9/8/09 1 Mapping PL Objects to the Machine – managing the address space David E. Culler CS61CL Feb 9, 2009 Lecture 3 UCB CS61CL F09 Lec 3!Big Ideas • Review: – Computers manipulate finite representations of things. – A bunch of bits can represent anything, it is all a matter of what you do with it. – Finite representations have limitations. • Today – Type constructors to compose complex type – Mapping of program objects to machine storage – An object, its value, its location, its reference • Pointers are THE most subtle concept in C – Very powerful – Easy to misuse – Completely hidden in Java 9/8/09 UCB CS61CL F09 Lec 3!2C Types - the big picture • Basic Types – “understood” by the machine • Array – sequence of indexed objects of homogeneous type • Struct – collection of named objects of heterogeneous types • Pointer – reference to an object of specified type • Union – an object of one of a specific collection of types 9/8/09 UCB CS61CL F09 Lec 3!3 char unsigned int float doubleComposing Complex Types in C • Complex types are really tools for composing new types – Strings – sequences of characters – Vectors – sequences of numbers – Matrixes – 2D collections of numbers – Records – finite sets of strings and numbers – Lists, Tables – Sounds, Images, Graphs – … – Think induction • Pointers are fundamentally “understood” by the machine as well – address 9/8/09 UCB CS61CL F09 Lec 3!4Where do Objects live and work? 9/8/09 UCB CS61CL F09 Lec 3!5 °°° Processor Memory 000..0: FFF..F: n: register load operate store word 0F..FAC0: 00..1AA0:Where do complex objects reside? • Arrays are stored in memory • The variable (i.e., name) is associated with the location (i.e., address) of the collection – Just like variables of basic type • Elements are stored consecutively – Can locate each of the elements • Can operate on the indexed object just like an object of that type – A[2] = x + Y[i] – 3; 9/8/09 UCB CS61CL F09 Lec 3!6 °°° 000..0: FFF..F: A:Where do complex objects reside? • Struct are stored in memory • The variable (i.e., name) is associated with the location (i.e., address) of the collection – Just like variables of any type • Elements are stored at fixed offsets – Can locate each of the elements • Can operate on the named member object just like an object of that type – S.row = x + S.col – 3; 9/8/09 UCB CS61CL F09 Lec 3!7 °°° 000..0: FFF..F: S:All objects have a size • The size of their representation • The size of static objects is given by sizeof operator 9/8/09 UCB CS61CL F09 Lec 3!8 #include <stdio.h>!int main() {! char c = 'a';! int x = 34;! int y[4];! printf("sizeof(c)=%d\n", sizeof(c) );! printf("sizeof(char)=%d\n",sizeof(char));! printf("sizeof(x)=%d\n", sizeof(x) );! printf("sizeof(int)=%d\n", sizeof(int) );! printf("sizeof(y)=%d\n", sizeof(y) );! printf("sizeof(7)=%d\n", sizeof(7) );!}!What can be done with a complex object? • Access its elements – A[i], S.row • Pass it around – Sort(A) – x = max(A, n) • Copy it – T = S – z = munge(S, 3) • Note the name of an array behaves as a reference to the object • The name of a struct behaves as the object 9/8/09 UCB CS61CL F09 Lec 3!9Administration • HW3 due at start of next week’s lab – Try to have it give practice in test tools • Lab changers and waitlisters must give target TA your prioritized lab request list this week • Readings are shifting for K&R to P&H • Project 1 goes out on Tuesday, Due Friday 10/1 9/8/09 UCB CS61CL F09 Lec 3!10An object and its value… X = X + 1; 9/8/09 UCB CS61CL F09 Lec 3!11 °°° 000..0: FFF..F: x: 3 °°° 000..0: FFF..F: x: 4 The value of variable X The storage that holds the value XEvery object in memory has an address • That address is a pointer to the object. • It is a fixed size object itself • Just like basic type 9/8/09 UCB CS61CL F09 Lec 3!12 °°° 000..0: FFF..F: S:What can be done with a reference? • Dereference it – Obtain the object that it refers (points) to – X = *P; Y = S->row; z = A[0]; z = A[i]; • Pass it around, copy it, store it – Q = P; – clearfields(S); • Do type-based arithmetic on it – P-1 – Q++ • Do both – S->next = P; – A[i] = 3; • Cast it to an uint and mess with it (!!!) 9/8/09 UCB CS61CL F09 Lec 3!13Array variables are also a reference for the object • Array name is essentially the address of (pointer to) the zeroth object in the array • There are a few subtle differences – Can change what c refers to, but not what ac refers to 9/8/09 UCB CS61CL F09 Lec 3!14 int main() {! char *c = "abc";! char ac[4] = "def";! printf("c[1]=%c\n",c[1] );! printf("ac[1]=%c\n",ac[1] );!}!°°° 000..0: c: * ‘a’ ‘b’ ‘c’ \0 ‘d’ ‘e’ ‘f’ \0 ac:What kinds of variables (storage)? • Visibility vs Lifetime • Variables declared within a function – Arguments and Local Variables – Visible in remainder of function – Lifetime = Function Call – Each call obtains a new set of variables » Recursive calls too – C “internals” • Variables declared outside any function – Visible in remainder of file (!!!) » include .h file » extern vs static – Lifetime = Whole Program – C “externals” • Malloc’d objects 9/8/09 UCB CS61CL F09 Lec 3!15 int ave(int A, int B) {! int C = (A + B)/2;! return C;!}!int count = 0;!…!int fib(int n) {! count++;! if (n <= 2) return 1;! return fib(n-1)+fib(n-2);!}!Where does the program itself reside? • In memory, just like the data • Processor contains a special register – PC – Program counter – Address of the instruction to execute (i.e. ptr) • Instruction Execution Cycle – Instruction fetch – Decode – Operand fetch – Execute – Result Store – Update PC 9/8/09 UCB CS61CL F09 Lec 3!16 °°° 000..0: FFF..F: n: 0020FAC0: PC main: 00401B20: Instruction Fetch ExecuteWhat’s a Process • Address Space + a thread of control 9/8/09 UCB CS61CL F09 Lec 3!17Logical Structure of an Executing Program 9/8/09 UCB CS61CL F09 Lec
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