C Compiler Basics_8051

7/27/2019 C Compiler Basics_8051

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C Compiler Training

that Sticks8051



Agenda

C Language Review

Examples

Programs



What is C?

High-level programming language

One of the most popular general purposeprogramming languages available

First developed in 1972 by Dennis Ritchie at AT&T Bell Labs

Advantages of CReadability

Maintainability

Portability



A Word of Caution

C IS CASE

SENSITIVE Var ≠ VAR

Use for, not FOR etc.



A Simple C Program

/*This is my 1st C program

Port 1 will be loaded with 0xAA

and Port 3 with 0xF6. Everything between

the star and forward slash is ignored by compiler*/

#include <stdio.h>

void main(void){

P1 = 0xAA;

P3 = 0xF6;

while(1){

asm{NOP;} //Assembler instruction

used }



Dissecting a C Program

The first line is a comment

A comment starts with a /* and ends with a */

Everything in between is ignored

Comments can cross more than one line

Document code, enhance readability

Comments affect only size of text file

Shortform notation for commenting out a single

line: two slashes, //

Comments cannot be nested




/* Comments do not increase the

size of the executable, nor

does it affect performance */

/* Comments do not increase the */

/* size of the executable, nor */

/* does it affect performance */

// Comments do not increase the

// size of the executable, nor

// does it affect performance




#include <stdio.h>

Line 2 is a preprocessor directive

Tells the compiler to look for a file and placethe contents of that file at the directive

stio.h is a header file

Contains prototypes and macros for functionsand registers




Angle brackets <stdio.h> tell the

preprocessor to look in another directory for the

file

Double quotes “stdio.h” tell the

preprocessor to look in the current directory for

the file




Every C program must have one,and only one, main( ) function

Can be anywhere in your source code Program execution starts with main( )

Programs always end with main( )

The body of the function main( ) islocated between the curly braces { and the }



The Basics of the C Program

A C program is composed of basic elementssuch as:

keywords

constants

variables expressions

statements and statement blocks



ANSI C Keywords

auto else register union

break enum return unsinged

case extern short void char float signed volatile

const for sizeof while

continue goto static

default if structdo int switch

double long typedef



Expressions

An expression is a combination of constants, variablesand operators

Examples(2 + 3) * 10 10 * (4 + 5)

80/4 6

i 6 + i

return 0



Precedence of Operators

All expressions are evaluated according to the precedence of it‟s operators

What do the following expressionsreduce to?

2 + 3 * 10 (2 + 3) * 10



Precedence of Operators

All expressions are evaluated according to the precedence of it‟s operators

What do the following expressionsreduce to?

2 + 3 * 10 (2 + 3) * 10

= 2+(3*10) = 32 =5*10 =50* has higher precedence than +() has higher precedence than *



Operator Precedence Table

( ) [ ] . -> l to r ^ l to r

- ~ ! * & r to l | l to r

++ -- sizeof r to l && l to r

* / % l to r || l to r+ - l to r ?: r to l

<< >> l to r = *= /= r to l

< > <= >= l to r %= += -= r to l

== != l to r <<= >>= r to l

& l to r &= ^= |= r to l



Statements

A statement is a complete instruction, ending with a semicolon

A statement is made up of one or moreexpressions

i = 1; Var1 = (2 + 3) * 10;

j = 6 % 4; BinValue = i + j;

return 0;



Statement Blocks

A group of statements is a statement block beginning with a curly brace ( { ) and ending with a

curly brace ( } )

A statement block is treated like a single statement by the compiler

for(…)

{

s3 = s1 + s2;

MulValue = s3 * c;

}



Data Types

Defines the size and type of data that a variableholds

Only five data types in ANSI C void

char -> 8 Bits ->1 Byte

int -> 16 Bits ->2 Bytes

long -> 32 Bits ->4 Bytes

float -> 32 Bits ->4 Bytes double



void Data Type

Represents no data type

Used most commonly with functions

void CombineValues(void){

//Changes global values

}



char Data Type

Represents a single character or an8-bit number, i.e., A or 7

To define a variable as a char use:char variablename;

