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

Differences Between 16-Bit Device C and ANSI C
auto_psv, no_auto_psv
The auto_psv attribute, when combined with the interrupt attribute, will cause the
compiler to generate additional code in the function prologue to set the PSVPAG SFR
to the correct value for accessing space(auto_psv) (or constants in the constants-in-code
memory model) variables. Use this option when using 24-bit pointers
and an interrupt may occur while the PSVPAG has been modified and the interrupt routine,
or a function it calls, uses an auto_psv variable. Compare this with
no_auto_psv. If neither auto_psv nor no_auto_psv option is specified for an
interrupt routine, the compiler will issue a warning and select this option.
The no_auto_psv attribute, when combined with the interrupt attribute, will cause the
compiler to not generate additional code for accessing space(auto_psv) (or constants
in the constants-in-code memory model) variables. Use this option if none of the
conditions under auto_psv hold true. If neither auto_psv nor no_auto_psv option
is specified for an interrupt routine, the compiler will issue a warning and assume
auto_psv.
boot
This attribute directs the compiler to allocate a function in the boot segment of program
Flash.
For example, to declare a protected function:
void __attribute__((boot)) func();
An optional argument can be used to specify a protected access entry point within the
boot segment. The argument may be a literal integer in the range 0 to 31 (except 16),
or the word unused. Integer arguments correspond to 32 instruction slots in the segment
access area, which occupies the lowest address range of each secure segment.
The value 16 is excluded because access entry 16 is reserved for the secure segment
interrupt vector. The value unused is used to specify a function for all of the unused
slots in the access area.
Access entry points facilitate the creation of application segments from different vendors
that are combined at run time. They can be specified for external functions as well
as locally defined functions. For example:
/* an external function that we wish to call */
extern void __attribute__((boot(3))) boot_service3();
/* local function callable from other segments */
void __attribute__((secure(4))) secure_service4()
{
boot_service3();
}
To specify a secure interrupt handler, use the boot attribute in combination with the
interrupt attribute:
void __attribute__((boot,interrupt)) boot_interrupts();
When an access entry point is specified for an external secure function, that function
need not be included in the project for a successful link. All references to that function
will be resolved to a fixed location in Flash, depending on the security model selected
at link time.
When an access entry point is specified for a locally defined function, the linker will
insert a branch instruction into the secure segment access area. The exception is for
access entry 16, which is represented as a vector (i.e, an instruction address) rather
than an instruction. The actual function definition will be located beyond the access
area; therefore the access area will contain a jump table through which control can be
transferred from another security segment to functions with defined entry points.
© 2008 Microchip Technology Inc. DS51284H-page 21