.

  1. char
  2. int
  3. unsigned long
  4. unsigned long long
  5. char*
  6. None of the above

3. Disassembly I

Here’s some assembly produced by compiling a C program with gcc.

	.string	"%d %d\n"

	.globl	f
	.type	f, @function
	pushl	%ebp
	movl	$1, %ecx
	movl	%esp, %ebp
	pushl	%edi
	pushl	%esi
	pushl	%ebx
	subl	$12, %esp
	movl	$1, %eax
	movl	%eax, %ebx
	imull	%eax, %ebx
	movl	%ecx, %esi
	imull	%ecx, %esi
	movl	$1, %edx
	movl	%edx, %edi
	imull	%edx, %edi
	addl	%ebx, %edi
	cmpl	%esi, %edi
	je	.L11
	incl	%edx
	cmpl	%eax, %edx
	jle	.L4
	incl	%eax
	cmpl	%ecx, %eax
	jle	.L9
	incl	%ecx
	jmp	.L13
	pushl	%ecx
	pushl	%eax
	pushl	%edx
	pushl	$.LC1
	call	printf
	leal	-12(%ebp), %esp
	movl	$1, %eax
	popl	%ebx
	popl	%esi
	popl	%edi
	popl	%ebp

QUESTION 3A. How many arguments might this function have? Circle all that apply.

  1. 0
  2. 1
  3. 2
  4. 3 or more

QUESTION 3B. What might this function return? Circle all that apply.

  1. 0
  2. 1
  3. −1
  4. Its first argument, whatever that argument is
  5. A square number other than 0 or 1
  6. None of the above

QUESTION 3C. Which callee-saved registers does this function save and restore? Circle all that apply.

  1.  %eax
  2.  %ebx
  3.  %ecx
  4.  %edx
  5.  %ebp
  6.  %esi
  7.  %edi
  8. None of the above

QUESTION 3D. This function handles signed integers. If we changed the C source to use unsigned integers instead, which instructions would change? Circle all that apply.

  1. movl
  2. imull
  3. addl
  4. cmpl
  5. je
  6. jge
  7. popl
  8. None of the above

QUESTION 3E. What might this function print? Circle all that apply.

  1. 0 0
  2. 1 1
  3. 3 4
  4. 4 5
  5. 6 8
  6. None of the above

4. Disassembly II

The questions in this section concern a function called ensmallen, which has the following assembly.

 1.          pushl   %ebx
 2.          movl    8(%esp), %ebx
 3.          movl    12(%esp), %eax
 4.          movb    (%eax), %cl
 5.          movb    %cl, (%ebx)
 6.          testb   %cl, %cl
 7.          jne     .L34
 8.          jmp     .L26
 9.  .L29:
10.          incl    %eax
11.  .L34:
12.          movb    (%eax), %dl
13.          cmpb    %dl, %cl
14.          je      .L29
15.          incl    %ebx
16.          movb    %dl, %cl
17.          movb    %cl, (%ebx)
18.          testb   %cl, %cl
19.          jne     .L34
20.  .L26:
21.          popl    %ebx
22.          ret

QUESTION 4A. How many arguments is this function likely to take? Give line numbers that helped you determine an answer.

QUESTION 4B. Are the argument(s) pointers? Give line numbers that helped you determine an answer.

QUESTION 4C. What type(s) are the argument(s) likely to have? Give line numbers that helped you determine an answer.

QUESTION 4D. Write a likely signature for the function. Use return type void.

QUESTION 4E. Write an alternate likely signature for the function, different from your last answer. Again, use return type void.

QUESTION 4F. Which callee-saved registers does this function use? Give line numbers that helped you determine an answer.

QUESTION 4G. The function has an “input” and an “output”. Give an “input” that would cause the CPU to jump from line 8 to label .L26, and describe what is placed in the “output” for that “input”.

QUESTION 4H. Give an “input” for which the corresponding “output” is not a copy of the “input”. Your answer must differ from the previous answer.

QUESTION 4I. Write C code corresponding to this function. Make it as compact as you can.

5. Machine programming

Intel really messed up this time. They’ve released a processor, the Fartium Core Trio, where every instruction is broken except the ones on this list.

