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S07: The C Language

int i;main (){for(; i [&quot;]&lt; i ;++ i ){-- i ;}&quot;];read('-'-'-', i +++&quot;hell o, world!<br>&quot;,'/'/'/'));}read( j,i,p ){write(j/ p+p,i --- j,i / i );} -- Dishonorable mention, Obfuscated C Code Contest, 1984. (Author requested anonymity .). S07: The C Language.

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S07: The C Language

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  1. inti;main(){for(;i["]<i;++i){--i;}"];read('-'-'-',i+++"hell\ o, world!\\n",'/'/'/'));}read(j,i,p){write(j/p+p,i---j,i/i);} -- Dishonorable mention, Obfuscated C Code Contest, 1984. (Author requested anonymity.) S07:The C Language Required: PM: Ch 6, pgs 63-80 PM: Ch 8.4, pgs114-118 Recommended: K&R, Chapters 1-4 C programC++ Compiler 6.5VariablesGlobal VariablesExpressionsOperatorsControl StatementsFunctionsLibrary Functions

  2. CS 124 The C Language

  3. Learning Objectives… After completing this section, you should be able to • Discuss the advantages of using a high level language. • Explain the difference between a compiler and an interpreter. • Outline the function of the C preprocessor. • Describe the compile/assembly/linker process. • List the main features of a C Language program. • Describe how C stream I/O works. The C Language

  4. Levels of Abstraction Problems Algorithms High Level Languages Language Assembly code Machine (ISA) Architecture Machine code Microarchitecture MSP430 Architecture Circuits Logic gates, multiplexers, memory, etc. Transistors Devices The C Language

  5. High Level Languages High Level Languages • The closer a language is to your original specification, the easier the program is to write. • Many, many programming languages • LISP - LISt Processing • PROLOG - logic programming • MATLAB - matrix and vector manipulations • BASIC – interpreter for small computers • APL – matrix and vectors • FORTRAN – formula translation • COBOL – business and accounting • PASCAL – procedural • Ada – DOD large systems • Java – Internet • C, C++ …. The C Language

  6. High Level Languages High Level Languages • Allow us to use symbolic names for values • Programmer simply assigns each value a name • Allow us to ignore many memory details. numberOfDays = 30; switch_A = ON; • Provide abstraction of underlying hardware • Hide low level details (ISA) from programmer • Portable software (works on different ISAs) printf("Hello World!"); • Provide expressiveness • Express complex tasks with smaller amount of code • English-like and human constructs if(isCloudy) get(umbrella); else get(sunglasses); main() { readInput(); checkForErrors(); doCalculation(); writeOutput(); } • Enhance code readability • Can read like a novel… • Easier to debug/maintain The C Language

  7. High Level Languages High Level Languages • Provide safeguards against bugs • Rules can lead to well-formed programs • structured programming (no GOTO statements) • Compilers can generate checks • array bounds checking • data type checking • Many languages provide explicit support for assertions • something that should be true - if it isn’t, then error assert(accountBalance >= 0); • High-level languages make complex programming simpler, while low-level languages tend to produce more efficient code • However, well-designed compilers frequently produce code comparable in efficiency to what most low-level programmers can produce by hand with better overall results The C Language

  8. Source code temp=v[i]; v[i]=v[i+1]; v[i+1]=temp; Interpreter Compilers vs Interpreters Compilers vs Interpreters Object code Assembly temp=v[i]; v[i]=v[i+1]; v[i+1]=temp; Compiler Application Assembler MOV.B 0x0001(SP),R14 MOV.W SP,R15 INCD.W R15 ADD.W R15,R14 MOV.B @R14,0x0000(SP) MOV.B 0x0001(SP),R14 INC.W R14 415E 0001 410F 532F 5F0E 4EE1 0000 415E 0001 531E High-level language statements = Data Path = Executable The C Language

  9. The C Language The C Programming Language • Developed between 1969 and 1973 by Dennis Ritchie at Bell Labs. • C first developed for use in writing compilers and operating systems (UNIX). • A low-level high-level language • Many variants of C • 1989, the American National Standards Institute standardized C (ANSI C, most commonly used C) • “The C Programming Language” by Kernighan and Ritchie is the C “Bible” (Also called the “White Book”.) • C is one of the most popular programming languages of all time – very few computer architectures exist for which there is no C. • C is predecessor to most of today’s procedural languages such as C++ and Java. The C Language