To define multiple variables use:char variablename1;char variablename2 ;

Another way of defining:char variablename3 , variablename4;



Character Constants

Single characters can be represented using singlequotes, such as: „A‟

The ASCII value of the character is the value used inthe expression

x = „A‟; //0x41



int Data Type

Integer numbers are whole numbers

Result of integer division is truncation

The length of an integer varies from one machine to

another, typically 16-bits

To define a variable as an int use:

int variablename;



float Data Type

Floating point numbers contain both real andfractional parts

Has at least 6 digits of precision

To define a variable as float use:

float variablename;

Supported as 32-bit value in MCS51



Data Type Modifiers

These four data type modifiers can be applied to datatypes char and int

signed

unsigned

short

long



Sign Bit

The difference between a signed number and anunsigned number is the interpretation of the MSb

For signed numbers, the MSb indicates a positive

number when 0 and a negative number when 1 For unsigned numbers all bits are used to determine

the value

The default value of data types is unsigned



Sign Bit

The range of unsigned char is 0 to 255

{0, 1, …, 255}

The range of signed char is -128 to +127

{0x80, 0x81, …, 0xFE,0xFF,0x00,…,0x7F}



Changing Data Sizes

long and short are used to increase ordecrease the size used by the data type

short int x; reduces a 16-bit integer to 8-bits may be used without ‘int’: short x;

long int x; doubles a 16-bit integer to 32-bits may be used without ‘int’: long x;



Anatomy of a C Function

char add (char x, char y){

char result;

result = x + y;

return (result);}

Function

Type

Function

Name

Argument List to

Function

Start of Function

Body of Function

End of Function Bodyand Function



Function Type

The function type is used to signify what typeof value the function returns

Can be any valid C data type

void functions do not return a value

The function add( ) returns a char



Function Names

A function name should be chosen to reflect what

the function does

Function names cannot start with numbers

Function names cannot contain asterisk,

arithmetic signs, or dot (.)

Functions cannot contain minus signs or

apostrophes



Function Arguments

Arguments are used to pass information TO a function

Multiple arguments are separated by commas

A function that does not receive any arguments is

considered to be void int status(void){

// This function will return data of

type int}



Function Body

The open curly brace ( { ) signifies the start of the

function body

The closed curly brace ( } ) denotes the end of the

function body The statements in between are the statement block

The function completes when the statement block isfinished executing



Global Variables

Defined outside functions Variable can be used by ALL functions

unsigned char FlagReg; // Globally defined register

void TestFlags(void){FlagReg = FlagReg & 0x86;

}// end of function

void main(void){

-FlagReg = FlagReg + 5;TestFlags(); //Call function-

}



Local Variables

Defined inside a functions e.g. Temp Variable can be used ONLY by that function

unsigned char Online; // Globally defined register

void ChangeVal(void){unsigned char Temp; // Local variable

Temp = P1 + 0x21;Online = Temp & 0x0F;

}// end of function

void main(void){-ChangeVal(); //Call function-

}



Making Function Calls

#include <stdio.h>

char add( char x, char y){ /* This function adds two chars */char result; //define a LOCAL result variable

result = x + y; /* add parameters */return (result); /* return result to caller */

}

void main(void){char sum; /* defines a local variable */

sum = add(5,12); // calculate sum of 5 and 12P1 = sum; // place result on Port 1

}



Quiz

What is wrong with this function?

int int_add(int x, int y){

char sum;

sum = x + y;

return (sum);

}



Quiz

Answer – Types different, answer limited tochar!

Change to same type

int int_add(int x, int y){int sum;

sum = x + y;return (sum);

}



Quiz

What is wrong with this function?

int AddUp(char x, int y){

int sum;

sum = x + y;

return (sum);

}



Quiz

Answer – Types different !

