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          27-Jun-11
         Parts of a computer
   For purposes of this talk, we will assume three main
    parts to a computer
       Main memory, once upon a time called core, but these days
        called RAM (Random Access Memory)
            RAM consists of a very long sequence of bits, organized into bytes
             (8-bit units) or words (longer units)
       Peripheral memory, these days called disks (even when
        they aren’t) or drives
            Peripheral memory consists of a very, very long sequence of bits,
             organized into pages of words or bytes
            Peripheral memory is thousands of times slower than RAM
       The CPU (Central Processing Unit), which manipulates
        these bits, and moves them back and forth between main
        memory and peripheral memory
                                                                                  2
         It’s all bits
   Everything in a computer is represented by a sequence
    of bits—integers, floating point numbers, characters,
    and, most importantly, instructions
       Bits are the ultimate flexible representation—at least until
        we have working quantum computers, which use qubits
        (quantum bits)
   Modern languages use strong typing to prevent you
    from accidentally treating a floating point number as a
    boolean, or a string as an integer
       A weakly typed language provides some protection, but
        there are ways around it
       But it wasn’t always this way...

                                                                       3
         Storage is storage
   At one time, words representing machine instructions
    and words representing data could be intermixed
       Strong typing was a thing of the future
       It was the programmer’s responsibility to avoid executing
        data, or doing arithmetic on instructions
       Both of these things could be done, either accidentally or
        deliberately
   Machine instructions are just a sequence of bits
       They can be manipulated like any other sequence of bits
       Hence, programmers could change any instruction into any
        other instruction (of the same size), or rewrite whole blocks
        of instructions
       A self-modifying program is one that changes its own
        instructions
                                                                        4
         Self-modifying programs
   Once upon a time, self-modifying programs were
    thought of as a good thing
       Just think of how flexible your programs could be!
       ...yes, and smoking was once considered good for your health
   The usual way to step through an array was by adding
    one to the address part of a load or store instruction
   You could write some really clever self-modifying
    programs
   But, as the poet Piet Hein says:
       Here’s a good rule of thumb:
        Too clever is dumb.


                                                                       5
         Preparation for next example
   In the next example, we will talk about how a higher-level
    language might be translated into assembly languages
   Here are some of the assembly instructions we will use:
       The load instruction copies a value from a memory location into a special
        register called the accumulator
            Example: load 53 gets whatever is in location 53 and puts it into the
             accumulator
       The enter instruction puts a given value into the accumulator
            Example: enter 53 puts 53 itself into the accumulator
       All arithmetic is done in the accumulator
            Example: add 53 adds the contents of location 53 to the accumulator
       The store instruction copies a value from the accumulator into memory
            Example: store 53 puts whatever is in the accumulator into location 53


                                                                                      6
        Procedure calls
   Consider the following:            20 [load from 43] // addr of b
   a = add(b, c);                      21 [store in 71]
                                        22 [load from 44] // addr of c
    ...
                                        23 [store in 72]
    function add(x, y) {                24 [enter 27] // the return addr
        return x + y;                   25 [store in addr part of 70]
    }                                   26 [jump to 73]
   Here’s how it might have            27 [store in 42] // addr of a
    been translated to assembly
    language in the old days (red      70 [jump to 27] // gets return addr
    values are filled in as the         71 [ 10] // will receive b
    program runs):                      72 [ 15] // will receive c
                                        73 [load value at addr 71]
   42 [ 0] // a                        74 [add value at addr 72]
    43 [ 10] // b                       75 [jump to 70]
    44 [ 15] // c
                                                                              7
         Problems with the previous code
   In this example, storage was static—you always knew
    where everything was (and it didn’t move around)
   If you called a function, you told it where to return to,
    by storing the return address in the function itself
       Hence, you could call the function from (almost) anywhere,
        and it would find its way back
   You stored the parameter values in the function itself
   This worked fine until recursion was invented
   Recursion requires:
       Multiple return addresses
       Multiple copies of parameters and local variables
   In other words, recursion requires dynamic storage
                                                                     8
         The end of an era
   What really killed off self-modifying programs was the advent of
    timesharing computers
       Multiple users, or at least multiple programs, could share the computer,
        taking turns
       But there isn’t always enough main memory to satisfy everybody
            When one program is running, another program (or parts of it) may need to be
             copied to disk, and brought back in again later
            This is really, really slow
            If only the data changed, not the program, we wouldn’t have to save the
             program (which is often the largest part) over and over and over...
            Besides, with the new emphasis on understandable programs, self-modifying
             programs were turning out to be a Really Bad Idea
   Besides, think about what a security nightmare self-modifying
    programs could be!

