The compiler is thus told to allocate storage for these variables. When the compiler processes any other .C files, the header file (uCOS_II.H) looks as shown below because OS_GLOBALS is not defined and thus OS_EXT is set to extern.

In this case, no storage is allocated and any .C file can access these variables. The nice thing about this technique is that the declaration for the variables is done in only one file, the .H file.

1.04 OS_ENTER_CRITICAL() and OS_EXIT_CRITICAL()

Throughout the source code provided in this book, you will see calls to the following macros:

OS_ENTER_CRITICAL() and OS_EXIT_CRITICAL(). OS_ENTER_CRITICAL()

is a macro that disables interrupts and OS_EXIT_CRITICAL() is a macro that enables interrupts. Disabling and enabling interrupts is done to protect critical sections of code. These macros are obviously processor specific and are different for each processor. These macros are found inOS_CPU.H. Listing 1.3

shows the declarations of these macros for the 80x86 processor. Section 9.03.02 discusses why there are two ways of declaring these macros.

Listing 1.3, Macros to access critical sections.

Your application code can make use of these macros as long as you realize that they are used to disable and enable interrupts. Disabling interrupts obviously affect interrupt latency so be careful. You can also protect critical sections using semaphores.

1.05 PC Based Services

1.05.02 PC Based Services, Elapsed Time Measurement

The elapsed time measurement functions are used to determine how much time a function takes to execute. Time measurement is performed by using the PC’s 82C54 timer #2. You make time measurement by wrapping the code to measure by the two functions and PC_ElapsedStop(). However, before you can use these two functions, you need to call the function PC_ElapsedInit(). PC_ElapsedInit()basically computes the overhead associated with the other two functions. This way, the execution time returned by PC_ElapsedStop() consist exclusively of the code you are measuring. Note that none of these functions are reentrant and thus, you must be careful that you do not invoke them from multiple tasks at the same time. The example in listing 1.4 shows how you could measure the execution time of PC_DispChar(). Note that time is in microseconds (µS).

INT16U time;

PC_ElapsedInit();

.

.

PC_ElapsedStart();

PC_DispChar(40, 24, ‘A’, DISP_FGND_WHITE); time = PC_ElapsedStop();

Listing 1.4, Measuring code execution time.

1.05.03 PC Based Services, Miscellaneous

A µC/OS-II application looks just like any other DOS application. In other words, you compile and link your code just as if you would do a single threaded application running under DOS. The .EXE file that you create is loaded and executed by DOS and execution of your application starts from main(). Because µC/OS-II performs multitasking and needs to create a stack for each task, the single threaded DOS environment must be stored in case your application wishes to quit µC/OS-II, and return to DOS. Saving the DOS environment is done by calling PC_DOSSaveReturn(). When your application needs to return to DOS (and exit µC/OS-II), you simply call PC_DOSReturn(). PC.C makes use of the ANSI C setjmp() and longjmp() functions to save and restore the DOS environment, respectively. These functions are provided by the Borland C/C++ compiler library and should be available on most other compilers.

You should note that a crashed application or invoking exit(0) without using PC_DOSReturn()

can leave DOS is a corrupted state. This may lead to a crash of DOS, or the DOS window within Windows 95.

PC_GetDateTime() is a function that obtains the PC’s current date and time, and formats this

information into an ASCII string. The format is “MM-DD-YY HH:MM:SS” and you will need at least 19 characters (including the NUL character) to hold this string. PC_GetDateTime() uses the Borland

Compiler:

Optimizations:

Optimizations:

Global register allocation

Invariant code motion

Induction variables

Loop optimization

Suppress redundant loads

Copy propagation

Dead code elimination

Jump optimization

Inline intrinsic functions

Register variables:

Automatic

Common subexpressions:

Optimize globally

Optimize for:

Speed

It is assumed that the Borland C/C++ compiler is installed in the C:\CPP directory. If your compiler is located in a different directory, you will need to change the path in the Options/Directories menu of the IDE.

µC/OS-II is a scalable operating system which means that the code size of µC/OS-II can be reduced if you are not using all of its services. Code reduction is done by setting the #defines OS_???_EN to 0 in OS_CFG.H. You do this to disable code generation for the services that you will not be using. The examples in this chapter makes use of this feature and thus, each example declares their OS_???_EN appropriately.