  10. Assembler Code C/C++ Code Object Code Machine Code The C Language Compiling a C Program The C Language

  11. C Preprocessor 1st Pass 2nd Pass Code Generation Source Code Analysis Symbol Table Library & Object Files ExecutableImage The C Language Compiling a C Program C Source Code Preprocessor Text C Compiler Preprocessed C source code Assembly Code Object Code Assembler Linker Machine Code The C Language

  12. 1st C Program A First Program Tells compiler to use all the definitions found in the msp430.h library. A .h file is called a header file and contains definitions and declarations. //************************************ // blinky.c: Software Toggle P1.0 //************************************ #include "msp430.h" volatile unsigned inti; // no optimization void main(void) { WDTCTL = WDTPW | WDTHOLD; // stop watchdog P4DIR |= 0x40; // P4.6 output for (;;) // loop { P4OUT ^= 0x40; // toggle P4.6 while (--i); // delay } } All C programs must have a main()routine. Stop WD w/Password Allocate a RAM variable (.bss i,2) Set P4.6 as output Loop forever Toggle P4.6 Delay 65,536 The C Language

  13. Style 2 if(a < b) { b = a; a = 0; } else { a = b; b = 0; } Style 1 if(a < b) { b = a; a = 0; } else { a = b; b = 0; } C Style Style • Use lots of comments /* This is a comment */ // This is a single line comment • Indents • Each new scope is indented 2 spaces from previous • Put { on end of previous line, or start of next line • Line matching } up below Style is something of a personal matter. Everyone has their own opinions… What is presented here is similar to that in common use and a good place to start... The C Language

  14. C Preprocessor The C Preprocessor • #define symbol code • The preprocessor replaces symbol with code everywhere it appears in the program below #define NUMBER_OF_MONKEYS 259 #define MAX_LENGTH 80 #define PI 3.14159 • #include filename.h • The preprocessor replaces the #include directive itself with the contents of header file filename.h #include <stdio.h> /* a system header file */ #include "myheader.h" /* a user header file */ • Macros #define add(x,y) x+=y #define doLoop(x,y) do {x} while(y); doLoop(add(z,2),z<10) do {z+=2} while(z<10); The C Language

  15. Streaming I/O in C

  16. C Stream I/O C I/O • I/O facilities are not part of the C language itself • Nonetheless, programs do interact with their environment! • Most digital I/O handled directly by C program • #include "msp430.h" • SPR’s, Ports, A/D, transponder, switches, LED’s, etc • The ANSI standard defines a set of I/O library functions for portability • Programs that confine their system interactions to facilities provided by the standard library can be moved from one system to another without change. • The properties of the C I/O library functions are specified in header files • #include <stdio.h> (C standard library) • #include "RBX430_lcd.h" The C Language

  17. C Stream I/O C Data Streams • C I/O is character based, using streams. • I/O streams must be opened / closed. • In standard C there are 3 streams automatically opened before main() is called: • stdin is the input stream • stdout is the output stream • stderr stream for error messages • printf function outputs formatted values to stdout stream • The printf function requires a format string followed by optional parameters: printf( "format string...", parameters... ); • The format string contains two object types: • Ordinary characters that are copied to the output stream • Conversion specifications which cause conversion and printing of the next argument in the argument list. The C Language

  18. C Stream I/O Printf Output in C String literal • printf( format_string, parameters ) printf("Hello World"); printf("\n%d plus %d is %d", x, y, x+y); printf("\nIn hex it is %x", x+y); printf("\nHello, I am %s. ", myname); printf("\nInascii, 65 is %c. ", 65); • Output: Hello world 5 plus 6 is 11 In hex it is b Hello, I am Bambi. In ascii, 65 is A. Decimal Integer Hex Integer Newline Character String The C Language

  19. LCD I/O LCD on Ports 2,3, & 4 Command/Data Read/Write MSP430F2274 Parallel 8-bit Data Enable The C Language

  20. LCD I/O RBX430_lcd.h Prototypes • uint8 lcd_init(void); • void lcd_clear(uint8 value); • void lcd_backlight(uint8 backlight); • void lcd_volume(uint8 volume); • uint16 lcd_mode(int16 mode); • uint8 lcd_cursor(uint16 x, uint16 y); • uint16 lcd_printf(const char* fmt, ...); • uint8 lcd_image(const uint8* image, int16 x, int16 y); • uint8 lcd_bitImage(constuint8* image, int16 x, int16 y, uint8 flag); • uint8 lcd_wordImage(const uint16* image, int16 x, int16 y, uint8 flag); • uint8 lcd_blank(int16 x, int16 y, uint16 w, uint16 h); • uint8 lcd_point(int16 x, int16 y, uint8 flag); • void lcd_circle(int16 x, int16 y, uint16 r, uint8 pen); • void lcd_rectangle(int16 x, int16 y, uint16 w, uint16 h, uint8 pen); The C Language

