interrupt

[mehak]

when i use the following syntax in my code it says interrupt declared void but as far as i know this is the correct syntax of interrupt

 void interrupt(*old1b)();
void interrupt(*old23)() ;

[Chervil]

What is the type of old1b ? ... and the type of old23 is?

[mehak]

far pointers

[Chervil]

ok, now specify that in the code.
Also you still did not say what type it is pointing to.

[Jaybob66]

c++ knows nothing of interrupts. handlers are static void functions with no parameters, the address of which goes into the interrupt vector.

static void my_handler();

isr_vec[4] = my_handler;

where did you find that declaration of an interrupt?

I notice that you have asked a few questions about legacy issues like far pointers. Tell us which compiler/version you are using because all of the default advice on these boards will be for modern c++.