1. cmpl %ecx, %edx
2. decl %edx
3. incl %eax
4. je L1
5. jl L2
6. jmp L3
7. movl 4(%esp), %ecx [movc]
8. movl 8(%esp), %edx [movd]
9. movl (%ecx,%eax,4), %ecx [movx]
10. ret
11. xchgl %eax, %ecx
12. xorl %eax, %eax

(In case you forgot, xchgl swaps two values—here, the values in two registers—without modifying condition codes.)

“So what if it’s buggy,” says Andy Grove; “it can still run programs.” For instance, he argues convincingly that this function:

 void do_nothing(void) {

is implemented correctly by this Fartium instruction sequence:


Your job is to implement more complex functions using only Fartium instructions. Your implementations must have the same semantics as the C functions, but may perform much worse than one might expect. You may leave off arguments and write instruction numbers (#1–12) or instruction names (for mov, use the bracketed abbreviations). Indicate where labels L1–L3 point (if you need them). Assume that on function entry, the stack is set up as on a normal x86.


 int return_zero(void) {
     return 0;


 int identity(int a) {
     return a;


 void infinite_loop(void) {
     while (1)
         /* do nothing */;


 typedef struct point {
     int x;
     int y;
     int z;
 } point;
 int extract_z(point *p) {
     return p->z;

So much for the easy ones. Now complete one out of the following 3 questions, or more than one for extra credit. (Question 5G is worth more than the others.)

QUESTION 5E. [Reminder: Complete at least one of 5E–5G for full credit.]

 int add(int a, int b) {
     return a + b;

QUESTION 5F. [Reminder: Complete at least one of 5E–5G for full credit.]

 int array_dereference(int *a, int i) {
     return a[i];

QUESTION 5G. [Reminder: Complete at least one of 5E–5G for full credit.]

 int traverse_array_tree(int *a, int x) {
     int i = 0;
     while (1) {
         if (x == a[i])
             return i;
         else if (x < a[i])
             i = a[i+1];
             i = a[i+2];

(This funky function traverses a binary tree that’s represented as an array of ints. It returns the position of the x argument in this “tree.” For example, given the following array:

 int a[] = {100, 3, 6,  50, 9, 12,  150, 0, 0,  25, 0, 0,  80, 0, 0};

the call traverse_array_tree(a, 100) returns 0, because that’s the position of 100 in a. The call traverse_array_tree(a, 80) first examines position 0; since a[0] == 100 and 80 < 100, it jumps to position a[0+1] == 3; since a[3] == 50 and 80 > 50, it jumps to position a[3+2] == 12; and then it returns 12, since a[12] == 80. The code breaks if x isn’t in the tree; don’t worry about that.)

6. Virtual memory

QUESTION 6A. What is the x86 page size? Circle all that apply.

  1. 4096 bytes
  2. 64 cache lines
  3. 512 words
  4. 0x1000 bytes
  5. 216 bits
  6. None of the above

The following questions concern the sizes of page tables. Answer the questions in units of pages. For instance, the page directories in WeensyOS (Assignment 4) each contained one level-1 page table page and one level-2 page table page, for a total size of 2 pages per page table.

QUESTION 6B. What is the maximum size (in pages) of an x86 page table?

QUESTION 6C. What is the minimum size (in pages) of an x86 page table that would allow a process to access 222 distinct physical addresses?

The x86 architecture we discussed in class has 32-bit virtual addresses and 32-bit physical addresses. Extensions to the x86 architecture have increased both these limits.

  • Physical Address Extensions (PAE) allow 32-bit machines to access up to 252 bytes of physical memory (which is about 4000000 GB). That is, virtual addresses are 32 bits, and physical addresses are 52 bits.
  • The x86-64 architecture evolves the x86 architecture to a 64-bit word size. x86-64 pointers are 64 bits wide instead of 32. However, only 48 of those bits are meaningful: the upper 16 bits of each virtual address are ignored. Thus, virtual addresses are 48 bits. As with PAE, physical addresses are 52 bits.

QUESTION 6D. Which of these two machines would support a higher number of concurrent processes?

