5.9.2 Global Register Variables

The C/C++ compiler extends the C language by adding a special convention to the register storage class specifier to allow the allocation of global registers. This special global declaration has the form:

The regid parameter can be __R5, __R6, or __R9. The identifiers _ _R5, _ _R6, and _ _R9 are each bound to their corresponding register R5, R6 and R9, respectively.

When you use this declaration at the file level, the register is permanently reserved from any other use by the optimizer and code generator for that file. You cannot assign an initial value to the register. You can use a #define directive to assign a meaningful name to the register; for example:

register struct data_struct *__R5 #define data_pointer __R5 data_pointer->element; data_pointer++;

There are two reasons that you would be likely to use a global register variable:

• You are using a global variable throughout your program, and it would significantly reduce code size and execution speed to assign this variable to a register permanently.
• You are using an interrupt service routine that is called so frequently that it would significantly reduce execution speed if the routine did not have to save and restore the register(s) it uses every time it is called.

You need to consider very carefully the implications of reserving a global register variable. Registers are a precious resource to the compiler, and using this feature indiscriminately may result in poorer code.

You also need to consider carefully how code with a globally declared register variable interacts with other code, including library functions, that does not recognize the restriction placed on the register.

Because the registers that can be global register variables are save-on-entry registers, a normal function call and return does not affect the value in the register and neither does a normal interrupt. However, when you mix code that has a globally declared register variable with code that does not have the register reserved, it is still possible for the value in the register to become corrupted. To avoid the possibility of corruption, you must follow these rules:

• Functions that alter global register variables cannot be called by functions that are not aware of the global register. Use the -r shell option to reserve the register in code that is not aware of the global register declaration. You must be careful if you pass a pointer to a function as an argument. If the passed function alters the global register variable and the called function saves the register, the value in the register will be corrupted.
• You cannot access a global register variable in an interrupt service routine unless you recompile all code, including all libraries, to reserve the register. This is because the interrupt routine can be called from any point in the program.
• The longjmp ( ) function restores global register variables to the values they had at the setjmp ( ) location. If this presents a problem in your code, you must alter the code for the function and recompile rts.src.

The -r register compiler command-line option allows you to prevent the compiler from using the named register. This lets you reserve the named register in modules that do not have the global register variable declaration, such as the run-time-support libraries, if you need to compile the modules to prevent some of the above occurrences.