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EE472 Lecture Notes Pack

Blake Hannaford, James Peckol, Shwetak Patel

Department of Electrical Engineering

The University of Washington

April 3, 2009

Contents

EE472 EE 472 Introduction

Embedded Microcomputer Systems

Embedded: Special purpose computing devices. Computers that are embedded or hidden inside something else. Examples: Xerox machine controller, Automotive Anti-lock braking system, Airliner autopilot, MP3 Player, DVD player, TiVo Box, CelPhone.

Microcomputer: A computer you can buy for $10 - $1000. Almost all of the same software and hardware issues we will study have been around since the 1960’s. The only thing different is the cost of the hardware. All of the following had essentially the exact same software environment to what we are studying in this class:

Era Technology Cost 1960’s Mainframe $1M 1970’s Minicomputer $100k 1980’s PC’s $5k 1990’s Cellphone $ 2000’s Microcontroller $

Systems: From “The Random House Dictionary”

1. a group or combination of things or parts forming a complex or unified whole. ...
2. a set of body organs or related tissues concerned with the same function. ...
3. Often: an organized set of computer programs.

Chapter 2

C Program Structure

C Programs in Multiple Files

A C program is typically written in multiple files. Simpler editing Helps modularity and multiple authors. File: main.c main () { ... code ... }

fcn1 () { ... code ... }

File: functions.c

fnc2 () { ... code ... }

fcn3 () { ... code ... }

... etc ...

Execution starts with first line of main(). All functions (except main()) require a function prototype in any file in which they will be called or defined. All functions are visible from any file (but prototype must be supplied in the file).

EE472 C Program Structure

... windows - specific code ...

endif

ifdef LINUX

... linux - specific code ...

endif

2.2.3 C pre-processor overview

(draw your own graphic)

2.3 Building a C program

For each “.c” file:

1. Run C pre-processor on the “.c” file
2. Compile output into “object” file.
   • Incomplete machine language program
   • “.o” file for each “.c” file

Then: “Link” all “.o” files, and library files, to create “executable” binary file.

2.4 Variable Scope

• Variable declared inside a function is local.
• Variable declared outside a function is visible anywhere inside that “.c” file.

EE472 C Program Structure

• Variable declared outside a function can also be visible in other “.c” files — if extern is used.
• Variables declared with prefix extern must be declared outside a function in another “.c” file.

2.5 Function Scope

• Functions cannot be used without a function prototype.
• Each “.c” file must have a function prototype for each function which is used in that “.c” file.
• #include can help manage your function prototypes.

EE472 C - Pointers

int szint = sizeof(myInt); int szflt = sizeof(myFloat);

// Assume intPtr has the value 0x // and floatPtr == 0x // and szint = 4, szflt = 8 (bytes)

printf("intPtr = %d\n", (int)intPtr); printf("intPtr+1 = %d\n", (int)intPtr+1); printf("fltPtr = %d\n", (int)fltPtr); printf("fltPtr++ = %d\n", (int)fltPtr++ );

3.1.3 Pointer Dereferencing

Consider

int *pa; // (assume (int)pa == 0x3000) *pa = 0x000B; *(pa++) = 0x0010; *(pa+1) = 0x00C0;

What does memory look like? What are the values of

*pa + 1 == ?? (int)pa == ??

Name Addr 3000

3.1.4 Example: Pointers and Arrays

(Lewis, Section 3.6) Consider the original IBM-PC 80 col x 40 rows, display buffer

This is the graphics hardware that generates the clunky screen that you use to configure your BIOS. addr col 1 col 2 col 3 row 1 B8000 ch cl ch cl ... row 2 B80A0 ch cl ... ... ch ... ...

where ch is a char display character, and cl is a char which encodes the color of that character position.

EE472 C - Pointers

Lewis Approach

#define DISP_BUFFER 0xB int row, col; char disp_char; char *p; // Let’s display "A" in row 4, col 20 disp_char = ’A’; row = 4; // for example ... col = 20; // " "

p = (char *) (DISP_BUFFER + 2 (80row + col)) *p = disp_char;

3.1.5 Exercise in Class

Complete this example: write code to set the byte which controls the color of the character A we just displayed. Assume the color byte should be assigned the value COLOR BYTE. The color byte has 2 4-bit fields which set the background color (16 possible colors) and the character color (16 possible colors).

3.1.6 Arrays

An array is an ordered set of memory elements. For example, int j[3] sets up memory for three integers known as j[0], j[1], j[2]. Graphically, if

int a[5]=0; // array of 5 ints // each int is 2 bytes

Element name a[0] a[1] a[2] a[3] a[4] Address 3000 3002 3004 3006 3008

3.1.7 Initializing Arrays

When you declare an array such as

int a[5];

Enough storage is allocated for 5 integers (typically 5×32 bits or 40 bytes). But you should not count on any initial value. To initialize the array use one of two methods:

EE472 C - Pointers

3.1.10 Exercise in Class

Modify the scheme in Lewis, sec 3.6, so that we can address display characters and display colors each in a separate row/column array.

3.2 Generic (void) Pointers

Sometimes we want a pointer which is not locked to a specific type. It can potentially point to anything.

void *name ; // declare a generic pointer called name

• name can point to anything in the computer.
• name cannot be dereferenced with *
• Must instead assign value of void pointer to a pointer of the type you want.

Examples

void* myGenericPtr; int t, *ip, myvalue = 3; myGenericPtr = &myvalue; // OK t = *myGenericPtr; // NO!!

