Introduction to C Programming for Embedded Systems

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Introduction to C Programming for Embedded Systems
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History

• C was developed in parallel with UNIX
• Martin Richards wrote BCPL in mid 1960s
• Ken Thompson wrote B in 1970
• Dennis Ritchie designed most creative parts of C
  in 1972
• C is used to re-write UNIX in 1973
• Dennis Ritchie and Brian Kernighan wrote The C
  Programming Language in 1978
• C was standardized during 1983-1988 by ANSI

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History

• C and its predecessors were designed as system
  programming languages
• BCPL needs to be compiled on a DEC PDP-7 machine
  with 8KB 18-bit words
• B was used to write utility programs on a DEC
  PDP-11 with 24KB memory running UNIX
• C was used to re-write that UNIX on the same
  machine
• It has to be simple!

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Hello World!

• include ltstdio.hgt
• main()
• printf(hello, world\n)
• To build and run on a Linux/unix machine
• gcc o helloworld helloworld.c
• ./helloworld
• hello, world

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Compare C and Java/C

• C is a procedural language
• No classes or objects
• Function is the building block
• C philosophy
• As simple as possible
• Uses a minimum set of language constructs

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Functions

• Equivalent to methods in Java, or
  function/procedure in FORTRAN
• ReturnType FunctionName (
• Type Parameter1Name,
• Type Parameter2Name, )
• return expression of ReturnType

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Functions

• / calculate the average of x and y /
• int average(int x, int y)
• int avg_value
• avg_value (x y) / 2
• return avg_value
• / a simpler style /
• int average2(int x, int y)
• return (x y)/2

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Functions

• Procedure equivalent function with void return
  type
• / a procedure that calls three others /
• void do_tasks(int x, int y, int z)
• do_task1()
• do_task2(x, y)
• do_task3(z)

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Functions

• main function the starting point of the
  program
• int main(int argc, char argv)
• int i
• for (i 0 i lt argc i)
• fprintf(parameter d is s\n, i, argvi)
• Other forms
• int main()
• main()

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Functions

• Prototype declare first, define later
• int callee_func(int x, int y)
• int caller_func()
• avg callee_func (a, b)
• int callee_func(int x, int y)

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Types, Variables, and Constants

• Scalar variables
• int i, j, k
• char c
• float x, y
• double z
• Arrays
• float X100
• float pixel1024768

• Structures and unions
• struct
• int x, y
• float weight
• union
• char ch_value
• int int_value
• float float_value

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Types, Variables, and Constants

• Rules on Variable Names
• What can be used A-Z, a-z, 0-9, _ (underscore)
• The first letter must be A-Z, a-z, or
  underscore
• Case sensitive MyVariable and myvariable are
  different
• Variable length as long as wish, but only first
  31 may be significant
• Convention is to use meaningful names
• int height
• float area
• int array_size
• Another style
• int nHeight
• float fArea
• int nArraySize

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Types, Variables, and Constants

• Scalar type integer, character and floating
  point
• Integer types
• int normal integer, 2-, 4- or 8-byte
• short short integer, 2-byte
• long long integer, 4- or 8-byte
• Character type
• char single character, one-byte
• Floating point types
• float single-precision floating point, 4-byte
• double double-precision floating point, 8-byte

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Types, Variables, and Constants

• unsigned Modifier
• unsigned short short_value
• unsigned int int_value
• unsigned char char_value
• It changes the effective range of value
• unsigned short is 0 65535 short is -32,768
  32,767.
• It affects the use of variables in conditions

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Types, Variables, and Constants
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Types, Variables, and Constants

• Initialization
• int n 100
• float x 20.5
• char ch X
• int avg average(n, m)
• int m n/2, k

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Types, Variables, and Constants

• Type casting convert a value from one type to
  another
• (type) expression
• pi (double) 3.1415926
• size (int) (3.14 radius radius)
• if ((unsigned) x gt (unsigned) y)

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Types, Variables, and Constants

• How to use constants?
• Use C macro
• define N 1024
• int arrayN
• Use constant variable
• const double Pi 3.1415926

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Types, Variables, and Constants

• Special constants enumerated type
• enum color_type RED, YELLOW, BLUE
• enum color_type wall_color
• Equivalent to
• define RED 0
• define YELLOW 1
• define BLUE 2

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Types, Variables, and Constants

• C constant values integer
• decimal 100
• hex 0x64
• octal 0144
• Hex starts with 0x, octal starts with 0

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Types, Variables, and Constants

• C constant values character
• \0 NULL character
• \n newline
• \r return
• a letter a
• \t tab
• \040 char of ASCII 0408 or 3210 (space)

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Types, Variables, and Constants

