c prog for embedded prog.ppt

7/27/2019 C prog for embedded prog.ppt

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Memory UsageCommon memory sections:

Code: holds binary instructions of each function

Static data: stores static data accessible to all functions Stack: stores run-time data local to each function

Heap: holds dynamically allocated data

I/O: does not really hold anything

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Memory Usagestatic char[] greeting=Hello world!;

main()

{int i;

char bVal;

LCD_init();

LCD_PutString(greeting);

}

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Static Data

Code

Heap (grows up)

Stack

(grows down)

Low end

High endI/O addresses


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Variable Scope and StorageGlobal data:

Declared outside all C functions

Visible to all C functions

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Variable Scope and StorageVariables with static storage

Storage space allocated in data sections

Life time is from the start to the end of programexecution

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Variable Scope and StorageVariables in stack:

Local variables of a function go into stack storage

Dynamically allocated and de-allocated every time thefunction is called

Variable life time is from the allocation to thedeallocation

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Variable Scope and Storagescope: Where a variable is visible

int m;

int any_func(){

int m;m = n = 5;

}

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Variable Scope and StorageC global variable (visible to all program files)int global_var;

C file-wide static variables (visible only in this file)

static int static_var;Local static variables

any_func()

{

static int static_var;

}

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Variable Scope and StorageExample: How to define and use global variables

In header file myvar.h

extern int global_var;In program file myvar.c

#include myvar.h

int global_var;

In program file usevar.c#include myvar.h

/* use myvar */

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Data Memory LayoutAlignment: A scalar variable ofn bytes must be aligned

on n-byte boundaries

Examples:

char: no restriction

short: two-byte boundaries

float: four-byte boundaries

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Data Memory LayoutExample: Whats the memory layout? Whats the address

of each variable? Assume a starts at address0x20000000.

char a[5];

short b;

int d;

double e;short f;

char *g;

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Data Memory LayoutByte-ordering: Big-endian vs. Little-endian

Big-endian: Byte 0 of a n-byte variable is the MostSignificant Byte (MSB)

Little-endian: Byte 0 of a n-byte variable is the Least

Significant Byte (LSB)

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Data Memory LayoutWhats the value in string with the two endian types?

static char string[8];* (int*) string = 0x41424344;

string[4] = \0;

Note: ASCII 0x41 is A.

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Data Memory LayoutAddress Memory Contents________30001FF8 25 24 5B 1A AC 57 9C 8D

30002000 00 FE 25 24 5B 1A EE 05

30002008 AC AE 81 83 45 67 89 25

30002010 01 23 55 3F AC CB F0 8D

What are the variable contents on MPC555?

char a[5];

short b;

int d;

double e;

short f;

char *g;14


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Data Memory LayoutWhat is the size of the following structure?

Each component must be aligned

Total size must be a multiple of eight

struct mystruct {

int a;

short b;

double c;

char d;

}

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Function and StackStack: LIFO structure (Last-in, First-out)

Stack top and stack base to track usage

Push operation: add an item at the top and update thetop

Pop operation: remove an item from the top and updatethe top

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Function and StackConventional stack grows downwards

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low address

high address

stack top

stack growth


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Function and StackFunction Frame: Local storage for a function

Example: 1. A is called; 2. A calls B; 3. B calls C; 4. C

returns

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Function and StackWhy stack?

The LIFO order matches perfectly with functionscall/return order

Efficient memory allocation and de-allocation

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Function and StackDetermine frame layout Each variable is aligned Each frame is on eight-byte boundaries

Example: What is the frame size? (Add 8 bytes for

return address and frame linking)int myfunc(){char byVal;

long lVal;char szName[10]; /* statements start here */

}

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Function and StackFunction pointer: How does it work? A function is translated into a block of machine

instructions

Calling the function is to transfer the program control to

the starting address of the block At hardware level this is done by setting the program counter

At assembly programming level this is to branch (jump) tothat address

A function pointer variable stores the pointer to thestarting address of the block

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Function and StackFunction pointer: Use exampleint average(int x, int y){

}

int main(){

int (*pFunc)(int x, int y) = average;int a = (*pFunc) (10, 20);}

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C Library FunctionsIn C many things are carried out by library functions

Simple language, rich libraries

Commonly used libraries File I/O (include user input/output)

String manipulations

Mathematical functions

Process management

Networking

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C Library FunctionsUse standard file I/O

/* include the header file for I/O lib */

#include

main()

{

/* use the fprintf function */fprintf(stdout, %s\n, Hello World\n);

}

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C Library FunctionsFormatted output: printf, fprintf, sprintf and more; useconversion specifiers as follows%s string%d signed decimal%u unsigned decimal%x hex%f floating point (float or double)

How to output the following variables in formata = , b =, c = , str = in a single line?

int a;float b;int *c;char str[10];

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C Library FunctionsFormatted input: scanf, fscanf, sscanf and more

Use similar conversion specifiers as the printf family

How to read the following variables in formata = , b =, c = , str = in a single line? (Usescanf)

int a, b;

float c;

char str[10];

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C Library FunctionsString operations: copy, compare, parse strings and more

#include

strcpy: copy one string to another strcmp: compare two strings

strlen: calculate the length of a string

strstr: search a string for the occurrence of another

string

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C Library FunctionsError processing and reporting: use exit function#include #include

...void myfunc(int x){if (x < 0) {fprintf(stderr, %s

\n,

x is out of range);exit(-1);

}}

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C Library FunctionsMath library functions

#include

n = round (x); /* FP round function */

To build:

gcc Wo myprogram

lm myprogram.c

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C Library FunctionsHow to find more?

On Linux machines: Use manman printf

man string

man string.h

man math.h

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c prog for embedded prog.ppt

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