On address vector processors, the CS3 library provides an array of pointers to interrupt handlers named __cs3_interrupt_vector_form, where form identifies the particular processor variant the vector is appropriate for. Each entry in the vector holds a reference to a symbol named __cs3_isr_name, where name is the customary name of that interrupt on the processor, or a number if there is no consistently used name. You can find the interrupt vector details in Section 5.6, “Interrupt Vector Tables”. The particular vector used by a given CS3-supported board is documented in the tables in Section 5.5, “Supported Boards for ARM EABI”.

CS3 provides a reasonable default definition for each __cs3_isr_name handler. Many of these symbols are aliased to a common handler routine. If your program stops at a default interrupt handler, its name as shown in backtraces may therefore not correctly reflect which interrupt occurred.

To override an individual handler, provide your own definition for the appropriate __cs3_isr_name symbol. The definition need not be placed in any particular object file section.

To override the entire interrupt vector, you can define __cs3_interrupt_vector_form. You must place this definition in a section named .cs3.interrupt_vector. The linker script reports an error if the .cs3.interrupt_vector section is empty, to ensure that the definition of __cs3_interrupt_vector_form occupies the proper section.

You may define the vector in C with an array of pointers using the section attribute to place it in the appropriate section. For example, to override the interrupt vector on Altera Cyclone III Cortex-M1 boards, make the following definition:

typedef void handler(void);
handler *__attribute__((section (".cs3.interrupt_vector")))
  __cs3_interrupt_vector_micro[] =
{ ... };

Interrupt handlers typically require special call/return and register usage conventions that are target-specific and beyond the scope of this document. In many cases, normal C functions cannot be used as interrupt handlers. For example, the EABI requires that the stack be 8-byte aligned, but on some ARMv7-M processors, only 4-byte stack alignment is guaranteed when calling an interrupt vector. This can cause subtle runtime failures, usually when 8-byte types are used.

As an alternative to writing interrupt handlers in assembly language, on ARM targets they may be written in C using the interrupt attribute. This tells the compiler to generate appropriate function entry and exit sequences for an interrupt handler. For example, to override the __cs3_isr_nmi handler, use the following definition:

void __attribute__ ((interrupt)) __cs3_isr_nmi (void)
{
  ... custom handler code ...
}