  1. x86 with PAE with 100 GB of physical memory.
  2. x86-64 with 20 GB of physical memory.

QUESTION 6E. Which of these two machines would support a higher maximum number of threads per process?

  1. x86 with PAE with 100 GB of physical memory.
  2. x86-64 with 20 GB of physical memory.

7. Virtual memory and kernel programming

These problems consider implementations of virtual memory features in a WeensyOS-like operating system. Recall the signatures and specifications of the virtual_memory_lookup and virtual_memory_map functions:

// virtual_memory_map(pagetable, va, pa, sz, perm)
//    Map virtual address range [va, va+sz) in pagetable.
//    When X >= 0 && X < sz, the new pagetable will map virtual address
//    va+X to physical address pa+X with permissions perm.
//    Preconditions:
//    * va, pa, and sz must be multiples of PAGESIZE (4096).
//    * The level-2 pagetables referenced by the virtual address range
//      must exist and be writable (e.g., va + sz < MEMSIZE_VIRTUAL).
//    Typically perm is a combination of PTE_P (the memory is Present),
//    PTE_W (the memory is Writable), and PTE_U (the memory may be
//    accessed by User applications). If !(perm & PTE_P), pa is ignored.
void virtual_memory_map(pageentry_t* pagetable, uintptr_t va, uintptr_t pa, size_t sz, int perm);

// virtual_memory_lookup(pagetable, va)
//    Returns information about the mapping of the virtual address va in
//    pagetable. The information is returned as a vamapping object,
//    which has the following components:
typedef struct vamapping {
    int pn;           // physical page number; -1 if unmapped
    uintptr_t pa;     // physical address; (uintptr_t) -1 if unmapped
    int perm;         // permissions; 0 if unmapped
} vamapping;

vamapping virtual_memory_lookup(pageentry_t* pagetable, uintptr_t va);

Also recall that WeensyOS tracks physical memory using an array of pageinfo structures:

typedef struct physical_pageinfo {
    int8_t owner;
    int8_t refcount; // 0 means the page is free
} physical_pageinfo;
static physical_pageinfo pageinfo[PAGENUMBER(MEMSIZE_PHYSICAL)];

The WeensyOS kernel occupies virtual addresses 0 through 0xFFFFF; the process address space starts at PROC_START_ADDR == 0x100000 and goes up to (but not including) MEMSIZE_VIRTUAL == 0x300000.

QUESTION 7A. True or false: On x86 Linux, like on WeensyOS, the kernel occupies low virtual addresses.

QUESTION 7B. On WeensyOS, which region of a process’s address space is closest to the kernel’s address space? Choose from code, data, stack, and heap.

QUESTION 7C. On Linux on an x86 machine, which region of a process’s address space is closest to the kernel’s address space? Choose from code, data, stack, and heap.

Recall that the WeensyOS sys_page_alloc(addr) system call allocates a new physical page at the given virtual address. Here’s an example kernel implementation of sys_page_alloc, taken from the WeensyOS interrupt function:

    uintptr_t addr = current->p_registers.reg_eax; // address is passed to kernel in %eax
    // [A]

    int free_pn = find_free_physical_page();
    if (free_pn < 0) { // no free physical pages
        console_printf(CPOS(24, 0), 0x0C00, "Out of physical memory!\n");
        current->p_registers.reg_eax = -1; // return result in %eax
        break; // will call run(current)
    // [B]

    // otherwise, allocate the page
    assert(pageinfo[free_pn].refcount == 0);
    pageinfo[free_pn].refcount += 1;
    pageinfo[free_pn].owner = current->p_pid;
    // [C]

    // and map it into the user’s address space
    virtual_memory_map(current->p_pagetable, addr, PAGEADDRESS(free_pn), PAGESIZE, PTE_P | PTE_U | PTE_W);
    current->p_registers.reg_eax = 0;
    // [D]


QUESTION 7D. Thanks to insufficient checking, this implementation allows a WeensyOS process to crash the operating system or even take it over. This kernel is not isolated. What the kernel should do is return −1 when the calling process supplies bad arguments. Write code that, if executed at slot [A], would preserve kernel isolation and handle bad arguments correctly.

