Processes and threads

A program on disk is just bytes. A process is that program running: its own memory, its own file descriptors, its own place in the CPU queue. Two copies of the same program are two separate processes that happen to share code.

A process is rarely simply "running". It is ready (waiting for CPU), running (on a core right now), or blocked (waiting for I/O or a lock). Most of a process's life is spent ready or blocked, not running.

The OS tracks each process with a process control block: its id, its state, where its memory starts, which files it has open, and the saved CPU registers. The PCB is the process, as far as the kernel is concerned.

/* PCB — то, что ОС помнит о процессе, пока он не на CPU. */
struct pcb {
  int   pid;             /* кто это                     */
  int   state;           /* READY / RUNNING / WAITING    */
  void *page_table;      /* где лежит его память        */
  struct regs saved;     /* PC, SP — где он остановился */
  int   open_files[16];  /* что он открыл               */
};