Typically, when a CPU starts up, it does some internal consistency checks and transfers control to a PROM or EPROM device that contains permanent coding meant to survive a power loss. In some computers, this is all the code they need. However, in many general computing devices, this read-only memory defines a BIOS or Basic I/O System, capable of finding a boot sector on a standard secondary memory device. In some cartridge type game machines, the BIOS transfers control to the cartridge after doing some preliminary tests to make sure the machinery is working correctly. On other machines like the PC and Mac, the BIOS calls a utility from read only memory and lets that check the machinery.
Power On Self-TestEdit
The POST, or Power On Self-Test triggers the initialization of peripheral devices and the memory of the computer, and may, if preset parameters are set to allow it, do a preliminary memory check. It also sets the bottom of memory so that the operating system knows how much memory it has to work with.
Once the Power On Self-Test is completed, the computer attempts to pass control to the boot sector of the secondary memory device. In cases like the PC where you may have multiple secondary memory devices, it can sample each device in turn, according to either a standard pattern, or according to parameters set in a battery protected static RAM device. The boot sector is part of the bootstrap system that loads the specific operating system.
Before boot sectors, the operating system had to be fully loaded before it could be run. The utility of the boot-strap process is that it uses a process analogous to a technique developed for climbing: climber's spare boots were used as weights to sling a light line called the bootstrap up over a promontory, and then by tying the bootstrap to a heavier line and pulling on the light line, eventually allow the climber to sling his heavier climbing rope up and over the promontory.
What the bootstrap program does is allow a small sector-sized program to load a loader that eventually loads the operating system. The exact number of intermediate loaders needed depends on the operating system. In DOS, the boot sector loads IO.sys and Msdos.sys, which loads config.sys to configure the computer, then loads command.com, and runs Autoexec.bat on it. The sector sized program is called the boot sector.
Once the kernel is fully loaded, the next step in initialization is to set the kernel parameters and options, and add any modules that have been selected in the kernel set-up file. Once the kernel is fully initialized it takes over control of the computer and continues initialization with the file-systems and processes.
File System InitializationEdit
The kernel starts up the processes, and loads the file-systems. The main file-system then includes initialization files, which can be used to set up the operating systems environment, and initialize all the services, daemons, and applications.
Plug and PlayEdit
The idea of Plug and Play is that during initialization, the system builds up a database of devices and afterwards an application can access the database, to find out details about any device on the system. Along with this form of initialization comes an extended BIOS on the device, which allows it to detect the operating system and set its parameters to be more compatible with that operating system. This combination is especially useful, for secondary initialization where you might want to initialize an application (like windows) to accept the drivers for that device.
A further extension of this concept was developed for the Universal Serial Bus, which allows devices to be hot socketed, or installed while the computer is running. The USB bus contacts the new device and learns from it the necessary information to match it to a driver. This information gets put into the database, and whenever that device is again plugged in, the same driver is found for it. When the device is unsocketted, the driver shuts itself down and removes itself from the list of active devices. It can do this because it can monitor the USB controller to make sure it's device is still attached.