System Software and Applications

When a music player opens an audio file, it does not drive the storage chips, decode raw keyboard signals, or decide when it gets CPU time. Someone else handles all of that, and the player simply asks.

Two kinds of program

An application helps a person do something: edit a document, browse the web, play music, work out a budget.

System software supports and runs the computer itself. The operating system is the main example: it manages shared resources and provides common services that applications can request.

Stack them up and every request travels down the same ladder:

person
application
operating system
hardware

Say you open a file. The application asks the operating system for it by name. The operating system finds it, checks whether the request is allowed, and coordinates the storage work. The application never needs to control the drive itself.

That is the whole trick. Each layer offers something simple upward and deals with the mess below. “Open this file” is the same sentence whether the file is on a drive inside the machine, on a memory stick, or on a computer in another country.

These layers let applications ask for stable services while system software handles the machine underneath. They do not make every application portable to every operating system, because applications still rely on the services and formats their target system provides.

One step in the chain is still missing. People write source code that reads almost like English. CPUs run machine instructions that read like nothing at all. Next: what happens in between.