                                                                                            9
         An aside—compilers and loaders
   Although self-modifying code is a bad idea, it is still
    necessary for computers to be able to create and
    modify machine instructions
   This is what a compiler does—it creates machine
    instructions
   A loader takes a compiled program and puts it
    somewhere in a computer memory
       It can’t always put it in the same place, so it has to be able to
        modify the addresses in the instructions
   Still, compilers and loaders don’t modify themselves

                                                                            10
         Static and dynamic storage
   In the beginning, storage was static—you declared your variables
    at the beginning of the program, and that was all you got
   A procedure or function with, say, three parameters, got three
    words in which to store them
       The parameters went in a fixed, known location in memory, assigned to
        them by the compiler
       Recursion had not yet been invented
   The programming language Algol 60 introduced recursive
    functions and procedures
       Parameters went onto a stack
       Hence, parameters were dynamically assigned to memory locations, not by
        the compiler, but by the running program itself
       Storage was dynamically allocated and deallocated as needed

                                                                                  11
         Stacks
   Stacks obey a simple regimen—last in, first out (LIFO)
   When you enter a function or procedure or method, storage is
    allocated for you on the stack
   When you leave, the storage is released
   In Java, this is even more fine-grained—storage is allocated and
    deallocated for individual blocks, and even for for statements
   Since this is so well-defined, your compiler writes the code to do
    it for you
       But it’s still dynamic—done by your running program
   Since virtually every language supports recursion these days
    (and all the popular languages do), computers typically provide
    machine-language instructions to simplify stack operations


                                                                         12
         Heaps
   Stacks are great, but they have their limitations
   Suppose you want to write a method to read in an array
       You enter the method, and declare the array, thus dynamically
        allocating space for it
       You read values into the array
       You return from the method and POOF! your array is gone
   You need something more flexible—something where
    you have control over allocation and deallocation
   The invention that allows this (which came somewhat
    later than the stack, I’m not sure when) is the heap
       You explicitly get storage via malloc (C) or new (Java)
       The storage remains until you are done with it
                                                                        13
         Stacks vs. heaps
   Stack allocation and deallocation is very regular
   Heap allocation and deallocation is unpredictable


   Stack allocation and deallocation is handled by the compiler
   Heap allocation is at the whim of the programmer
   Heap deallocation may also be up to the programmer (C, C++)
    or by the programming language system (Java)
   Values on stacks are typically small and uniform in size
       In Java, arrays and objects don’t go in the stack—references to them do
   Values on the heap can be any size
   Stacks are tightly packed, with no wasted space
   Deallocation can leave gaps in the heap
                                                                                  14
         Implementing a heap
   A heap is a single large area of storage
   When the program requests a block of storage, it is given a pointer
    (reference) to some part of this storage that is not already in use
   The task of the heap routines is to keep track of which parts of the heap
    are available and which are in use
   To do this, the heap routines create a linked list of blocks of varying sizes
   Every block, whether available or in use, contains header information
    about the block
   We will describe a simple implementation in                 pointer to next
    which each block header contains two items of                size of block
    information:
      A pointer to the next block, and                           User data
                                             user gets
      The size of this block
                                            from here            (an Object)
                                               on down

                                                                                    15
         Anatomy of a block
                                                   ptr-2   pointer to next
   Here is our simple block:                      ptr-1    size of block
                                                     ptr
                        user gets N words          ptr+1
                                                   ptr+2     User data
                        from here (ptr) to          :       (an Object)
                           end of block             :
                                                 ptr+N-1

   Java Objects hold more information than this (for example, the
    class of the object)
   Notice that our implementation will return a pointer to the first
    word available to the user
   Data with negative offsets are header data
       ptr-1 contains the size of this block, including header information
       ptr-2 will be used to construct a free space list of available blocks


                                                                                16
       The heap, I
        0   next = 0       Initially, the user has no blocks,
        1   size = 20
free    2                   and the free space list consists
        3
        4                   of a single block
        5
        6                  In our implementation, we will
        7
        8                   allocate space from the end of
        9
       10                   the block
       11
       12                  To begin, let’s assume that the
       13
       14                   user asks for a block of two
       15
       16                   words
       17
       18
       19


                                                                 17
         The heap, II
          0 next = 0          The user has asked for a block
          1 size = 16
 free     2                    of size 2
          3
          4
          5
                              The “free” block is reduced in
          6                    size from 20 to 16 (two words
          7
          8                    asked for by the user, plus two
          9
         10                    for a new header)
         11
         12                   The new block has size 4 and
         13
         14                    the next field is not used
         15
         16 next = 0          Next, assume the user asks for a
given to 17 size = 4
user     18 ////////////       block of three words
         19 ////////////


