Embedded systems programming.ppt

Chapter 25
Embedded systems programming
Bjarne Stroustrup
www.stroustrup.com/Programming

Abstract
 This lecture provides a brief overview of what
distinguishes embedded systems programming
from “ordinary programming.” It then touches
upon facilities that become prominent or
problems when working “close to the
hardware” such as free store use, bit
manipulation, and coding standards.
Remember: not all computers are little grey
boxes hiding under desks in offices.
2
Stroustrup/Programming

Overview
 Embedded systems
 What’s special/different
 predictability
 Resource management
 memory
 Access to hardware
 Absolute addresses
 Bits – unsigned
 Coding standards
3

Embedded systems
 Hard real time
 Response must occur before the deadline
 Soft real time
 Response should occur before the deadline most of the time
 Often there are plenty of resources to handle the common cases
 But crises happen and must be handled
 Predictability is key
 Correctness is even more important than usual
 “correctness” is not an abstract concept
 “but I assumed that the hardware worked correctly” is no excuse
 Over a long time and over a large range of conditions, it simply doesn’t
4

Embedded systems
 Computers used as part of a larger system
 That usually doesn’t look like a computer
 That usually controls physical devices
 Often reliability is critical
 “Critical” as in “if the system fails someone might die”
 Often resources (memory, processor capacity) are limited
 Often real-time response is essential
5

Embedded systems
 What are we talking about?
 Assembly line quality monitors
 Bar code readers
 Bread machines
 Cameras
 Car assembly robots
 Cell phones
 Centrifuge controllers
 CD players
 Disk drive controllers
 “Smart card” processors
6
 Fuel injector controls
 Medical equipment monitors
 PDAs
 Printer controllers
 Sound systems
 Rice cookers
 Telephone switches
 Water pump controllers
 Welding machines
 Windmills
 Wrist watches
 …

Do You Need to Know This Stuff ?
 Computer Engineers – You will build and oversee the building
of these systems
 All “close to he hardware” code resembles this
 The concern for correctness and predictability of embedded systems
code is simply a more critical form of what we want for all code
 Electrical Engineers – You will build and oversee the building
of these systems.
 You have to work with the computer guys
 You have to be able to talk to them
 You may have to teach them
 You may have to take over for them
 Computer scientists – you’ll know to do this or only work on
web applications (and the like)
7

Predictability
 C++ operations execute in constant, measurable time
 E.g., you can simply measure the time for an add operation or a virtual
function call and that’ll be the cost of every such add operation and every
virtual function call (pipelining, caching, implicit concurrency makes this
somewhat trickier on some modern processors)
 With the exception of:
 Free store allocation (new)
 Exception throw
 So throw and new are typically banned in hard real-time
applications
 Today, I wouldn’t fly in a plane that used those
 In 5 years, we’ll have solved the problem for throw
 Each individual throw is predictable
 Not just in C++ programs
 Similar operations in other languages are similarly avoided
8

Ideals/aims
 Given the constraints
 Keep the highest level of abstraction
 Don’t write glorified assembler code
 Represent your ideas directly in code
 As always, try to write the clearest, cleanest, most
maintainable code
 Don’t optimize until you have to
 People far too often optimize prematurely
 John Bentley's rules for optimization
 First law: Don’t do it
 Second law (for experts only): Don’t do it yet
9

 You (usually) have to be much more aware of the resources
consumed in embedded systems programming than you have
to in “ordinary” programs
 Time
 Space
 Communication channels
 Files
 ROM (Read-Only Memory)
 Flash memory
 …
 You must take the time to learn about the way your language
features are implemented for a particular platform
 Hardware
 Operating system
 Libraries
10

 A lot of this kind of programming is
 Looking at specialized features of an RTOS (Real Time
Operating System)
 Using a “Non-hosted environment” (that’s one way of
saying “a language right on top of hardware without an
operating system”)
 Involving (sometimes complex) device driver architectures
 Dealing directly with hardware device interfaces
 …
 We won’t go into details here
 That’s what specific courses and manuals are for
11