  21. LCD I/O LCD – 160 x 160 x 5 Pixels // 5 x 8 pixel Characters lcd_cursor(40, 60); lcd_printf("Hello World!"); Hello World! Y (0-159)  X (0-159)  The C Language

  22. Terms… • Activation Record – A block of memory on the stack that is created when a function is called and contains all the local variables for a given invocation of a function. • Arithmetic Operator – Operator that returns a numerical value. • Associativity – The execution order of same precedence operators. • Bitwise Operator – Operator that performs bitwise logical operations. • Data type – Representation and valid operations of data object. • Expression – Combination of variables / operators that returns a single value. • Global (static) – Variable permanently assigned to a memory location. • Literal – An immutable data object. • Local (automatic) – Variable stored in a functions activation record. • Logical Operator – Operator that returns a logical (true/false) value. • Operator – Performs an operation on operand(s). • Scope - Extent of a variable/function’s availability in a program. • Precedence – The execution order of operators. • Variable - Symbolic name for a memory location that hold a value. • Variable Coercion – Forcing mixed data type variables to a common type. • Volatile – Variable modifier that prohibits optiminization by compiler. The C Language

  23. C Program A C Program • What is a C program? • Functions • Global variables • Variables are symbolic names for memory locations that hold values • 2 types of variables • Local (automatic) • Global (static) • Variable declarations include • A symbolic name • Data type (int, char, double) • Scope (code region where the variable is defined) • Variables are stored in memory or in registers. • The compiler keeps track of where a variable’s value is currently stored. • Operators manipulate values The C Language

  24. Variables & Operators The C Symbol Table • The C compiler keeps track of variables in a program during compilation in a symbol table • A symbol table entry is created when a variable is declared. • Specifically, each symbol table entry contains: • Variable name • Variable data type (int, float, char, etc.) • Variable storage class (auto, static) • Where in memory the variable is stored (an offset) • An identifier to indicate the variable’s scope • Variables must be declared and in scope before they can be used (referenced) by a program The C Language

  25. C Preprocessor 1st Pass 2nd Pass Code Generation Source Code Analysis Symbol Table Library & Object Files ExecutableImage The C Language Compiling a C Program Preprocessor Text C Source Code C Compiler Preprocessed C source code Assembly Code Object Code Linker Assembler Machine Code The C Language

  26. Variables & Operators MSP430 C Variable Data Types The C Language

  27. Variables & Operators Variable Declarations int i,j,k; // declaring more than one variable int i1, i2, i3, c3po; // numbers OK, except for first letter int bananas = 10; // using an initializer int monkey_count = 0; // two ways of doing ... int monkeyCount = 0; // ... multi-word names int ab, Ab, aB, AB; // case sensitive names int _compilerVar; // compiler uses _ as first char char newline = ‘\n’; // a character with an initializer char lineBuffer[32]; // an array of 32 chars (a string) double bananasPerMonkey; // floating point declarations double hugeNumber = 1.0E33; // positive exponent double tinyNumber = 1.0E-33; // negative exponent double fractionThing = 3.33333; // no exponent The C Language

  28. Scope Scope: Local versus Global • Extent of a variable/function’s availability in a program • Local Variables (automatic) • Declared at the beginning of a block • Stored in activation record on the stack • Scope is from point of declaration to the end of the block • Un-initialized • Global Variables (static) • Declared outside of a function • Stored in Global Data Section of memory • Scope is from point of declaration to the end of the program • May be initialized to zero { // begin block int chimp; ...} int chimp; { // begin block ...} The C Language

  29. Variables Literals/ Constants • Literal Values • Unnamed constant values used in programs • area = 3.14159 * radius * radius; • Constant Variables • Variable declarations prefixed with the const qualifier • Immutable named variables • const double pi = 3.14159; • Symbolic Values • Created using preprocessor directive #define • #define PI 3.14159 • How are the above the same? • How are the above different? The C Language

