Question

How is it possible that many popular programs, such as Microsoft Office, are written in languages such as C, C++ and assembly yet the same installer works on any computer with the appropriate OS? Is it because the installer actually compiles the source code on the computer? Is it because AMD and Intel share basically the same instruction set?

I have heard C is portable, but in what sense is Java more portable than C? What then is the point of a virtual machine?

Answer 1

1. There is nothing inherantly nonportable about C or C++. Portability concerns only arise when calling operating system or compiler-specific functions. In the case of assembly language programming, the lack of portability is generally due to instruction set concerns. If restricting the target machine set appropriately, those portability problems may or not matter.
2. The “appropriate OS” is a hint that portability (in the strictest sense) is not a concern, since a well-defined execution environment exists. That is, the binary provided will only run on Intel ISA machines running Windows.
3. The installer almost certainly does not compile source code for the computer.
4. Yes, because of the shared instruction set (and operating system in this case), the program should run equally well on both. If you tried to run that binary on a computer with a different archicture (ARM, for example), or under a different operating system (Linux, perhaps), you’d be out of luck.
5. Java provides a bunch of APIs and a virtual machine that can (sometimes) allow code to be compiled once and then deployed on multiple architectures and operating systems. Because they abstract the interface to the system through Java API, you don’t have to worry about porting all of that code when you want to deploy on a different system. C provides only some standard libraries, but doesn’t specify a particular machine architecture.
6. The point of the virtual machine is precisely this abstraction – instead of writing a program that compiles to code running natively in the instruction set of the host processor, you write one that compiles to code that runs in the virtual machine. The implementer of the VM takes care of the necessary translation from VM instructions to native host instructions and the translation back of the results.