Embedded C

Embedded C .

1. C Language

1.1. Introduction

• Programming language is a set of instructions that used to communicate with a computer and control its behavior.
• C is a general-purpose procedural programming language.

1.2. Expression

• An expression is a combination of operators, constants and variables that evaluate to a value.
• There are several types of expressions, including:
  • constant expr: 5, 10+3
  • integral expr: x*y
  • floating expr: x + 10.75
  • relational expr: x <=y
  • logical expr: x > y && z == 6
  • pointer expr: &x
  • bitwise: x<<3

1.3. Variables & Data Types

• A variable is a named location in memory that stores data. Every variable must be declared to specify its type. e.g. int a;
• Common Data Types in C:

On a 32-bit machine, short is not efficient because the CPU is designed to process 32-bit data in one step.

1.4. Operators

• An operator is a symbol that performs an operation on value.
• There are several operators, including:
  • Arithmetic operators are used to perform arithmetic operations on operands, include addition +, subtraction -, multiplication *, division /, and remainder %
  • Logical operators are used to combine two or more conditions into a single expression, include and &, or |, and not !
  • Bitwise operators are used to perform bit-level operations on the operands, include and &, or |, exclusive-or ^, negation ~, left shift <<, and right shift >>
  • Relational operators are used for the comparison of the two operands, include equal ==, not equal !=, greater/e >/ >=, and less/e </<=
  • Assignment operators are used to assign value to a variable, include = , and =`
  • Casting operators convert one data to another (new_type) operand;
  • Other: & * …

When adding/subtracting integer to pointer, the integer is multiplied by the size of the pointed data. e.g. long *p; p+1 adds 4 to the address value p. Thus, p[i] is an abbreviation of *(p+i)

1.5. Statements

• A statement is a complete instruction that the program executes.

An expression produces a value, but a statement performs an action.

• There are several kinds of statements, including:
  • Expression statements a=b
  • Condition statements if(con), switch
  • Iteration statements while(ter_con), for(init,ter_con,do_end_step), do{state} while(ter_con);
  • Compound statements { // list of statements }
  • Jump statements goto, break, continue

1.6. Data Structures

• Data structures is a way of organizing data in memory for efficient use.
• It can be classified into two main types: built-in and user-defined
• Built-in data structures includes arrays, structures and unions.
• User-defined data structures are implemented using structures and pointers.

1.6.1. Array

• An array is a collection of elements of the same type stored in contiguous memory locations.

A multi-dimensional array is an array with more than one dimension, 2-D,3D etc.

1.6.2. Structure

• A structure is a user-defined data type that groups different types of data together.
• Data alignment means placing data in memory at addresses that match the CPU’s preferred boundaries. (short 2 - others 4)
• Padding is extra unused memory added by the compiler to satisfy alignment requirements.
• e.g.

// ======== Memory layout ========
// address	content
// 0	    a         (1 byte)
// 1–3	    padding   (3B)
// 4–7	    b         (4 bytes)
struct Example {
    char a;   // 1 byte
    int b;    // 4 bytes
};

1.6.3. Union

• A union is like a structure, but its members are allocated at the same address and share the same memory area.
• e.g.

// ======== Memory layout (union) ========
// All members share the same starting address
// address    content
// 0–3        i (4 bytes)
//            f (4 bytes)
//            c (1 byte, but overlaps)

union Data {
    int i;     // 4 bytes
    float f;   // 4 bytes
    char c;    // 1 byte
};

enum Type {
    INT_TYPE,
    FLOAT_TYPE
};

struct TaggedData {
    enum Type type;   // tag
    union {
        int i;
        float f;
    } data;
};

1.6.4. Enum

• An enumeration (or enum) is a user defined data type that contains a set of named integer constants.

1.7. Function

• A function is a reusable block of code used to perform a specific task.
• A function pointer is a pointer that stores the address of a function, allows indirect function calls.
• A function prototype tells the compiler what the function looks like before it is actually defined. int f(int, int *);

1.8. Preprocessing Directives

• Declarations mean giving types to variables or functions, using keyword extern. e.g. extern int x;

• Definitions mean defining the contents for variables or functions. e.g. int x = 1;

• The keyword extern is used to declare a variable or function defined elsewhere.

• The keyword static is used to specify internal linkage at file scope or to extend the lifetime of a variable inside a function.

2. C Embedded

2.1. Program Memory Model

• The memory layout of a program shows how its data is stored in memory during execution. It typically include the following segments:
  • Code (Text) Segment: stores executable code and is usually read-only.
  • Data Segment:
    • Initialized Data Segment: contains global and static variable that are initialized.
    • Uninitialized Data Segment (BSS): stores global and static variable that are not initialized.
  • Heap Segment: is used for dynamic memory allocation.
  • Stack Segment: is used for storing local variables, functions params, and return addresses for each function call.