• Floating point IEEE 754 standard
• used less frequently in embedded programs
• very expensive versus integer operations
• (Single precision is 32-bit, and double precision
  is 64-bit)
• Example
• 68HC11 floating point op vs. integer op
• 160 cycles vs. 3 cycles

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Arrays and Pointers

• Type VariableName ArraySize
• Sequence of a specific variable type stored in
  memory
• Zero-indexed starts at zero and ends at N-1 
•  
• int vector8
• int n
• vector0 1 / first element /
• n vector7 / last element /
• vector8 -1 / what happens?! /

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Arrays and Pointers

• Be careful of boundaries in C
• No guard to prevent you from accessing beyond
  array edge
• Write beyond array Potential for disaster
• What exactly is an array?
• Not a specific type
• Pointer to a block of memory
• No built-in mechanism for copying arrays

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Arrays and Pointers

• Example
• / This works /
• nTestArray10 nTestArray20  
• / This does not work /
• nTestArray1 nTestArray2

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Arrays and Pointers

• A variable is just a block of memory
• Address (starting address) Where the block
  starts in the address space
• Size How much space it occupies
• Type How to interpret its value
• Pointer is the starting address of a memory block
• Pointer value memory address
• Pointer variable a variable that holds a memory
  address

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Arrays and Pointers

• Declare Type VarName
• char pLED
• char pSwitch
• Initializes a pointer variable
• pLED 0x0f001000
• pSwitch 0x0f001004
• Uses (Dereferences) a pointer variable
• pLED 0xff / clear all LED bars /
• ch pSwitch / read switch states /

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Array and Pointers

• int nVal
• int pnVal
• pnVal nVal / let it be 0x20000000 /nVal
  10
• pnVal is 0x2000000
• pnVal is 10
• pnVal 5
• pnVal is still 0x2000pnVal is 5nVal is 5

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Arrays and Pointers

• Three key steps when using pointers
• 1. Declare the pointer
• type pName
•   char pChar
• long pHistory
• 2. Initialize the pointer
• In order to use the pointer, we need to point
  it somewhere.
•   pChar (char ) 0x00001800
• pHistory lValue
•   The (char ) tells the compiler this is a
  32-bit memory address, not a 32-bit value.
• 3. Access the pointer (Read/Write)
• In order to get the value, we must use a in
  front of the name.
•   n pChar 0x80
• if((pHistory 25) gt TOL_HISTORY)
• pHistory TOL_MINIMUM

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Arrays and Pointers

• What does the pointer point to?
• Depends upon the system, may not always be RAM
• Two types of architecture
• Unified Memory - Motorola
• All devices, RAM, etc. share the same address
  space
• 0x2000 may be memory, a temperature sensor, hard
  disk
• Split I/O Intel
• Separate addresses for I/O and memory
• Hard disk, PCI cards I/O address space, special
  assembly instructions to access

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Arrays and Pointers

• A memory word read always returns the value of
  the last write
• A device can choose to respond to reads and
  writes however it wants.
• Thus, a write with bit 7 set may behave
  differently than a write with bit 7 clear
• Need to understand the devices programming model
  or interface

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Arrays and Pointers

• Pointer arithmetic and comparison
• Can add and subtract change by the size of
  element
• Can test equality or inequality
• int pInt 0x0f000000
• char pChar 0x0f000004
• (pInt 1) points to 0x0f000004
• (pChar1) points to 0x0f000005
• pChar gt pInt is true
• pChar pInt 1 is true

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Arrays and Pointers

• An array variable has a pointer value
• int XN
• int pX X
• The following are all equivalent
• X2 100
• (X2) 100
• (pX2) 100
• pX2 100
• An array is not really a variable
• X 0xf0000000/ compiler error! /

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Structure and Union

• To define a complex component
• struct point
• int x
• int y
• struct point p1 10, 20
• struct point p2, p3
• p2.x 10 p2.y -p1.y
• p3 p2

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Structure and Union

• Structure and pointer
• struct point p1 10, 20
• struct point ptr p1
• ptr-gtx 20
• (ptr).y 50

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Structure and Union

• Structure and array
• struct key
• char word
• int count
• keytabNKEYS
• keytab0.word auto
• keytab0.count 5

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Structure and Union

• Union Merge multiple components
• union u_tag
• int ival
• float fval
• char sval
• The size of a union variable is the size of its
  maximum component.

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Structure and Union

• Use of union structure
• Struct
• char name
• int flags
• int utype
• union
• int ival
• float fval
• char sval
• symtabNSYM

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Object-Oriented Programming

• typedef struct
• int x
• int y
• point
• init(point p, int x, int y)
• p-gtx x
• p-gty y
• add(point p, int x, int y)
• p-gtx x
• p-gty y