//*****************

ip = nyGenericPtr; t = *ip; // OK!!

3.2.1 Null Pointer

If a pointer has the value NULL, it points to nothing. NULL is a predefined constant in <stddef.h> or use #define NULL 0 NULL is illegal to dereference. NULL can be tested for:

EE472 C - Pointers

int i,*ip = NULL;

[...]

if(ip == NULL) { // I haven’t defined ip yet } else { // OK, now I can use it! i = *ip;}

3.2.2 Function Pointers

C can have pointers to functions.

type (* functionpointer)(arg list)

Examples

int(*IntFuncPtr)(); \ IntFuncPtr is a pointer to a function with \ no arguments which returns an int

double(*doubleFuncPtr)(int arg1, char arg2) \ doubleFuncPtr is a pointer to a function with \ an int and a char arg which returns a double.

Assignment to function pointers:

int (* IFP)(int x) = NULL; // empty function pointer int realfunction( int x); // an actual function

IFP = &realfunction; IFP = realfunction; // can skip &

Dereferencing function pointers:

(*IFP)(5) // call function realfunction with arg 5

IFP(5) // can also use function pointer just like // original function name

3.3 Pointer Arithmetic

A powerful feature of pointers is the ability to compute with them like integers. However only some operations are allowed with pointers. Allowed:

• Add a scalar to a pointer
• Subtract pointers

Not Allowed:

• Add two pointers

Chapter 4

Constants and Defines in C

C Constants, Defines and Integers 1

4.1 Review of C Constants

We will often need to specify the content of memory very precisely.

4.1.1 HEX and OCTAL review

Entering binary numbers can wear out the 1 and 0 keys on your computer! HEX is a good shorthand for binary since each HEX character maps to a specific bit pattern.

bits HEX Octal 0000 0 0 0001 1 1 0010 2 2 ... ... ... 0111 7 7 1000 8 10 1001 9 11 1010 A 12 1011 B 13 ... ... ... 1111 F 17

Hex Place value: To convert 0xD2FB to decimal:

D 2 F B 212 28 24 20 4096 256 16 1

Octal can also be used and is easier to memorize because there are only 8 symbols (0-7). (^1) B. Kernigan and D. Ritchie, “The C Programming Language,,” 2nd Edition, Prentice Hall, 1988. Daniel W. Lewis, Fundamentals of Embedded Software, Where C and Assembly Meet, Prentice Hall, 2002.

EE472 C - Constants, Defines and Integers

However, to fill a byte of memory requires 2.5 octal characters since each character only encodes 3 bits. For example, HEX 9A = Binary 1011010 = Octal 132

• Ordinary numbers are interpreted by the C compiler as decimal values. Example: 25 = 1101
• Hex values are indicated to the C compiler by the prefix Ox for example: Ox2F
• Octal values are indicated by a leading zero: 028 is a syntax error (why?).

4.1.2 Constant Syntax in C

It is very important to use symbolic constants (#define’s ) for all your numeric constants. Here are the main reasons:

• Code is more readable by humans.
• Code is much easier to change.

Example and Exercise

// Constants #define EXAMPLE_CONST_D 1234 ; /* DECIMAL value / #define EXAMPLE_CONST_H 0x4D2 ; / HEX value / #define EXAMPLE_CONST_O 02322 ; / OCTAL value */

int x = EXAMPLE_CONST_D; int y = EXAMPLE_CONST_H; int z = EXAMPLE_CONST_O;

// test yourself here: (which of these print?) if (x == y) printf ("Hello there ... \n"); else {};

if (y == 1234) printf ("What’s up doc?? \n"); else {};

if (z == 2322) printf ("The Rain in Spain ... \n"); else {};

if (z = 2322) printf ("Falls mainly on the plains.\n"); else {};

Example 2

Suppose you are going to control a CD-ROM drive and you want to open and close the drive door. The bit to open and close the drawer is the 4th bit in a register located at address 0x2DA. Let’s say that to set that bit we use the function

IO_Register_Set([addr],[value])

where [addr] is the bus address of the I/O register you want to manipulate and [value] is the bit pattern you want to put there. It is “technically” correct to use the following code:

EE472 C - Constants, Defines and Integers

/* Use of bitwise logical operators */

#define Bit_Zero 0x #define Bit_One 0x #define Bit_Two 0x #define Bit_Three 0x #define Bit_Four 0x #define Bit_Five 0x #define Bit_Six 0x

// etc etc etc ..

int x = 0x0B; // x = [... 0 0 0 0 1 0 1 1]

int y = 011; // y = [... 0 0 0 0 1 0 0 1]

int z = x << 2; // z = [... 0 0 1 0 1 1 0 0]

int z1 = y & Bit_Three; // z1 = [... 0 0 0 0 1 0 0 0]

int z2 = z | Bit_Four; // z2 = [... 0 0 1 1 1 1 0 0]

EE472 C - Constants, Defines and Integers

4.3 In-class Exercise:

Convert 0x1A to decimal: __________

Convert 0xA2 to decimal: __________

Convert 020 to Hex: _____________________

Convert 0x20 to Octal: _____________________

/* Give the binary value of the least significant 8 bits of the following values: */

int aa = 0xC5; // aa = [ ]

int ab = 017 + 1; // ab = [ ]

int a = x | y; // a = [ ]

int b = Bit_Two | Bit_Five | Bit_Seven; // b = [ ]

int c = ~((z | Bit_Zero) << 1); // c = [ ]

int d = !(z1 | Bit_Two); // d = [ ]