__fiq: Fast Interrupt

The CARM Compiler supports C fast interrupt functions and eliminates the need to write them in assembly language. Fast interrupt functions are defined using the __fiq function attribute.

void funcname (void) __fiq

Where:

The __fiq attribute affects the generated code of the function as follows:

• The function is generated using ARM instructions even if the THUMB directive is specified.
• On function entry, the registers R0-R7 (when modified) are saved on the stack.
• When exiting the function, all register contents saved on the stack are restored.
• The SUBS PC,R14,#4 instruction is used to return from the __fiq function.

When a fast interrupt is triggered, the ARM device jumps to the __fiq vector location (some ARM devices provide a Vectored Interrupt Controller (VIC) that supports fiq interrupts). By default, the CPU startup code (STARTUP.S) jumps to the function named FIQ_Handler. So, if you name your __irq function FIQ_Handler, no changes to the STARTUP.S file are required.

The following example shows how to use the __fiq attribute with a VIC on a Philips LPC2000 device. The code sequences generated are shown in the Assembler Listing.

stmt  level    source

   1          /*
   2           *  IRQ Handler for ADuC7000 Timer0 Interrupt
   3           */
   4
   5          #include <ADuC7024.H>
   6
   7          volatile int T0_ticks;
   8
   9          void IRQ_Handler (void)  __fiq  {
  10   1        if (IRQSIG & 0x00000004)  {   // Timer0 Interrupt
  11   2          T0CLRI = 1;
  12   2          T0_ticks++;
  13   2        }
  14   1      }

ASSEMBLY LISTING OF GENERATED OBJECT CODE

*** PUBLICS:
 PUBLIC         IRQ_Handler?A
 PUBLIC         T0_ticks



*** DATA SEGMENT '?DT0?irq':
 00000000          T0_ticks:
 00000000            DS          4



*** CODE SEGMENT '?PR?IRQ_Handler?A?irq':
    9: void IRQ_Handler (void)  __irq  {
 00000000  E92D0003  STMDB       R13!,{R0-R1}
   10:   if (IRQSIG & 0x00000004)  {   // Timer0 Interrupt
 00000004  E5100000  LDR         R0,=0xFFFF0004
 00000008  E5900000  LDR         R0,[R0,#0x0]
 0000000C  E3A01004  MOV         R1,#0x4
 00000010  E1100001  TST         R0,R1
 00000014  0A000006  BEQ         L_1  ; Targ=0x34
   11:     T0CLRI = 1;
 00000018  E3A01001  MOV         R1,#0x1
 0000001C  E5100000  LDR         R0,=0xFFFF030C
 00000020  E5801000  STR         R1,[R0,#0x0]
   12:     T0_ticks++;
 00000024  E5100000  LDR         R0,=T0_ticks ; T0_ticks
 00000028  E5901000  LDR         R1,[R0,#0x0] ; T0_ticks
 0000002C  E2811001  ADD         R1,R1,#0x0001
 00000030  E5801000  STR         R1,[R0,#0x0] ; T0_ticks
   13:   }
 00000034          L_1:
 00000034  E8BD0003  LDMIA       R13!,{R0-R1}
 00000038  E25EF004  SUBS        R15,R14,#0x0004
 0000003C          ENDP ; 'IRQ_Handler?A'

Note

• The __fiq attribute is required when declaring or prototyping external functions. For example:

  extern void func (void) __fiq;
  

• Passing parameters to an interrupt function is not possible. Any attempt to define an interrupt function with arguments generates an error message.
• Interrupt function declarations may not include a return value. They must be declared with a void return type. The compiler generates an error message if the return type is not void.
• The CARM run-time library contains a default FIQ_Handler that generates an endless loop using the B $ instruction.