Declaration of SFR (Special Function Register) declaration

The SFR type is a special case of the standard BYTE type, designing a processor Special Function Register, at an absolute address.

The SFR keyword allows the programmer to declare and manipulate an alias of a SFR.

To the program, an SFR appears to be an ordinary RAM variable.  

The syntax for an SFR declaration is:

sfr-declaration = VAR identifier ":" SFR "@" address-constant-expression ";" .

Mainly, this is equivalent to declaring a VOLATILE BYTE variable, but there's some differences:

• The variable name is not internally prefixed by the module or procedure/function name.
• The variable is always global.

<identifier> is treated as a variable name and <address-constant-expression> is the file register address of the particular SFR.

User-declared SFR variables are always treated as bankable by PMP. The compiler will automatically handle bank switching as needed.

• PMP automatically defines all processor's registers definitions as absolute SFR variables (such as INDF, TMR0 …), so these identifiers cannot be redefined in the source code.
• A user-declared SFR is always assumed to be VOLATILE: there's never any read/write optimizations on SFRs.
• A pointer to SFR or a procedure / function argument of type SFR are also treated as volatile.

It is possible to declare a pointer to an SFR:

VAR MySfrPtr : ^SFR;