How to live without new
 What’s the problem
 C++ code refers directly to memory
 Once allocated, an object cannot be moved (or can it?)
 Allocation delays
 The effort needed to find a new free chunk of memory of a
given size depends on what has already been allocated
 Fragmentation
 If you have a “hole” (free space) of size N and you allocate an
object of size M where M<N in it, you now have a fragment of
size N-M to deal with
 After a while, such fragments constitute much of the memory
12
Free space
New object
old object
old object

 Solution: pre-allocate
 Global objects
 Allocated at startup time
 Sets aside a fixed amount of memory
 Stacks
 Grow and shrink only at the top
 No fragmentation
 Constant time operations
 Pools of fixed sized objects
 We can allocate and deallocate
 No fragmentation
 Constant time operations
13
Pool:
Stack:
Top of stack

 No new (of course)
 And no malloc() (memory allocation during runtime) either (for those
of you who speak C)
 No standard library containers (they use free store indirectly)
 Instead
 Define (or borrow) fixed-sized Pools
 Define (or borrow) fixed-sized Stacks
 Do not regress to using arrays and lots of pointers
14

Pool example
// Note: element type known at compile time
// allocation times are completely predictable (and short)
// the user has to pre-calculate the maximum number of elements needed
template<class T, int N>class Pool {
public:
Pool(); // make pool of N Ts – construct pools only during startup
T* get(); // get a T from the pool; return 0 if no free Ts
void free(T*); // return a T given out by get() to the pool
private:
// keep track of T[N] array (e.g., a list of free objects)
};
Pool<Small_buffer,10> sb_pool;
Pool<Status_indicator,200> indicator_pool;
15

Stack example
// Note: allocation times completely predictable (and short)
// the user has to pre-calculate the maximum number of elements needed
template<int N>class Stack {
public:
Stack(); // make an N byte stack – construct stacks only during startup
void* get(int N); // allocate n bytes from the stack; return 0 if no free space
void free(void* p); // return the last block returned by get() to the stack
private:
// keep track of an array of N bytes (e.g. a top of stack pointer)
};
Stack<50*1024> my_free_store; // 50K worth of storage to be used as a stack
void* pv1 = my_free_store.get(1024);
int* pi = static_cast<int*>(pv1); // you have to convert memory to objects
void* pv2 = my_free_store.get(50);
Pump_driver* pdriver = static_cast<Pump_driver*>(pv2);
16

Templates
 Excellent for embedded systems work
 No runtime overhead for inline operations
 Sometimes performance matters
 No memory used for unused operations
 In embedded systems memory is often critical (limited)
17

How to live with failing hardware
 Failing how?
 In general, we cannot know
 In practice, we can assume that some kinds of errors are more common than
others
 But sometimes a memory bit just decides to change
 Why?
 Power surges/failure
 The connector vibrated out of its socket
 Falling debris
 Falling computer
 X-rays
 …
 Transient errors are the worst
 E.g., only when the temperature exceeds 100° F. and the cabinet door is closed
 Errors that occur away from the lab are the worst
 E.g., on Mars
18

How to live with failing hardware
 Replicate
 In emergency, use a spare
 Self-check
 Know when the program (or hardware) is misbehaving
 Have a quick way out of misbehaving code
 Make systems modular
 Have some other module, computer, part of the system
responsible for serious errors
 In the end, maybe a person i.e., manual override
 Remember HAL ?
 Monitor (sub)systems
 In case they can’t/don’t notice problems themselves
19

Absolute addresses
 Physical resources (e.g., control registers for external devices)
and their most basic software controls typically exist at
specific addresses in a low-level system
 We have to enter such addresses into our programs and give a
type to such data
 For example
Device_driver* p = reinterpret_cast<Device_driver*>(0xffb8);
Serial_port_base *COM1 =
reinterpret_cast<Serial_port_base*>(0x3f8);
20