  30. Variables Variable Usage • Make your variable names meaningful • Common naming conventions • Hungarian notation (prefix hints) • gVariable, hMyRoutine • UpperCamelCase / lowerCamelCase for most identifiers • MyInputByte, buzzerCounter • Underscores • last_variable_used, number_of_days • all-upper-case for constants • #define TRUE 1 • Names beginning with underscore are reserved for compilers/libraries • __reserved, _Reserved • Encapsulate your variables • Avoid global variables - explicitly pass parameters to functions • Keep the scope as small as you can The C Language

  31. Variables volatile • volatile proceeding a variable name instructs the compiler to • prohibit caching the variable’s contents when optimizing code. • always re-read the variable’s value when accessing the variable. • not use computer registers to store a variable’s content. Sample P1IN when dcnt equals 0 Pressing a switch sets dcnt volatile intswitches,dcnt void main(void) { if (switches & 0x01) {...} } #pragma vector=PORT1_VECTOR __interrupt void Port_1_ISR(void) { P1IFG &= ~0x0f; // P1.0-3 IFG cleared dcnt= DEBOUNCE_CNT; // enable debounce } #pragma vector = WDT_VECTOR __interrupt void WDT_ISR(void) { if (dcnt&& (--dcnt== 0)) switches = (P1IN ^ 0x0f) & 0x0f; } Inform the compiler that integers switches and dcnt are not to be optimized. The C Language

  32. Operators Operators and Expressions • Expressions are formed by combining variables with operators and ALWAYS return a single value in C. i = 5; i < j; a = (a < b); • Operators • Assignment – • changes the values of variables • Arithmetic – • add, subtract, multiply, divide • Bitwise – • AND, OR, XOR, NOT, and shifts on Integers • Relational – • equality, inequality, less-than, etc. • Logical – • AND, OR, NOT on Booleans • Increment/Decrement C supports a rich set of operators that allow the programmer to manipulate variables The C Language

  33. Operators The Assignment Operator • The operator symbol is the equal sign • The expression on the right-hand side is evaluated and assigned to the left-hand variable int x = 9; x = x + 4; Stack sub.w #2,sp mov.w #9,0(sp) 0x05fa 0x05fc 0x05fe sp 0x05f0 X sp ... add.w #4,0(sp) 0x0600 The C Language

  34. Operators Arithmetic / Relational Operators x + y x – y x * y x / y x % y • Arithmetic Operators • Add (+), subtract (–), multiply (*), divide (/) • Integer; 5/3 = 1 (truncated to int) • Floating point : 5.0 / 3.0 = 1.66666666 • Modulus (%) • Integer; remainder after integer division; 5 % 3 = 2 • Relational operators return Boolean values: • 0 if relation is FALSE • 1 if relation is TRUE • Comparisons x == y equality x != y inequality x < y less-than x <= y less-than-or-equal x > y greater-than x >= y greater-than-or-equal The C Language

  35. Operators Bitwise Operators • Perform bitwise logical operations across individual bits of a value. • AND & • OR | • XOR ^ • NOT ~ (1’s complement) • Shifts are bitwise operators • SHIFT LEFT << • SHIFT RIGHT >> x : 1 0 1 0 (binary) y : 1 1 0 0 (binary) x & y : 1 0 0 0 (binary) x | y : 1 1 1 0 (binary) x ^ y : 0 1 1 0 (binary) ~x : 0 1 0 1 (binary) x << y shift x y-places to the left (add zeros) x >> y shift x y-places to the right (sign extend) The C Language

  36. Same Same Operators Logical Operators • Logical operators evaluate to Boolean • AND && • OR | | • NOT ! • Don’t confuse with Bitwise operators • Operate on Boolean inputs and produce Boolean outputs • Boolean inputs (how values are interpreted): • Value not equal to zero  TRUE • Value equal to zero  FALSE 10 && 20  110 && 0  0 if( 'a' <= x <= 'z' ) statement; // wrong! if(('a' <= x) && (x <= 'z')) statement; if(!x) statement;if(x == 0) statement; if(x) statement;if(x != 0) statement; The C Language

  37. Expressions Order of Evaluation • Variable Coercion • When executing expressions of mixed types, C automatically converts integer to floating point and back again as needed. • Avoid the use of forced data conversion as operators may yield unanticipated results. • Order of expression evaluation: • Precedence – higher precedence operators evaluate first. • Associativity – operators of same precedence evaluate left to right (with a few exceptions). • Parentheses override all other evaluation rules. int x = 1; x is declared an integer x = x + 4.3; integer + floating point ?? (result is x = 5) The C Language