QUESTION 7E. This implementation has another problem, which the following process would trigger:

void process_main(void) {
    heap_top = ROUNDUP((uint8_t*) end, PAGESIZE); // first address in heap region
    while (1) {

This process code repeatedly allocates a page at the same address. What should happen is that the kernel should repeatedly deallocate the old page and replace it with a newly-allocated page. But that’s not what will happen given the example implementation.

What will happen instead? And what is the name of this kind of problem?

QUESTION 7F. Write code that would fix the problem, and name the slot in the INT_SYS_PAGE_ALLOC implementation where your code should go.

8. Cost expressions

In the following questions, you will reason about the abstract costs of various operations, using the following tables of constants.

Table of Basic Costs

S System call overhead (i.e., entering and exiting the kernel)
F Page fault cost (i.e., entering and exiting the kernel)
P Cost of allocating a new physical page
M Cost of installing a new page mapping
B Cost of copying a byte

Table of Sizes

nk Number of memory pages allocated to the kernel
Per-process sizes (defined for each process p)
np Number of memory pages allocated to p
rp Number of read-only memory pages allocated to p
wp = nprp Number of writable memory pages allocated to p
mp Number of memory pages actually modified by p after the previous fork()

One of our tiny operating systems from class (OS02) included a program that called a recursive function. When the recursive function’s argument grew large enough, the stack pointer moved beyond the memory actually allocated for the stack, and the program crashed.

QUESTION 8A. In our first solution for this problem, the process called the sys_page_alloc(void *addr) system call, which allocated and mapped a single new page at address addr (the new stack page). Write an expression for the cost of this sys_page_alloc() system call in terms of the constants above.

QUESTION 8B. Our second solution for this problem changed the operating system’s page fault handler. When a fault occurred in a process’s stack region, the operating system allocated a new page to cover the corresponding address and restarted the process. Write an expression for the cost of such a fault in terms of the constants above.

QUESTION 8C. Design a revised version of sys_page_alloc that supports batching. Give its signature and describe its behavior.

QUESTION 8D. Write an expression for the cost of a call to your batching allocation API.

In the remaining questions, a process p calls fork(), which creates a child process, c.

Assume that the base cost of performing a fork() system call is Φ. This cost includes the fork() system call overhead (S), the overhead of allocating a new process, the overhead of allocating a new page directory with kernel mappings, and the overhead of copying registers. But it does not include overhead from allocating, copying, or mapping other memory.

QUESTION 8E. Consider the following implementations of fork():

A. Naive fork: Copy all process memory (Assignment 4, Step 5).
B. Eager fork: Copy all writable process memory; share read-only process memory, such as code (Assignment 4, Step 6).
C. Copy-on-write fork: initially share all memory as read-only. Create writable copies later, on demand, in response to write faults (Assignment 4 extra credit).

Which expression best represents the total cost of the fork() system call in process p, for each of these fork implementations? Only consider the system call itself, not later copy-on-write faults.

(Note: Per-process variables, such as n, are defined for each process. So, for example, np is the number of pages allocated to the parent process p, and nc is the number of pages allocated to the child process c.)

  1. Φ
  2. Φ + np × M
  3. Φ + (np + wp) × M
  4. Φ + np × 212 × (B + F)
  5. Φ + np × (212B + P + M)
  6. Φ + np × (P + M)
  7. Φ + wp × (212B + P + M)
  8. Φ + np × (212B + P + M) − rp × (212B + P)
  9. Φ + np × M + mc × (P + F)
  10. Φ + np × M + mc × (212B + F + P)
  11. Φ + np × M + (mp+mc) × (P + F)
  12. Φ + np × M + (mp+mc) × (212B + F + P)

QUESTION 8F. When would copy-on-write fork be more efficient than eager fork (meaning that the sum of all fork-related overheads, including faults for pages that were copied on write, would be less for copy-on-write fork than eager fork)? Circle the best answer.

  1. When np < nk.
  2. When wp × F < wp × (M + P).
  3. When mc × (F + M + P) < wp × (M + P).
  4. When (mp+mc) × (F + M + P + 212B) < wp × (P + 212B).
  5. When (mp+mc) × (F + P + 212B) < wp × (P + M + 212B).
  6. When mp < mc.
  7. None of the above.