Change to same type

int int_add(char x, int y){

int sum;

sum = (int)x + y; //change x to an integer

return (sum);

}



for Loop

General form:

for(expr1;expr2;expr3){

statement1;statement2;

. . .

}

Example



for Loop

expr1

?-expr2

expr3

statement1

statement2

...

F

T



while Loop

General form:

while(expr1){

statement1;

statement2;. . .

}



while Loop

?-expr1

statement1

statement2

...

F

T



do-while Loop

General form:

do{

statement1;

statement2;. . .

} while(expr1);



do-while Loop

expr1

statement1

statement2

...

F

T



break Statement

break is used to jump out of a loop

General form: break;

i=0;

do{if (i==10){ break;

}i++;

}while(1);



continue Statement

continue allows you to stay in loop but skip over

statements within loop

General form: continue;

for (i=1;i<6;i++){if (i==3){

continue;}sum += i;

}



if Statement

Executes a statement block based on a condition

General form:

if (expr){

statement1;statement2;. . .

}

else{statement3;statement4;

} } Optional



if Statement

Executes a statement block based on a condition

General form:

if (Value == 0x34){

statement1;statement2;. . .

}

else{statement3;statement4;

} } Optional

NB!! Not Value = 0x34

False

} True



switch Statement

Replaces a group of if-else-if statements

switch(expr){

case expr1: statement1; break;

case expr2: statement2; break;

. . .default: default-statement; //if no case

break;}

Optional



Arrays

An array is a collection of values with the same data type

Each value in the array is an element

All elements are referenced by the name of the array with anoffset

Elements are stored in consecutive memory locations



Arrays

General form:

data-type Array-name[array_size];

data-type indicates what data type the elements in the

array will be

Array-name is the name of the array

Array_size defines how many elements are in the array

E.g. char Keyboard[16]; Array name is Keyboard

All data in Array is of type char

Array has 16 locations (0 to 15)



Arrays

Arrays are accessed using an index Index starts at 0 and ends at arraysize - 1 The elements of the array are:

day[0], day[1], day[2], day[3], day[4], day[5], day[6]

Data[0], Data[1], Data[2], Data[3], Data[4], Data[5],Data[6]

Elements may be initialized individually day[0]=„S‟;

Data[1]= 56;

Or all together char day[7]={„S‟,‟M‟,‟T‟,‟W‟,‟T‟,‟F‟,‟S‟};

char Data[6]= {3,5,4,1,0,2};



Arrays

Accessing arrays using an index variable

int List[10], i;

for(i=0;i<10;i++)

{

List[i] = i;

}

Array with name of List

and 10 elements –int’s



Arrays and Strings

An array of characters is a string Strings must be terminated by a null character \0 Null characters have a value of 0 char Greet[7]={„H‟,‟e‟,‟l‟,‟l‟,‟o‟,‟\0‟};

char Greet[7]={„H‟,‟e‟,‟l‟,‟l‟,‟o‟,0};

Strings may be initialized individually as shown above

Or all togetherchar Greet[]=“Hello”;

Compiler fill positions



Strings

What is the difference between the two statements?

char ch = „x‟;

char str[ ] = „x‟;

ch is a character constant

str is a string constant

str includes a null character



Pointers

Used for indirect access to program elements single variables, arrays, structures, functions, etc.

Declared using “splat” notation: data-type *name; where data-type is the data type of data the pointer is pointing to andname is the pointer name

Pointer functionality is similar to the @ use in assembler Pointers may be operated on just like any other variable

compiler automatically adjusts pointer registers to data size similar to array and index combination



Pointer Initialization

Pointers contain addresses

pointers must always be manually initialized

“&” notation used to initialize pointers to single variables

array name may be used to initialize array pointersint Var; //declare a variable

int *VPtr; //declare pointer to int variable

char Array1[] = “Hello”; //declare and init an array

char *APtr; //declare pointer to array

VPtr = &Var; //initialize variable pointer

APtr = Array1; //initialize array pointer

Aptr = &Array1[0]; //does same as previous line

OR char *APtr = Array1;



