17.3.1 Vector Attribute

A handler function can be associated with one or more exception vector addresses via an attribute. The at_vector attribute indicates that the handler function should itself be placed at the exception vector address. The vector attribute indicates that a dispatch function should be created at the exception vector address(es) which will transfer control to the handler function.

For example, the following declaration specifies that function foo will be created as an interrupt handler function of priority four. foo will be located at the address of exception vector 54.

void __attribute__ ((interrupt(IPL4SOFT))) __attribute__ ((at_vector(54))) foo void);

The following declaration specifies that function foo will be created as an interrupt handler function of priority four. Define dispatch functions targeting foo at exception vector addresses 52 and 53.

void __attribute__ ((interrupt(IPL4SOFT))) __attribute__ ((vector(53, 52))) foo void)

Handler functions that are linked directly to the vector will be executed faster. Although the vector spacing can be adjusted, there is limited space between vectors and linking a substantial handler function directly at a vector may cause it to overlap the higher vector locations, preventing their use. In such situations, using a dispatch function is a safer option.

The newer devices family features variable offsets for vector spacing. The compiler and linker work together to treat the OFFnnn SFRs as initialized data so that they are initialized at startup. This means there is no need for application code to initialize the OFFnnn SFRs. This also means that it is often more efficient to place the ISR within the vector table rather than using a dispatch function.

#include <xc.h>
#include <sys/attribs.h>
void
__ISR_AT_VECTOR(_CORE_TIMER_VECTOR, IPL7SRS)
CoreTimerHandler(void)
{
   // ISR code here
}