Multiprogramming in Embedded Software

SA,..
I know this is pretty long topic, and discussed heavily while writing multi-threaded applications for desktop, but I have found few books that tackle that subject in Embedded Software. I will also review some basic operating system concepts, that can be found in any real time operating system book.

Embedded Systems Architecture

The minimum software architecture to construct a simple embedded system or even fifa2012 is the following code:

for games, the game ends when the user quits the game by himself

while(!exit)

{

render_graphics();

updateAI();

updatePhysics();

}

The same for an Embdded System, except that the system will exit or shutdown when the user switches off the power.

while(1)

{

readAdc();

calculateWeight();

sendtoPC();

}

This is AKA endless loop, and in games it's called Game Loop.

For Embedded Systems, that basic architecture is simple, efficient (no need for timers or other uC resources). On the other hand, if your application requires to read an ADC data precisely for example 2ms, that architecture won't provide you with the flexibility or the accuracy for that task.

Another issue, that the uC will be busy all the time and it will use it's full power, and that have a dramatic impact on the power consumption, especially if you run your system using batteries...

Basically, you need a scheduler to solve the previous issues.    

Basic  OS Concepts

I remember when I got my hands on windows 98, and I finally managed to play, and was surprised by the computer tech-guy while explaining to me how nice was windows98 that it can play Fifa98, and at the same time you can listen to win-amp. It was interesting feature of the windows series, as the previous operating system, windows 3.11,  it didn't support multi-tasking. 

Scheduling is the method that let processes control the cpu time or its working power. By letting the CPU to switch among the processes like a game or windows media player, it can let the computer be more productive. In a comptuer which has only a single CPU, it can only run one process at a time, like a weight a scale, it only can read the weight from the load cell,..etc. The idea of multi-tasking or multi-threaded applications is to have some process running at all times, in order to utilize the full speed of the CPU at max. The basic idea to achieve that, is to execute the process until it waits for a completion of an I/O request. In a simple system like a small embedded system, the CPU just sits idle, and all that time waiting for an I/O Completion is wasted. Most OSs, several processes are kept in memory at one time, when a process in a wait condition, the OS let the CPU to switch to another process. That previous selection process is a property of a CPU Scheduler.

There are two different types of CPU Scheduling 

1. Non-Preemptive Scheduling, once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU either by terminating (ESC) or by switching to the waiting state (ALT-TAB ;) ). This scheduling method was used by windows 3.1. It provides a single tasking system architecture. 

2 Preemptive Scheduling,  scheduling is prioritized. The highest priority process should always be the process that is currently utilized by the CPU, Windows 95,98..etc works by that technique. it provides a Multitasking system Architecture.

There are different scheduling algorithms like round-robin, you can read more about them at any OS Book.

Next blog isA, will discuss Critical Sections, and issues in multi-tasking, and form a design pattern for that.