I am writing an assembly FIQ ISR using GCC complier. I want to change return address in FIQ ISR so that after ISR, it returns to the requested address. the reason for doing that is because i am writing a RTOS.
what are the instruction for ISR return in assembly? is it
SUBS PC, R14, #4
? i know in 8051, noraml function call return is RET, ISR return is RETI.
will cause CPSR to be replaced by SPSR_FIQ
SUB PC, R14, #4
will cause CPSR to remain the same (not be replaced by SPSR_IRQ)
an LDM instruction that references the PC will cause CPSR to be replaced by SPSR_FIQ.
If you are doing a task switch, you definately want to set the CPSR to the correct value, and not what it currently is in the FIQ routine (this usually means that you will need to put into SPSR_FIQ the value that you would like CPSR to be after the return).
thanks, Robert.
You mean if it is
LDMFD R13!, {R0-R7,R14}
the CPSR will remain the same.
But if it is:
LDMFD R13!, {R0-R7,PC}
the CPSR will replaced by SPSR_IRQ.
I have tried the second one, CPSR still remains the same, still in FIQ mode.
Change
to
LDMFD R13!, {R0-R7,PC}^