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GCC Signal Attribute
[RTOS Implementation Building Blocks]

The GCC development tools allow interrupts to be written in C. A compare match event on the AVR timer 1 peripheral can be written using the following syntax.
void SIG_OUTPUT_COMPARE1A( void ) __attribute__ ( ( signal ) );

void SIG_OUTPUT_COMPARE1A( void ) { /* ISR C code for RTOS tick. */ vPortYieldFromTick(); }


The '__attribute__ ( ( signal ) )' directive on the function prototype informs the compiler that the function is an ISR and results in two important changes in the compiler output.

  1. The 'signal' attribute ensures that every processor register that gets modified during the ISR is restored to its original value when the ISR exits. This is required as the compiler cannot make any assumptions as to when the interrupt will execute, and therefore cannot optimize which processor registers require saving and which don't.
  2. The 'signal' attribute also forces a 'return from interrupt' instruction (RETI) to be used in place of the 'return' instruction (RET) that would otherwise be used. The AVR microcontroller disables interrupts upon entering an ISR and the RETI instruction is required to re-enable them on exiting.

Code output by the compiler:

;void SIG_OUTPUT_COMPARE1A( void ) ;{ ; --------------------------------------- ; CODE GENERATED BY THE COMPILER TO SAVE ; THE REGISTERS THAT GET ALTERED BY THE ; APPLICATION CODE DURING THE ISR.

PUSH R1 PUSH R0 IN R0,0x3F PUSH R0 CLR R1 PUSH R18 PUSH R19 PUSH R20 PUSH R21 PUSH R22 PUSH R23 PUSH R24 PUSH R25 PUSH R26 PUSH R27 PUSH R30 PUSH R31 ; ---------------------------------------

; CODE GENERATED BY THE COMPILER FROM THE ; APPLICATION C CODE.

;vPortYieldFromTick(); CALL 0x0000029B ;Call subroutine ;} ; ---------------------------------------

; CODE GENERATED BY THE COMPILER TO ; RESTORE THE REGISTERS PREVIOUSLY ; SAVED.

POP R31 POP R30 POP R27 POP R26 POP R25 POP R24 POP R23 POP R22 POP R21 POP R20 POP R19 POP R18 POP R0 OUT 0x3F,R0 POP R0 POP R1

RETI ; ---------------------------------------



Next: RTOS Implementation - The GCC Naked Attribute


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