                                                                  18
         The heap, III
          0 next = 0          The user has asked for a block
          1 size = 11
 free     2                    of size 3
          3
          4
          5
                              The “free” block is reduced in
          6                    size from 16 to 11 (three words
          7
          8                    asked for by the user, plus two
          9
         10                    for a new header)
         11 next = 0
given to 12 size = 5          The new block has size 5 and
user     13 ////////////
         14 ////////////       the next field is not used
         15 ////////////
         16 next = 0          Next, assume the user asks for a
         17 size = 4
         18 ////////////       block of just one word
         19 ////////////


                                                                  19
        The heap, IV
          0    next = 0         The user has asked for a block
          1    size = 8
 free     2                      of size 1
          3
          4
          5
                                The “free” block is reduced in
          6                      size from 11 to 8 (one word for
          7
given to 9
          8   next = 0           the user, plus two for a new
              size = 3
user     10   ////////////       header)
         11    next = 0
         12    size = 5         The new block has size 3 and
         13   ////////////
         14   ////////////       the next field is not used
         15   ////////////
         16    next = 0         Next, the user releases the
         17    size = 4
         18   ////////////       second block (at 13)
         19   ////////////

                                                                   20
       The heap, V
        0    next = 0         The user has released the block
        1    size = 8
        2                      of size 5
        3
        4                     The freed block is added to the
        5                      front of the free space list:
        6
        7                         Its next field is set to the old
        8    next = 0
        9    size = 3              value of free
       10   ////////////          free is set to point to this block
       11   next = 2
       12   size = 5          Next, the user requests a block
free   13
       14                      of size 4
       15
       16    next = 0         The first block on the free list
       17    size = 4
       18   ////////////       isn’t large enough, so we have
       19   ////////////       to go to the next free block
                                                                        21
        The heap, VI
          0   next = 0          The user requests a block of
          1   size = 2
          2   next = 0           size 3
given to 3    size = 6
user      4   ////////////      The size of the first free block
          5   ////////////       is now 3, and its next field
          6   ////////////
          7   ////////////       does not change
          8    next = 0
          9    size = 3         The user gets a pointer to the
         10   ////////////
         11    next = 2          new block
         12    size = 5
 free    13                     Now the user releases the
         14
         15                      smallest block (at 10)
         16    next = 0
         17    size = 4         Again, this will be added to the
         18   ////////////       beginning of the free space list
         19   ////////////

                                                                    22
       The heap, VII
        0 next = 0          The user releases the smallest block
        1 size = 2           (at 10)
        2 next = 0
        3 size = 6          The freed block is added to the front of
        4 ////////////
        5 ////////////       the free space list:
        6 ////////////
        7 ////////////
                                Its next field is set to the old value of free
        8 next = 13             free is set to point to this block
        9 size = 3
free   10                   Now the user requests a block of size 4
       11 next = 2
       12 size = 5          Currently, we cannot satisfy this
       13                    request
       14
       15                       We have enough space, but no single block
       16 next = 0               is large enough
       17 size = 4
       18 ////////////          However, free blocks 10 and 13 are
       19 ////////////           adjacent to each other
                                We can coalesce blocks 10 and 13
                                                                                  23
       The heap, VIII
        0    next = 0         Blocks at 10 and 13 have now
        1    size = 2
        2    next = 0          been coalesced
        3    size = 6
        4   ////////////      The size of the new block is the
        5   ////////////       sum of the sizes of the old
        6   ////////////
        7   ////////////       blocks
        8    next = 2
free
        9    size = 8         We had to adjust the links
       10
       11                     Now we can give the user a
       12
       13                      block of size 4
       14
       15
       16    next = 0
       17    size = 4
       18   ////////////
       19   ////////////

                                                                  24
         Pointers
   Allocating storage from the heap is easy
       Person p = new Person ( );
   In Java, you request storage from the heap with new; there is no
    other way to get storage on the heap
       All Objects are on the heap
   In C and C++ you get a pointer to the new storage; in Java you
    get a reference
       The implementation is identical; the difference is that there are more
        operations on pointers than on references
   C and C++ provide operations on pointers
       C and C++ let you do arithmetic on pointers, for example, p++;
   Pointers are pervasive in C and C++; you can't avoid them

