Files, Processes & Memory

Every program you use begins as a file, an inert sequence of bytes resting on a disk, and only becomes a living, computing thing when the operating system turns it into a process. Learning to separate the file, the process, and the memory that process runs in is one of the clearest "aha" moments in understanding how computers actually work. These three ideas are the operating system's core abstractions, and almost everything else in computing is built on top of them.

The three abstractions

A file is a named, persistent sequence of bytes stored on a device such as an SSD or hard drive. Its defining trait is persistence: a file survives power off, reboots, and even the program that created it. It has a name and a path (for example C:\Users\me\report.txt), and it simply sits there until something reads, writes, or deletes it.

A process is a running program. When the OS launches a program, it creates a process and gives it three essential things: its own private virtual address space (so it behaves as if it owns all of memory), a program counter (a CPU register holding the address of the next instruction to run), and a set of resources such as open file handles. A process is active and temporary, it exists only while the program is running.

Memory, in this context, means the RAM a process uses while it runs. RAM is fast, volatile working storage: its contents vanish when power is lost. Each process reaches its memory through virtual memory, an OS mechanism that maps the process's virtual addresses onto physical RAM, giving every process the illusion of a clean, private address space.

The key distinction is this: code and data at rest live in a file on disk; to run, the OS creates a process and loads that code into RAM. The file is the recipe; the process is the meal being cooked.

Two related ideas, briefly. Threads: a single process may run several threads, independent streams of execution that share the same memory and resources. the OS maps each process's virtual addresses to physical RAM, paging inactive pages out to disk when RAM runs short.