Pointer Operations

Use “splat” to get at the data pointed to by the pointer interpret as “the contents of” called „de-referencing‟ the pointer

Suppose we have: char *Ptr1;

Ptr1=0x20; sets the pointer to point to address 0x20 *Ptr1=0x34; sets the contents of address 0x20 to

0x34 Ptr1 += 5; adds 5 to the pointer to point to

address 0x25

*Ptr1=0x67; 0x67 loaded into 0x25 *Ptr1 = *Ptrl + 2; adds 2 to the contents of address

( *Ptr1 += 2) 0x25. Will become 0x67 +2 = 0x69



Pointer Operations

Pointer arithmetic may increment or decrement pointers

may add values to or subtract values from pointers

pointer will always be modified by the number of bytes in the data type it

is pointing to:

int *IntPtr;char *ChPtr;IntPtr++; //pointer incremented by 2

bytesChPtr++; //pointer incremented by 1

byte



Pointers vs Arrays

Pointers are a more efficient way to pass data tofunctions

Arrays are always passed by reference (i.e. using the

base address of the array)

Array names may be used interchangeably withpointer names

array names used without indices are interpreted as thebase address of the array



Pointers vs Arrays

Code example:

void Add(char *Ptr1, char Length)

{ while(Length != 0x00) //check if all elements complete{

*Ptr1 += 0x03; //add 3 to array elementLength--; //decrement loop countPtr1++; //move to next element

}}void main(void){

char Array[] = {0,1,2,3,4}; //define and initialize arraychar *APtr = Array; //define and init pointer to array

Add(Array,5); //call function using array variable Add(APtr,5); //call function using pointer Add(&Array[0],5); //call function using array element

}



Structures

Structures provide a method of grouping data may contain data of different types, including pointers and arrays

Structure declaration

members may be of different types

may have multiple variables defined

struct struct-name {

type member1;type member2;. . .

} variable-name;

Structure Tag

Member variables}Variable



Declaring Structures

1 - First declare the template, then the variable namestruct Data{

int Cntrl;int Speed;

char Fault;};

struct Data Motor1;

void main (void){

Motor1.Cntrl = 0x7854; Motor1.Speed = 0xF38C;

Motor1.Fault = 0x56;------; }

Structure Tag

Member variables

} No Variable



Declaring Structures

2 - Declare the template + The variable namestruct Data{

int Cntrl;int Speed;

char Fault;} Motor1, Motor2;

void main (void){

Motor1.Cntrl = 0x7854;

Motor1.Speed = 0xF38C;Motor1.Fault = 0x56;

Motor2.Speed = 0xF4CC;

------; }

Structure Tag

Member variables

} Variable Names



Accessing Structures

Structure members are accessed using “dot” notation

variable-name.memberx

Member may be used as a variable of the type of memberx struct MyStruct

{char Ch[20]; //character string variableint Length; //string length variable

} StrData;

StrData.Ch[0]=„H‟; //initialize element 0 of string

StrData.Ch[1]=„i‟; //initialize element 1 of stringStrData.Ch[2]=„\0‟; //initialize element 2 of string

StrData.Length=3; //initialize string length



Casting Variables

Allows variables of one type to be converted to a variable of another type “square peg in a round hole” precede variable to be cast with the desired data type enclosed in parens

What happens in this situation?

upper byte of x is unknown Different types!!!!

unsigned int x; //define 16-bit integerunsigned char y; //define 8-bit character

y=0x32; //initialize y

x=y; //set x equal to y



Casting Variables

Previous situation can be avoided by casting

y is now treated like an integer during assignment

upper byte of Y set to 0

unsigned int x; //define 16-bit integerunsigned char y; //define 8-bit character

y=0x32; //initialize y

x=(unsigned int)y; //set x equal to y



Casting Variables

Situations in which to cast copying smaller data type to a larger one

char -> int

int -> long

float -> double assigning a pointer to a constant

when in doubt...



ASK !!!!!!

C Compiler Basics_8051

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