GCC Signal Attribute
[RTOS Implementation Building Blocks]

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
    ; ---------------------------------------