BOOTSTRAP: The Goal
Bootstrap is a term that refers to starting up the computer. The term comes from the phrase "pull yourself up by the bootstraps". The goal most Unix systems shoot for is to boot up, get all the devices running, launch all the appropriate software applications, connect to the network, start a User session, and let the user log in.
Power On Self Test (POST)
Virtually every computer out there goes through the POST process. When powered on, a computer will load a stored program from it's chipset to perform this test. This usually entails verifying that hardware previously found is still there and still available. In some cases, it involves probing the computer to find new haredware, and performing simple testing to see if it is functioning correctly, such as a memory parity test. If hardware fails to respond, the POST sends a message to the console, or beeps and halts the bootup process. If no errors are found it proceeds with the bootstrap process.
BASIC I/O
If the system passes the POST test, it then launches the local machine's basic input/output software, that is also stored on chips within the computer. On any system with an Intel based mainboard (motherboard) there is a chip called the "BIOS". On other systems it may go by the name of the "PROM" or even "PRAM". This chip loads a tiny program into RAM. That program searches the disk drives for a copy of the operating system.
DISK DRIVES
Most disk drives (EIDE and SCSI) contain a master boot record. Within this master boot record is contained another program that is loaded by the basic I/O program and then run. This program finds the actual operating system filesystem and begins loading the kernel of the Unix operating system.
KERNEL
The Kernel loads into memory and begins execution. The Kernel initializes all it's devices via drivers that are either built in or loaded dynamically into RAM. Once the drivers are loaded, various startup scripts are run. On System V unix systems, the computer starts with the lowest level run command script and works it's way up to the last script identified by a value in the inittab file.
DAEMONS
The system may or may not launch various demons at this point. What demons are launched and when depends on the exact run level chosen by the user the last time the machine was reset.
USER INTERFACE
Next a User interface is provided, somtimes before the Network connectivity has been established, sometimes after, sometimes not at all. This is dependent upon what "run level" the system is booted in. A graphical interface may be launched at this point, but regardless, a console session is opened to allow the user to log into the system via the LOGON application.