FIQ handler environment

AN3129

4/12

Doc ID 16927 Rev 1

3

FIQ handler environment

The FIQ registration, the ISR function and the runtime environment of a FIQ handler are

different from a standard IRQ handler. A working example of an FIQ handler can be found in

latency measurement module in Section 4: IRQ latency measurement.

3.1

FIQ registration

For FIQ registration ‘set_fiq_handler’ API is used.

The two arguments of this API are:

●

Function pointer

●

Size of handler function in bytes

This handler function is copied to the FIQ vector reserved area. The FIQ vector area is last

in the memory area reserved for interrupt vectors. About five hundred (500) bytes are

available in this area for copying the ISR.

In the SPEAr Linux Support Package (LSP), the following API must be used to select the

configuration of the IRQ source:

●

vic_unmask_irq(int number) – enables interrupt source on VIC as IRQ

●

vic_select_fiq(int number) – selects interrupt source on VIC as FIQ instead of IRQ

3.2

FIQ mode registers setup

When the FIQ handler runs, it utilizes FIQ mode-specific registers (r8-r14) and general

mode registers (r0-r7). This set of FIQ mode registers is exclusive to FIQ mode and not

available to other handlers.

To pass any information to the FIQ handler, the FIQ mode registers have to be set using the

call: ‘set_fiq_regs’.

3.3

Return from FIQ mode

To return from FIQ mode on completion of the code, use the following assembly instruction:

subs

pc, lr, #4;

This command restores the instruction pointer to the original location it was before FIQ

arrived.

3.4

FIQ ISR in C

Writing the FIQ handler function in ARM assembly language can be cumbersome, so it is

highly recommended to use C language instead. This is especially true for large functions

when the handler function is larger than the maximum size available for FIQ vector (500

bytes). In such cases branching to the C function can be possible. This involves saving of

the register context (r0-r12 general mode registers) and initialization of the function frame

pointer.