in isr there is no return value but in function call there 
is return value

In function call the main program and the funtion have some 
relations but in ISR there is no need for this so there is 
no need of return value in ISR.

In case of a function call the arguments,local variables and
return address is stored in the stack where as in case of an
ISR, after executing the current instruction the context is
saved with no return value.

isr will be executed only when the system is interrupted 
whereas the function call meant for any operations to 
perform eventhough when there is no interrupt

Simply, an ISR (interrupt service routine) can be defined 
as a hardware function call. Generally, a CPU must 
immediately pay attention to a hardware interrupt call. An 
ISR is always given higher priority than a normal function 
call.

this is with reference to ARM7. The function call is made 
when cpu is in User mode(non-priveleged). 
 But an ISR is called from an Interrupt handler and cpu is 
in one of the priveleged modes. Whenever an interrupt 
occurs the control moves to a particular address in vector 
table.

regards
Achal

Here are some of the differences:
 1. Fuction call has no inputs and return args
 2. ISR is triggered by external events
 3. ISR may need special interrupt handling upon entry and  
    exit.
    i.e. IRQs masking to enable or disable interupts, 
 4. Function call and ISR stack frame are different
 5. ISR returns with an interupt return (IRET) to
    restored not only call stack context but also 
    interrupt level.

isr : is the priority oriented. all other functions are 
waiting ,to complete these routene, whenever we can call.

dont u think isr is define in particular sector of memory 
with return and all task are suspended at this time but 
function call is user define with the main program with 
return value.?

There is no comparison between function call and an ISR.
Both are used in a completely different paradigm. Hence
comparison doesn't exist.

ISR is interrupt service routine. it is seperate code 
written some where in the memory. if any  interrupt comes 
mp checks the priority of the interrupt and service it 
meanwhile the content of the progaram counter value is 
written in stack after executing isr ,execution transferred 
to main program. 
FUNCTION CALL is seperate thing. function call is seperate 
code that has to be written at the end of function . for 
example, in some programs we want to perform one operation 
repeatedly at that time we perform that operation by 
calling that function instead of writting many times.

adding to d ans-11 i wud say that....an interrupt has no 
relation with the current program or task being performed 
by the microcontroller....it is an independent task or 
program that is executed whenever the interrupt is invoked.

however, a function is a part of the program that is being 
currently executed by the microcontroller, hence it is 
local.it can be called only when the microcontroller is 
executing the program in which it is contained..or is a 
part of.

ISR has priority associate whereas function call doesn't
because of this stack structure of them are different.
If we look assembly code : function call has RET and ISR have
IRET.