  38. Bitwise Relational Relational Bitwise Bitwise Bitwise Logical Logical Expressions Operator Precedence/Associativity The C Language

  39. Expressions Combined Assignment Operators • Arithmetic and bitwise operators can be combined with the assignment operator. x += y; x = x + (y);x -= y; x = x – (y);x *= y; x = x * (y);x /= y; x = x / (y);x %= y; x = x % (y);x &= y; x = x & (y);x |= y; x = x | (y);x ^= y; x = x ^ (y);x <<= y; x = x << (y);x >>= y; x = x >> (y); Note: All of the expression on the right is considered parenthesized. The C Language

  40. y z 0 1 x x ? y : z Expressions Conditional Expressions • Conditional expression • C multiplexor operation • Format: <boolean> ? <true expression> : <false expression> • Example: x ? y : z This expression returns the value of y if x != 0, otherwise it returns the value of z printf("%d dog%s", dogs, (dogs == 1) ? "" : "s"); The C Language

  41. Compilation Examples C to Assembly – Example 1 { int x = 10; int y = 20; int z = 30; x = x + 4; y = x + y - z; } 0x8696: 8031 0006 SUB.W #0x0006,SP 0x869a: 40B1 000A 0000 MOV.W #0x000a,0x0000(SP) 0x86a0: 40B1 0014 0002 MOV.W #0x0014,0x0002(SP) 0x86a6: 40B1 001E 0004 MOV.W #0x001e,0x0004(SP) 0x86ac: 52A1 0000 ADD.W #4,0x0000(SP) 0x86b0: 411F 0002 MOV.W 0x0002(SP),R15 0x86b4: 512F ADD.W @SP,R15 0x86b6: 811F 0004 SUB.W 0x0004(SP),R15 0x86ba: 4F81 0002 MOV.W R15,0x0002(SP) x05f4 x05f6 x05f8 SP x x05fa y x05fc z x05fe ret adr x0600 Stack The C Language

  42. Compilation Examples C to Assembly – Example 2 main: 0x8040: 8031 000A SUB.W #0x000a,SP 0x8044: 4D81 0002 MOV.W R13,0x0002(SP) 0x8048: 4C81 0000 MOV.W R12,0x0000(SP) 0x804c: 40B1 0007 0004 MOV.W #0x0007,0x0004(SP) 0x8052: 40B1 0005 0006 MOV.W #0x0005,0x0006(SP) 0x8058: 411C 0004 MOV.W 0x0004(SP),R12 0x805c: 411D 0006 MOV.W 0x0006(SP),R13 0x8060: 12B0 80DA CALL #__mpyi 0x8064: 4C81 0008 MOV.W R12,0x0008(SP) 0x8068: 430C CLR.W R12 0x806a: 5031 000A ADD.W #0x000a,SP 0x806e: 4130 RET __mpyi: 0x80da: 430E CLR.W R14 mpyi_add_loop: 0x80dc: C312 CLRC 0x80de: 100C RRC R12 0x80e0: 2801 JLO (shift_test_mpyi) 0x80e2: 5D0E ADD.W R13,R14 shift_test_mpyi: 0x80e4: 5D0D RLA.W R13 0x80e6: 930C TST.W R12 0x80e8: 23F9 JNE (mpyi_add_loop) 0x80ea: 4E0C MOV.W R14,R12 0x80ec: 4130 RET int main(int argc, char** argv) { unsigned int x = 7; unsigned int y = 5; unsigned int z; z = x * y; return 0; } x05f4 SP argc (r12) x05f6 argv (r13) x05f8 x x05fa y x05fc z x05fe ret adr x0600 Stack The C Language

  43. IdentifierTypeStorage ClassOffsetScope inGlobal int Static absolute global inLocalA int Auto 2(SP) main inLocalB int Auto 4(SP) main outLocal int Auto 0(SP) main SymbolTable Compilation Examples C to Assembly– Example 3 main: SUB.W #0x0006,SP MOV.W 0x0002(SP),R15 ADD.W &inGlobal,R15 ADD.W 0x0004(SP),R15 MOV.W R15,0x0000(SP) ADD.W #0x0006,SP RET int inGlobal; void main(void) { int outLocal; int inLocalA; int inLocalB; outLocal = inGobal + inLocalA + inLocalB; return; } The C Language

  44. Quiz 7.1 • Fill in the resulting values for x, y, and z after evaluating the construct. • Assume for each row, x, y, and z are initialized to 10, 20, and 30 respectively. The C Language

  45. The C Language

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