9. Processes

This question builds versions of the existing system calls based on new abstractions. Here are three system calls that define a new abstraction called a rendezvous.

int newrendezvous(void)
Returns a rendezvous ID that hasn’t been used yet.
int rendezvous(int rid, int data)
Blocks the calling process P1 until some other process P2 calls rendezvous() with the same rid (rendezvous ID). Then, both of the system calls return, but P1’s system call returns P2’s data and vice versa. Thus, the two processes swap their data. Rendezvous acts pairwise; if three processes call rendezvous, then two of them will swap values and the third will block, waiting for a fourth.
void freezerendezvous(int rid, int freezedata)
Freezes the rendezvous rid. All future calls to rendezvous(rid, data) will immediately return freezedata.

Here's an example. The two columns represent two processes. Assume they are the only processes using rendezvous ID 0.

int result; int result;
result = rendezvous(0, 5); printf("About to rendezvous\n");
result = rendezvous(0, 600);
/* The processes swap data; both become runnable */
printf("Process A got %d\n", result); printf("Process B got %d\n", result);

This code will print

About to rendezvous
Process B got 5
Process A got 600

(the last 2 lines might appear in either order).

QUESTION 9A. How might you implement pipes in terms of rendezvous? Try to figure out analogues for the pipe(), close(), read(), and write() system calls (perhaps with different signatures), but only worry about reading and writing 1 character at a time.

QUESTION 9B. Can a rendezvous-pipe support all pipe features?

10. Process management

Here’s the skeleton of a shell function implementing a simple two-command pipeline, such as “cmd1 | cmd2”.

void simple_pipe(const char* cmd1, char* const* argv1, const char* cmd2, char* const* argv2) {
    int pipefd[2], r, status;
    pid_t child1 = fork();
    if (child1 == 0) {
        execvp(cmd1, argv1);
    assert(child1 > 0);
    pid_t child2 = fork();
    if (child2 == 0) {
        execvp(cmd2, argv2);
    assert(child2 > 0);

And here is a grab bag of system calls.

[1] close(pipefd[0]);
[2] close(pipefd[1]);
[3] dup2(pipefd[0], STDIN_FILENO);
[4] dup2(pipefd[0], STDOUT_FILENO);
[5] dup2(pipefd[1], STDIN_FILENO);
[6] dup2(pipefd[1], STDOUT_FILENO);
[7] pipe(pipefd);
[8] r = waitpid(child1, &status, 0);
[9] r = waitpid(child2, &status, 0);

Your task is to assign system call IDs, such as “1”, to slots, such as “A”, to achieve several behaviors, including a correct pipeline and several incorrect pipelines. For each question:

  • You may use each system call ID once, more than once, or not at all.
  • You may use zero or more system call IDs per slot. Write them in the order they should appear in the code.
  • You may assume that no signals are delivered to the shell process (so no system call ever returns an EINTR error).
  • The function should wait for both commands in the pipeline to complete before returning.
  • It may help to detach the last “Reference material” page of the exam.

QUESTION 10A. Implement a correct foreground pipeline.

A B (child1) C D (child2) E

QUESTION 10B. Implement a pipeline so that, given arguments corresponding to “echo foo | wc -c”, the wc process reads “foo” from its standard input but does not exit thereafter. For partial credit describe in words how this might happen.

A B (child1) C D (child2) E

QUESTION 10C. Implement a pipeline so that, given arguments corresponding to “echo foo | wc -c”, “foo” is printed to the shell’s standard output and the wc process prints “0”. (In a correctly implemented pipeline, “wc” would print 4, which is the number of characters in “foo\n”.) For partial credit describe in words how this might happen.

A B (child1) C D (child2) E

QUESTION 10D. Implement a pipeline that appears to work correctly on “echo foo | wc -c”, but always blocks forever if the left-hand command outputs more than 65536 characters. For partial credit describe in words how this might happen.

