how come i can run something on two different processors ?

[stav]

if i write my code and compile it on a intelx64 bit processor how come im still able to run it on another processor with a different architecture?

[mbozzi]

In general, you can't.

It comes down to binary compatibility.
If the processors both support the subset of the instruction set used by the program with the same semantics, then you won't have to recompile the code.

[SamuelAdams]

I think you'll have to be a little more specific.

[49iURXSz]

Depends on the scope of the question. You can write code and run it through a compiler that converts it into machine-code specific to a particular architecture: x86, amd64, ARM (and its many revisions), mips, etc...