With a mental model of program execution in mind, what happens when you turn on your computer?
- Read-only memory (ROM) on the mainboard is always available for read once the power is on.
- Once power is plugged in, CPU starts executing instructions from ROM. The first program is BIOS.
- BIOS performs Power On Self Test (POST) which checks that necessary hardware devices are present.
- BIOS finds the boot drive (the hard disk) and starts the bootloader program.
- The bootloader loads the operating system image (its programs and data) into memory.
- The operating system takes control.
Now let's dive into the operating system—the first piece of system software.