Bit manipulation: Unsigned integers
 How do you represent a set of bits in C++?
 unsigned char uc; // 8 bits
 unsigned short us; // typically 16 bits
 unsigned int ui; // typically 16 bits or 32 bits
// (check before using)
// many embedded systems have 16-bit ints
 unsigned long int ul; // typically 32 bits or 64 bits
 std::vector<bool> vb(93); // 93 bits
 Use only if you really need more than 32 bits
 std::bitset bs(314); // 314 bits
 Use only if you really need more than 32 bits
 Typically efficient for multiples of sizeof(int)
21

Bit manipulation
 & and
 | inclusive or
 ^ exclusive or
 << left shift
 >> right shift
 ~ one’s complement
22
0 1 0 0
1
0
1
1 0xaa
0 0 0 1
1
1
1
0 0x0f
0 0 0 0
1
0
1
0 0x0a
a:
a&b:
b:
0 0 0 1
1
0
0
0 0x03
b>>2:
1 1 1 0
0
0
0
1 0xf0
~b:
0 1 0 1
1
1
1
1 0xaf
a|b:
0 1 0 1
0
1
0
1 0xa5
a^b:
1 0 1 0
0
1
0
0 0x54
a<<1:

Bit manipulation
 Bitwise operations
& (and)
| (or)
^ (exclusive or – xor)
<< (left shift)
>> (right shift)
~ (one's complement)
Basically, what the hardware provides right:
 For example
void f(unsigned short val) // assume 16-bit, 2-byte short integer
{
unsigned char right = val & 0xff ; // rightmost (least significant) byte
unsigned char left = (val>>8) & 0xff ; // leftmost (most significant) byte
bool negative = val & 0x8000 ; // sign bit (if 2’s complement)
// …
}
23
1 1 0
0 1
0 1 0 1 1
0 0
1
1
0
Sign bit
1 1 1 1
1
1
1
1
0xff:
8 bits == 1 byte
0
1 0 0
1
0
val
0 1 1
false
true

Bit manipulation
 Or |
 Set a bit
 And &
 Is a bit set? Select (mask) some bits
 For example:
enum Flags { bit4=1<<4, bit3=1<<3, bit2=1<<2, bit1=1<<1, bit0=1 };
unsigned char x = bit3 | bit1; // x becomes 8+2
x |= bit2; // x becomes 8+4+2
if (x&bit3) { // is bit3 set? (yes, it is)
// …
}
unsigned char y = x &(bit4|bit2); // y becomes 4
Flags z = Flags(bit2|bit0); // the cast is necessary because the compiler
// doesn’t know that 5 is in the Flags range
24
1 1 1 1
1
1
1
1
0xff:
0 1 1
1
1
val 0 0 0

Bit manipulation
 Exclusive or (xor) ^
 a^b means (a|b) & !(a&b) “either a or b but not both”
unsigned char a = 0xaa;
unsigned char b = 0x0f;
unsigned char c = a^b;
 Immensely important in graphics and cryptography
25
0 1 0 0
1
0
1
1 0xaa
0 0 0 1
1
1
1
0 0x0f
0 1 0 1
0
1
0
1 0xa5
a:
a^b:
b:

Unsigned integers
 You can do ordinary arithmetic on unsigned integers
 Avoid that when you can
 Try never to use unsigned just to get another bit of precision
 If you need one extra bit, soon, you’ll need another
 Don’t mix signed and unsigned in an expression
 You can’t completely avoid unsigned arithmetic
 Indexing into standard library containers uses unsigned
(in my opinion, that’s a design error)
vector<int> v;
// …
for (int i = 0; i<v.size(); ++i) …
for (vector<int>::size_type i = 0; i<v.size(); ++i) …
for (vector<int>::iterator p = v.begin(); p!=v.end(); ++p) …
26
unsigned
correct, but pedantic
Yet another way
signed

Complexity
 One source of errors is complicated problems
 Inherent complexity
 Another source of errors is poorly-written code
 Incidental complexity
 Reasons for unnecessarily complicated code
 Overly clever programmers
 Who use features they don’t understand
 Undereducated programmers
 Who don’t use the most appropriate features
 Large variations in programming style
27

