Why ISR contains no arguments and return type is void

HI all,

Can any one clear me why cant we pass the arguments to an
ISR and also why the ISR returns nothing.

With Regards,
Sudheervemana.

 

Because "we" don't call the ISR --- the CPU does that by itself, and
it has no arguments in hand to pass in, nor would it know what to with
a result value, if there were one.

 

I'm curious. What arguments would you want to pass to an ISR? And where
would the returned thing go?

Keep in mind that generally (except on some CPUs, where 'software
triggered' interrupts are used for OS calls) you have no control exactly
when an interrupt would occur. It doesn't make sense to pass it things,
or have things returned.

Al

 

The ISR gets called by the CPU in the middle of the running code. The
CPU pushes all of the current program's execution information (eg.
program counter, stack pointer and others) and then runs the ISR. So
there really is no calling code for the ISR...thus nobody to pass
arguments or fetch results.

Technically I suppose a CPU could pass an argument to an ISR, but I
don't know if it would be meaningful at all.

Why did you want to? Or are you just curious?

If you want to return information from an ISR to your main program, you
can set global flags in the ISR. Just make sure nobody else is modifying
the global flags when the ISR gets called. You might get an inconsistent
state.

 

This is not true of all processors. With some, the parameter
passed (by the CPU) is the Interrupt number which you can
then use inside the ISR.

tim

 

I'm not the OP, but the interrupt number/index/vector/whatever
might be handy. That way you could hook the same ISR to
multiple interrupts and have it behave slightly differently
based on which interrupt caused it.

For example, you've got 4 UARTs and the receive processing for
all of the is identical (except for which UART you access and
which buffer you put data into). Hooking one ISR to all four
and then using the "parameter" to determine which
UART/buffer-pair to service would be elegent.

 

I honestly don't see the big gain between that and 4 micro-ISRs
that each do essentially just

ACC := {this interrupt's number}
jump to commonISR

 

Can any one clear me why cant we pass the arguments to an

The ISR can't return a value because technically there is no "caller" to the
ISR, so it's declared as type void.

For this same reason no arguments can be passed to the ISR since there is
no caller, no telling where program flow is at the time of the interrupt, etc,,.


-Bruce
http://www.rentron.com

 

Not a big gain, but sometimes a few instructions matter.

 

If they do, then nice style will have to yield to efficiency. On a
CPU with individual ISR vectors, you just have to not use the trick I
just showed. On a single-ISR platform, you'ld have to start worrying
about how to squeeze single instructions out of a multi-way branch
statement.

 

You've got a lot of answers already, so you might have grasped the
concept. In some cases though, when the interrupt handler is called by
the OS, the former can have arguments and return value. In QNX6 we (tinw)
use the interrupt handlers that have the following prototype

const struct sigevent* handler (void* area, int id);

The handler is called by the OS kernel and returns to the OS kernel. It
does not (and should not) deal with interrupt controller directly, has
it's own rather limited stack and can be preempted by a handler of higher
priority interrupt.

BTW, in QNX4 the prototype was

pid_t handler (void);


HTH,
Vadim

 

what about polling the "interrupt" flag (of all the devices connected to
that interrupt line) which is set when an interrupt gets serviced?