A B (child1) C D (child2) E

QUESTION 10E. Implement a pipeline so that, given arguments corresponding to “echo foo | wc -c”, both echo and wc report a “Bad file descriptor” error. (This error, which corresponds to EBADF, is returned when a file descriptor is not valid or does not support the requested operation.) For partial credit describe in words how this might happen.

A B (child1) C D (child2) E

11. Networking

QUESTION 11A. Which of the following system calls should a programmer expect to sometimes block (i.e., to return after significant delay)? Circle all that apply.

1. socket 5. connect
2. read 6. write
3. accept 7. usleep
4. listen 8. None of these

QUESTION 11B. Below are seven message sequence diagrams demonstrating the operation of a client–server RPC protocol. A request such as “get(X)” means “fetch the value of the object named X”; the response contains that value. Match each network property or programming strategy below with the diagram with which it best corresponds. You will use every diagram once.

1. Loss 4. Duplication 7. Exponential backoff
2. Delay 5. Batching
3. Reordering 6. Prefetching
Finalfig2012 1.gif Finalfig2012 2.gif Finalfig2012 3.gif Finalfig2012 4.gif
Finalfig2012 5.gif Finalfig2012 6.gif Finalfig2012 7.gif

12. Threads

The following code performs a matrix multiplication, c = ab, where a, b, and c are all square matrices of dimension sz. It uses the cache-friendly ikj index ordering.

 #define MELT(matrix, sz, row, col) (matrix)[(row)*(sz) + (col)]
 void matrix_multiply(double* c, const double* a, const double* b, size_t sz) {
     for (size_t i = 0; i < sz; ++i)
         for (size_t j = 0; j < sz; ++j)
             MELT(c, sz, i, j) = 0;
     for (size_t i = 0; i < sz; ++i)
         for (size_t k = 0; k < sz; ++k)
             for (size_t j = 0; j < sz; ++j)
                 MELT(c, sz, i, j) += MELT(a, sz, i, k) * MELT(b, sz, k, j);

But matrix multiplication is a naturally parallelizable problem. Here’s some code that uses threads to multiply even faster on a multicore machine. We use sz parallel threads, one per row of c.

     typedef struct matrix_args {
         double* c;
         const double* a;
         const double* b;
         size_t sz;
         size_t i;
     } matrix_args;
     void* matrix_multiply_ikj_thread(void* arg) {
 (α)     matrix_args* m = (matrix_args*) arg;
 (β)     for (size_t j = 0; j < m->sz; ++j)
 (γ)         MELT(m->c, m->sz, m->i, j) = 0;
 (δ)     for (size_t k = 0; k < m->sz; ++k)
 (ε)         for (size_t j = 0; j < m->sz; ++j)
 (ζ)             MELT(m->c, m->sz, m->i, j) += MELT(m->a, m->sz, m->i, k) * MELT(m->b, m->sz, k, j);
 (η)     return NULL;
     void matrix_multiply_ikj(double* c, const double* a, const double* b, size_t sz) {
 (1)     pthread_t* threads = (pthread_t*) malloc(sizeof(pthread_t) * sz);
 (2)     for (size_t i = 0; i < sz; ++i) {
 (3)         matrix_args m = { c, a, b, sz, i };
 (4)         int r = pthread_create(&threads[i], NULL, &matrix_multiply_ikj_thread, &m);
 (5)         assert(r == 0);
 (6)     }
 (7)     for (size_t i = 0; i < sz; ++i)
 (8)         pthread_join(threads[i], NULL);
 (9)     free(threads);

But when run, this code gives wildly incorrect results.

QUESTION 12A. What is wrong? Describe why the problem is a synchronization issue.

QUESTION 12B. Write C code showing how the problem could be fixed with changes only to matrix_multiply_ikj. Refer to the numbered lines to indicate replacements and/or insertions. Use one or more additional heap allocations and no additional calls to pthread functions. Free any memory you allocate once it is safe to do so.