Coding standards
 A coding standard is a set of rules for what code should
look like
 Typically specifying naming and indentation rules
 E.g., use “Stroustrup” layout
 Typically specifying a subset of a language
 E.g., don’t use new or throw to avoid predictability problems
 Typically specifying rules for commenting
 Every function must have a comment explaining what it does
 Often requiring the use of certain libraries
 E.g., use <iostream> rather than <stdio.h> to avoid safety problems
 Organizations often try to manage complexity through
coding standards
 Often they fail and create more complexity than they manage
28

Coding standards
 A good coding standard is better than no standard
 I wouldn’t start a major (multi-person, multi-year) industrial project
without one
 A poor coding standard can be worse than no standard
 C++ coding standards that restrict programming to something like the C
subset do harm
 They are not uncommon
 All coding standards are disliked by programmers
 Even the good ones
 All programmers want to write their code exactly their own way
 A good coding standard is prescriptive as well as restrictive
 “Here is a good way of doing things” as well as
 “Never do this”
 A good coding standard gives rationales for its rules
 And examples
29

Coding standards
 Common aims
 Reliability
 Portability
 Maintainability
 Testability
 Reusability
 Extensibility
 Readability
30

Some sample rules
 No function shall have more than 200 lines (30 would be even better)
 that is, 200 non-comment source lines
 Each new statement starts on a new line
 E.g., int a = 7; x = a+7; f(x,9); // violation!
 No macros shall be used except for source control
 using #ifdef and #ifndef
 Identifiers should be given descriptive names
 May contain common abbreviations and acronyms
 When used conventionally, x, y, i, j, etc., are descriptive
 Use the number_of_elements style rather than the numberOfElements style
 Type names and constants start with a capital letter
 E.g., Device_driver and Buffer_pool
 Identifiers shall not differ only by case
 E.g., Head and head // violation!
31

Some more sample rules
 Identifiers in an inner scope should not be identical to identifiers
in an outer scope
 E.g., int var = 9; { int var = 7; ++var; } // violation: var hides var
 Declarations shall be declared in the smallest possible scope
 Variables shall be initialized
 E.g., int var; // violation: var is not initialized
 Casts should be used only when essential
 Code should not depend on precedence rules below the level of
arithmetic expressions
E.g., x = a*b+c; // ok
if( a<b || c<=d) // violation: parenthesize (a<b) and (c<=d)
 Increment and decrement operations shall not be used as
subexpressions
 E.g., int x = v[++i]; // violation (that increment might be overlooked)
32

An example of bit manipulation
 The Tiny Encryption Algorithm (TEA)
 Originally by David Wheeler
 http://143.53.36.235:8080/tea.htm
 Don’t look too hard at the code (unless you happen to need a
good simple encryption algorithm for an application); it’s
simply to give you the flavor of some bit manipulation code
 It takes one word (4 bytes at a time)
 E.g., 4 characters from a string or an image file
 It assumes 4-byte long integers
 Explanation is at the link (and in the book)
 Without the explanation this is just an example of how bit manipulation
code can look. This code is not meant to be self-explanatory.
33

TEA
void encipher(
const unsigned long *const v,
unsigned long *const w,
const unsigned long * const k)
{
unsigned long y = v[0];
unsigned long z = v[1];
unsigned long sum = 0;
unsigned long delta = 0x9E3779B9;
unsigned long n = 32;
while(n-->0) {
y += (z << 4 ^ z >> 5) + z ^ sum + k[sum&3];
sum += delta;
z += (y << 4 ^ y >> 5) + y ^ sum + k[sum>>11 & 3];
}
w[0]=y;
w[1]=z;
} 34

TEA
void decipher(
const unsigned long *const v,
unsigned long *const w,
const unsigned long * const k)
{
unsigned long y = v[0];
unsigned long z = v[1];
unsigned long sum = 0xC6EF3720;
unsigned long delta = 0x9E3779B9;
unsigned long n = 32;
// sum = delta<<5; in general, sum = delta * n
while(n-->0) {
z -= (y << 4 ^ y >> 5) + y ^ sum + k[sum>>11 & 3];
sum -= delta;
y -= (z << 4 ^ z >> 5) + z ^ sum + k[sum&3];
}
w[0]=y;
w[1]=z;
} 35

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