What does executable file actually contain?

Question

What does executable actually contain ? .. Does it contain instructions to processor in the form of Opcode and Operands ? If so why we have different executables for differe

Answer 1

To do any form of IO, an executable needs to interface with the Operating System using sys-calls. in Windows these are calls to the Win32 API and on linux/unit these are mostly posix calls.

Furthermore, the executable file format differs with the OS the same way a PNG file differs from a GIF file. the data is ordered differently and there are different headers and sub-headers.