111CMSC 212 – F05 (lect 28)AnnouncementsProgram #6– due TODAYFinal– Saturday, May 12 4-6 in PHY 14122CMSC 212 – F05 (lect 28)Review LectureFinal covers all material from the class– Comprehensive– relatively equally weighted between 1st, 2ndand after 2ndexamToday's lecture is set of highlights– Be aware that there are things not in today's review which will likely also be on the final!223CMSC 212 – F05 (lect 28)Why Study Low-level Programming?and Why Study the C language?Provides Understanding of How Things Work– Compilers, Processors, Data StructuresAllows access to Hardware when requiredAllows efficiency and control– Writing device drivers and operating systems– If you are careful and good it will be faster– BUT could also be Dangerous or SlowerWidely Used for System Programming“Middle Aged” Language – created in late 1970’sC’s design goals GoalsMajor Differences from JAVA– Explicit Memory Allocation and Deallocation– Procedural rather than object oriented– Compiles directly to machine code (rather than byte codes)4CMSC 212 – F05 (lect 28)Phases of Compilation Preprocessor– Not everything is in one file• Definitions of routines are in separate files– Function Prototypes• Define name, parameters, and return types– #include, #if, #endif, #ifndef– macros, constants, conditional compilationtranslation (compilation)– source to object code– of individual files– making sure prototypes match definitions and callslinking– combining object files– making sure all parts are there335CMSC 212 – F05 (lect 28)Readability, Reliability and Efficiency of Your programReadability– Comments• Should be surrounded by /* and */• May span multiple lines– White Space• vertical and horizontalTesting– Write Tests as you write software– When the software is “done”, tests continue as part of it– Types of Testing• line coverage, white box and black boxEfficiency– Code motion– Loop Unrolling– Dead-Code Elimination– human changes vs -O option on compiler6CMSC 212 – F05 (lect 28)Make: A Tool to Compile ProgramsProblem: Compiling programs is tediousSolution: Automate it!Done in Unix by the make command– Uses a file called Makefile• A makefile is a file (script) containing :– Project structure (files, dependencies)– Instructions for files creationMake is not limited to C programsAdapted from: www.cs.tau.ac.il/~dcor/Software1/Makefile.ppt447CMSC 212 – F05 (lect 28)Stored Program ComputerA Program:– Consists of a sequence of operations – Is loaded into a computer’s memory to executeStandardize set of allowed operations, called Instructions– each instruction performs a specific operation– Instructions can • read or change memory• compute a value• read or write output– What instruction to execute next:• normally it is the next instruction• provide control to select alternate next instructionProgram and data are both stored in memoryCompleteness of Instructions8CMSC 212 – F05 (lect 28)What do Instructions Look Like?For Project #1, instructions:– are 32 bits long– contain three partsExtension Later – Maintaining the Stack– Calling Functions• for calls to the same function from other places in the program• for recurrsion– parameter passing to functions– return values of functionsPurpose Opcode Register1Register2Register3MemorySize (bits) 4 4 4 4 16559CMSC 212 – F05 (lect 28)Function ArgumentsComma separated list of valuesAll parameters are passed by value– called function can modify values– arrays are passed by the address of their 0th element• modifying elements of array seen by calling functionCan pass address of variable to change valuesint foo(int *a) {*a = 3;}int x;foo(&x);10CMSC 212 – F05 (lect 28)struct, union, and typedefOften useful to use typedef and struct together:typdef struct {int a;char b;} Simple;Declarations then look like:Simple x;Simple y[20], z;Header Files– If structure used by more than one file, put it in a header file.– Use #include to include definition in each file where used. Unions– Rather than storing each field, only stores one of the fields– Syntax: union tag { member-list } variable-list6611CMSC 212 – F05 (lect 28)Bit FieldsPart of the Power of C is control over data layout– If you need a field with exactly 13 bits, you can define it– Useful for:– defining fields that interact with hardware– managing pre-defined file formats– saving every last bit of space– Syntax: type variable:size;Examples:int foo:13;unsigned int foo:4;typedef struct { /* from project #1 */unsigned int opCode:4;unsigned int r1:4;unsigned int r2:4;unsigned int r3:4;unsigned int address:16;} instruction;12CMSC 212 – F05 (lect 28)Bit Shift OperatorC has operators to bit shift numbers– number of bits changed depends on size of variableLeft Shift (number << xxx)– Move each bit of number to the left by xxx bit positions– Leftmost xxx bits discarded– Rightmost xxx bits gets 0Right Shift (number >> xxx)– Move each bit of number to the right by xxx bit positions– Rightmost xxx bits discarded– Leftmost xxx bits gets 0 or replicate sign bit• for unsigned gets 0• for signed, its implementation dependent7713CMSC 212 – F05 (lect 28)Type ConversionPromotion of char and short– in expressions, char and short are promoted to intchar a, b, c;a = b + c;• b and c are converted to int, then the sum is truncatedArithmetic Conversions– can't be performed on different types• converted to "higher" typelong doubledoublefloatunsigned long intlong intunsigned intintHighest Type14CMSC 212 – F05 (lect 28)PrecedenceThere is a set of rules about precedence of operator– Most important precedence rule:• When in doubt, put in () to ensure correct order– Order (highest to lowest):• ()• Function call, subscript, postfix increment/decrement• rest of unary operators• type conversion• Arithmetic operators• Relational operators• Bit operators• Assignment operators• , (comma operator)8815CMSC 212 – F05 (lect 28)Standard I/O Library#include <stdio.h>– includes prototypes for standard I/O routinesMost I/O is stream based– buffered to allow efficient operations • mostly to disks or networks– interactive I/O is normally flushed at useful points• printf("foo\n"); /* \n causes a flush