MPLAB C Compiler for PIC24 MCUs and dsPIC DSCs ... - Microchip

16-Bit C Compiler User’s Guide
For example, to place a variable in the auto_psv space, which will cause storage to
be allocated in Flash in a convenient way to be access by a single PSVPAG setting,
specify:
unsigned int FLASH_variable __attribute__((space(auto_psv)));
Other user spaces that relate to Flash are available:
• space(psv) - a PSV space that the compiler does not access automatically
• space(prog) - any location in Flash that the compiler does not access automatically
Note that both the psv and auto_psv spaces are appropriately blocked or aligned so
that a single PSVPAG setting is suitable for accessing the entire variable.
6.2.2 Managed PSV Access
Just placing something into Flash using the space attribute does not mean the compiler
will be able to manage the access. The compiler requires that you identify variables in
a special way. This is done because the managed PSV can be less efficient than managing
the PSVPAG by hand (though far less complicated).
The compiler introduces several new qualifiers (CV-qualifiers for the language lawyers
in the audience). Like const-volatile qualifier, the new qualifiers can be applied to
pointers or objects. These are:
• __psv__ for accessing objects that do not cross a PSV boundary, such as those
allocated in space(auto_psv) or space(psv)
• __prog__ for accessing objects that may cross a PSV boundary, specifically
those allocated in space(prog), but it may be applied to any object in Flash
Typically there is no need to specify __psv__ or __prog__ for an object placed in
space(auto_psv), though there is no reason why it could be not done.
Moving the FLASH_variable, from the previous section, into an normal Flash space
and requesting that the compiler manage the space is easy:
__psv__ unsigned int FLASH_variable __attribute__((space(psv)));
Reading from the variable now will cause the compiler to generate code that adjusts
the PSVPAG SFR as necessary to access the variable correctly. These qualifiers can
equally decorate pointers:
__psv__ unsigned int *pFLASH;
produces a pointer to something in PSV, which can be assigned to a managed PSV
object in the normal way. For example:
pFLASH = &FLASH_variable;
6.2.3 ISR Considerations
A data access using managed PSV pointers is definitely not atomic, meaning it can
take several instructions to complete the access. Care should be taken if an access
should not be interrupted.
Furthermore an interrupt service routine (ISR) never really knows what the current
state of the PSVPAG register will be. Unfortunately the compiler is not really in any
position to determine whether or not this is important in all cases.
The compiler will make the simplifying assumption that the writer of the interrupt service
routine will know whether or not the automatic, compiler managed PSVPAG is required
by the ISR. This is required to access any constant data in the auto_psv space or any
string literals or constants when the default -mconst-in-code option is selected.
When defining an interrupt service routine, it is best to specify whether or not it is necessary
to assert the default setting of the PSVPAG SFR.
This is achieved by adding a further attribute to the interrupt function definition:
DS51284H-page 82 © 2008 Microchip Technology Inc.