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Differences between a ISR and a function[procedure call]:
1. ISR call can occur even between instructions.
2. ISR are transparent to the program.
3. Address generation for ISR [Vectored and non vectored interrupts]can be different from funtion call address generation
4. ISR should not take any parameters and should not return anything [Ideally].
5. ISR should be as short as possible.

Sometimes, ISR can be coded in assembly level language as well, for faster execution, as there is no need to convert the high level language into assembly level language. They can also be written in C, it depends on what type of ISR it is.

Another questions related to this is it possible for ISR to be written in user space ?

ISR run in Interrupt context. Function calls run in user-space context.