On single-core machines, the kij order performs almost as fast as the ikj order. Here’s a version of the parallel matrix multiplication code that uses kij.

     typedef struct matrix_args_kij {
         double* c;
         const double* a;
         const double* b;
         size_t sz;
         size_t k;
     } matrix_args_kij;
     void* matrix_multiply_kij_thread(void* arg) {
 (α)     matrix_args_kij* m = (matrix_args_kij*) arg;
 (β)     for (size_t i = 0; i < m->sz; ++i)
 (γ)         for (size_t j = 0; j < m->sz; ++j)
 (δ)             MELT(m->c, m->sz, i, j) += MELT(m->a, m->sz, i, m->k) * MELT(m->b, m->sz, m->k, j);
 (ε)     return NULL;
     void matrix_multiply_kij(double* c, const double* a, const double* b, size_t sz) {
 (1)     pthread_t* threads = (pthread_t*) malloc(sizeof(pthread_t) * sz);
 (2)     for (size_t i = 0; i < sz; ++i)
 (3)         for (size_t j = 0; j < sz; ++j)
 (4)             MELT(c, sz, i, j) = 0;
 (5)     for (size_t k = 0; k < sz; ++k) {
 (6)         matrix_args_kij m = { c, a, b, sz, k };
 (7)         int r = pthread_create(&threads[k], NULL, &matrix_multiply_kij_thread, &m);
 (8)         assert(r == 0);
 (9)     }
(10)     for (size_t k = 0; k < sz; ++k)
(11)         pthread_join(threads[k], NULL);
(12)     free(threads);

This problem has the same problem as the previous version, plus another problem. Even after your fix from 8A–8B is applied, this version produces incorrect results.

QUESTION 12C. What is the new problem? Describe why it is a synchronization issue.

QUESTION 12D. Write pseudocode or C code that fixes this problem. You should refer to pthread functions. For full credit your solution should have low contention.

13. Synchronization and concurrency

Most synchronization objects have at least two operations. Mutual-exclusion locks support lock and unlock; condition variables support wait and signal; and from section notes you may remember the semaphore synchronization object, one of the earliest synchronization objects ever invented, which supports P and V.

In this problem, you’ll work with a synchronization object with only one operation, which we call a hemiphore. Hemiphores behave like the following; it is very important that you understand this pseudocode.

typedef struct hemiphore {
    int value;
} hemiphore;

// Initialize the hemiphore to value 0.
void hemiphore_init(hemiphore* h) {
    h->value = 0;

// Block until the hemiphore has value >= bound, then atomically increment its value by delta.
void H(hemiphore* h, int bound, int delta) {
    // This is pseudocode; a real hemiphore implementation would block, not spin, and would
    // ensure that the test and the increment happen in one atomic step.
    while (h->value < bound)
    h->value += delta;

Once a hemiphore is initialized with hemiphore_init, application code should access the hemiphore only through the H operation.

QUESTION 13A. Use hemiphores to implement mutual-exclusion locks. Fill out the code below. (You may not need to fill in every empty slot. You may use standard C constants; for example, INT_MIN is the smallest possible value for a variable of type int, which on a 32-bit machine is −2147483648.)

typedef struct mutex {                      // Initialize the mutex to the unlocked state.
    hemiphore h;                            void mutex_init(mutex* m) {

} mutex;


// Lock the mutex.                          // Unlock the mutex.
void mutex_lock(mutex* m) {                 void mutex_unlock(mutex* m) {

}                                           }

QUESTION 13B. Use hemiphores to implement condition variables. Fill out the code below. You may assume that the implementation of mutex is your hemiphore-based implementation from above (so, for instance, cond_wait may access the hemiphore m->h). See the Hints at the end of the question.

typedef struct condvar {                    // Initialize the condition variable.
    mutex m;                                void cond_init(condvar* c) {
    hemiphore h;                                mutex_init(&c->m);


} condvar;                                  }

// Signal the condition variable.
void cond_signal(condvar* c) {


// Block until the condition variable is signaled. The mutex m must be locked by the current
// thread. It is unlocked before the wait begins and re-locked after the wait ends.
// There are no sleep-wakeup race conditions: if thread 1 has m locked and executes
// cond_wait(c, m), no other thread is waiting on c, and thread 2 executes
// mutex_lock(m); cond_signal(c); mutex_unlock(m), then thread 1 will always receive the
// signal (i.e., wake up).
void cond_wait(condvar* c, mutex* m) {


Hints. For full credit:

  • If no thread is waiting on condition variable c, then cond_signal(c) should do nothing.
  • Assume N threads are waiting on condition variable c. Then N calls to cond_signal(c) are both necessary and sufficient to wake them all up.
  • Your solution must not suffer from sleep-wakeup race conditions.

