Operating Systems · topic
How a program enters the kernel (dual mode, traps, system calls), what the OS keeps for each process (PCB, states, context switch), how fork() multiplies processes and what it prints, and how threads share an address space under the many-to-one, one-to-one and many-to-many models.
An operating system (OS) is the program that controls the hardware and gives every other program a cleaner, safer machine to run on. It plays two roles at once. As a resource manager it shares the CPU, memory, disks and devices among competing programs and keeps them from damaging each other.
A program is a passive file of instructions on disk. A process is a program in execution: the code together with a program counter, register values, a stack, a heap, open files and everything else the OS must remember to stop it and later continue it exactly where it left off.
In UNIX-like systems a new process is made in two separate steps. fork() creates a near-exact copy of the calling process; exec() then replaces the copy's program with a new one. A shell runs a command by forking, letting the child exec the command, and waiting for the child to finish.
A thread is a single stream of execution inside a process. A traditional process has one thread; a multithreaded process has several, all running the same program in the same address space.
One-page visual sheets for this topic, in the sheet library on the store. Previews are free; the full sheets come with the subject pack.