                                                                                 25
        Advantages/disadvantages
   Pointers give you:
       Greater flexibility and (maybe) convenience
       A much more complicated syntax
       More ways to create hard-to-find errors
       Serious security holes
   References give you:
       Less flexibility (no pointer arithmetic)
       Simpler syntax, more like that of other variables
       Much safer programs with fewer mysterious bugs
   Pointer arithmetic is inherently unsafe
       You can accidentally point to the wrong thing
       You cannot be sure of the type of the thing you are pointing to


                                                                          26
         Deallocation
   There are two potential errors when de-allocating (freeing)
    storage yourself:
       De-allocating too soon, so that you have dangling references (pointers
        to storage that has been freed and possibly reused)
            A dangling reference is not a null link—it points to something (you just
             don’t know what)
       Forgetting to de-allocate, so that unused storage accumulates and you
        have a memory leak
   If you have to de-allocate storage yourself, a good strategy is
    to keep track of which function or method “owns” the storage
       The function that owns the storage is responsible for de-allocating it
       Ownership can be transferred to another function or method
            You just need a clearly defined policy for determining ownership
            In practice, this is easier said than done

                                                                                        27
         Discipline
   Most C/C++ advocates say:
       It's just a matter of being disciplined
       I'm disciplined, even if other people aren't
       Besides, there are good tools for finding memory problems
   However:
       Virtually all large C/C++ programs have memory problems




                                                                    28
         Garbage collection
   Garbage is storage that has been allocated but is not longer
    available to the program
   It's easy to create garbage:
       Allocate some storage and save the pointer to it in a variable
       Assign a different value to that variable
   A garbage collector automatically finds and de-allocates garbage
       This is far safer (and more convenient) than having the programmer do it
       Dangling references cannot happen
       Memory leaks, while not impossible, are pretty unlikely
   Practically every modern language, not including C++, uses a
    garbage collector


                                                                                   29
         Garbage collection algorithms
   There are two well-known algorithms (and several not
    so well known ones) for doing garbage collection:
       Reference counting
       Mark and sweep




                                                           30
        Reference counting
   When a block of storage is allocated, it includes header data
    that contains an integer reference count
       The reference count keeps track of how many references the program
        has to that block
       Any assignment to a reference variable modifies reference counts
            If the variable previously referenced an object (was not null), the
             reference count of that object is decremented
            If the new value is an object (not null), the reference count for the new
             object is incremented
       When a reference count reaches zero, the storage can immediately be
        garbage collected
   For this to work, the reference count has to be at a known
    displacement from the reference (pointer)
       If arbitrary pointer arithmetic is allowed, this condition cannot be
        guaranteed

                                                                                         31
         Problems with reference counting
   If object A points to object B, and object B points to
    object A, then each is referenced, even if nothing else
    in the program references either one
       This fools the garbage collector, which doesn't collect either
        object A or object B
   Thus, reference counting is imperfect and unreliable;
    memory leaks still happen
   However, reference counting is a simple technique and
    is occasionally used



                                                                         32
        Mark and sweep
   When memory runs low, languages that use mark-and-
    sweep temporarily pause the program and run the garbage
    collector
       The collector marks every block
       It then does an exhaustive search, starting from every reference
        variable in the program, and unmarks all the storage it can reach
   When done, every block that is still marked must not be
    accessible from the program; it is garbage that can be freed
   In order for this technique to work,
       It must be possible to find every block (so they are in a linked list)
       It must be possible to find and follow every reference
       The mark has to be at a known displacement from the reference
            Again, this is not compatible with arbitrary pointer arithmetic

                                                                                 33
       Problems with mark and sweep
   Mark-and-sweep is a complex algorithm that takes
    substantial time
   Unlike reference counting, it must be done all at once—
    nothing else can be going on
   The program stops responding during garbage
    collection
   This is unsuitable for many real-time applications




                                                              34
        Garbage collection in Java

   Java uses mark-and-sweep
   Mark-and-sweep is highly reliable, but may cause
    unexpected slowdowns
   You can ask Java to do garbage collection at a time
    you feel is more appropriate
       The call is System.gc();
       But not all implementations respect your request
   This problem is known and is being worked on
       There is also a “Real-time Specification for Java”



                                                             35
         No garbage collection in C or C++
   C and C++ do not have garbage collection—it is up to
    the programmer to explicitly free storage when it is no
    longer needed by the program
   C and C++ have pointer arithmetic, which means that
    pointers might point anywhere
       There is no way to do reference counting if the programming
        language does not have strict control over pointers
       There is no way to do mark-and-sweep if the programming
        language does not have strict control over pointers
   Pointer arithmetic and garbage collection are
    incompatible--it is essentially impossible to have both


                                                                      36
The End




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