How can a program compiled to machine language run on different machines?

Question

In school we've been taught that compilers compile a computer program to machine language. We've also been taught that the machine language consists of direct instructions to the hardware. Then how can the same compiled program run on several computer configurations with different hardware?

Answer 1

Depends what you mean by 'different hardware' if it is the same processor (or same family eg Intel x86) then the machine code instructions are the same.

If the extra hardware is different peripherals (screens, disks printers etc) then the operating system hides those details by giving you a consistent set of instructions to drive them

If you mean, how can you run a program for an ARM cpu on an Intel x86, then you can't - except by some sort of virtual machine emulator that reads each of the ARM instructions and either translates them into x86 or runs the same functionality as a set of x86 funcs and then returns the same answer that the ARM ones would have done.

Edit: I assume you mean PCs with different hw - ie different peripherals but the same processor family?

Talking to hardware doesn't involve specific instructions as such - it's mostly a matter of moving memory to specific locations where the operating system and/or device driver have specifically reserved for data going to that device. In the old days of DOS and BIOS you would then trigger an interupt to call a specific bit of code in the BIOS to act on that data and send it to the HW.

Answer 2

With an emulator or a virtual machine, either of which effectively translates the machine language on the fly.

Answer 3

I think it is more accurate to say that native compilers compile to a specific instruction set of a processor. Since there are families of processors that keep backwards compatibility: 8086 - 80386 - 80486 - 80586 - Dual Core - Quad Core...; then each processor runs the instructions of its ancestors. If you want to port your code across processor architectures, then you need for sure a virtual machine or emulator, like it was mentioned previously.