QUESTION 13C. Use pthread mutexes and condition variables to implement hemiphores. Fill out the code below. See the hints after the question.

typedef struct hemiphore {
    pthread_mutex_t m;
    int value;
    pthread_cond_t c;

} hemiphore;

void hemiphore_init(hemiphore* h) {
    h->value = 0;


void H(hemiphore* h, int bound, int delta) {


Hints. The pthread mutex and condition variable operations have the following signatures. You should pass NULL for any attributes arguments. Don’t worry about the pthread_mutex_destroy and pthread_cond_destroy operations, and feel free to abbreviate (e.g. “lock” instead of “pthread_mutex_lock”).

  • pthread_mutex_init(pthread_mutex_t* m, const pthread_mutexattr_t* attributes)
  • pthread_mutex_lock(pthread_mutex_t* m)
  • pthread_mutex_unlock(pthread_mutex_t* m)
  • pthread_cond_init(pthread_cond_t* c, const pthread_condattr_t* attributes)
  • pthread_cond_signal(pthread_cond_t* c) (wakes up at most one thread waiting on c)
  • pthread_cond_broadcast(pthread_cond_t* c) (wakes up all threads waiting on c)
  • pthread_cond_wait(pthread_cond_t* c, pthread_mutex_t* m)

QUESTION 13D. Consider the following two threads, which use a shared hemiphore h with initial value 0.

Thread 1                      Thread 2
H(&h, 1000, 1);               while (1) {
printf("Thread 1 done\n");        H(&h, 0, 1);
                                  H(&h, 0, -1);

Thread 2 will never block, and the hemiphore’s value will alternate between 1 and 0. Thread 1 will never reach the printf, because the hemiphore’s value never reaches 1000. However, in most people’s first implementation of hemiphores using pthread mutexes and condition variables, Thread 1 will not block. Every call to H in Thread 2 will effectively wake up Thread 1. Though Thread 1 will then check the hemiphore’s value and immediately go back to sleep, doing so wastes CPU time.

Design an implementation of hemiphores using pthread mutexes and condition variables that solves this problem. In your revised implementation, Thread 1 above should block forever. For full credit, write C code. For partial credit, write pseudocode or English describing your design.

Hint. One working implementation constructs a linked list of “waiter” objects, where each waiter object is on a different thread’s stack, as initially sketched below. You can use such objects or not as you please.

typedef struct hemiphore_waiter {           typedef struct hemiphore {
    struct hemiphore_waiter* next;              pthread_mutex_t m;
                                                int value;
                                                hemiphore_waiter* waiters;

} hemiphore_waiter;                         } hemiphore;

void hemiphore_init(hemiphore* h) {
    h->value = 0;
    h->waiters = NULL;


void H(hemiphore* h, int bound, int delta) {
    hemiphore_waiter w;


14. Miscellany

QUESTION 14A. True or false: Any C arithmetic operation has a well-defined result.

QUESTION 14B. True or false: Any x86 processor instruction has a well-defined result.

QUESTION 14C. True or false: By executing a trap instruction, a process can force an operating system kernel to execute arbitrary code.

QUESTION 14D. True or false: By manipulating process memory and registers, an operating system kernel can force a process to execute arbitrary instructions.

QUESTION 14E. True or false: All signals are sent explicitly via the kill() system call.

QUESTION 14F. True or false: An operating system’s buffer cache is generally fully associative.

QUESTION 14G. True or false: The least-recently-used eviction policy is more useful for very large files that are read sequentially than it is for stacks.

QUESTION 14H. True or false: Making a cache bigger can lower its hit rate for a given workload.

QUESTION 14I. True or false: x86 processor caches are coherent (i.e., always appear to contain the most up-to-date values).

QUESTION 14J. True or false: A socket file descriptor supports either reading or writing, but not both.