Giving your Computer the Boot
By this, I don't mean kicking your computer when it freezes or fails to respond! Booting is the initial process that happens when you turn on or power on your computer. The term was coined by comparing the start of a computer with getting dressed.
Correction: Sorry, I forgot to mention in last month's table a very important component – the power supply. It changes your household current into energy necessary for your computer to run.
What happens every morning just after you get dressed, yet before you're ready to leave the house? Why you put your shoes or boots on, of course! Similarly, the booting of a computer is the last step before a computer is ready to interact with the user – after it is dressed with hardware, but before it's operational.
Last month we discussed the primary categories of hardware and their purpose. Granted, it was an overwhelming list, but there is a method in my madness. In order to know what occurs during booting, you must first be familiar with hardware components as well as what they do.
Booting is essentially a two-step process that consists of two primary steps. The IML or Initial Machine Load followed by the IPL (Initial Program Load). Briefly, the IML loads the hardware, while the IPL loads the software. These two processes normally flow seamlessly together during the entire boot process, unless there is a problem in one of the steps. In that case, you will get an error message describing the problem. The diagram below illustrates the hierarchy in how these processes operate (bottom to top). The IML and IPL overlap with the processes in the diagram, as follows:
(bottom to top)
In the Power On Self Test, the power supply checks the electrical current levels to make sure they are sufficient for all of the basic system components – CPU and motherboard.
(Read Only Memory-Basic Input Output System) Also known as, Firmware, ROM-B.I.O.S. is a set of initial boot instructions that are implanted into the ROM chip on the motherboard.
During this phase, your computer reads code instructions that identify all partitions on the hard drive and specifically the boot partition – where in your operating system the boot process starts.
The Operating System performs three initial actions:
1. Loads kernel HW and registry information.
2. Configures all HW.
3. Searches for additional SW drivers that run other HW devices.
The Operating System loads the log on and security SW that prompts you for a user profile and password. After this is complete, the OS will run the main file management system. (Desktop)
The source for the above and some of the below material is http://220.127.116.11/search?q=cache:kSOpvQRPOukJ:www.adrc.com/ckr/windows_bootup_process.html+study+guide+bootup+process&hl=en&strip=1
I will now describe the procedure for each of these mega-processes. I urge you to refer to the table in last month's article, http://elephant.org.il/do_it_yourself/, so as not to get lost in the shuffle. Keep in mind that I will not describe every process – for that you can go to Wikipedia.
The IML (also called a pre-booting sequence or P.O.S.T [Power on Self Test])
- When you turn on the computer (power on), the power supply (see note above) sends current to the motherboard and the CPU. The power supply will continue to send current as long as the computer is on.
- If these machine components are working properly, the computer emits a beep (in PCs) or another type of sound (in Macs) indicating that the current is sufficient for the rest of the IML. The CPU now initiates the ROM-BIOS.
- P.O.S.T. continues to perform additional checks on the system, such as:
- Whether the boot is from a previously off system (cold) or a reboot using the OS (warm).
- If there are peripheral (read external) devices on the system such as a printer.
- How much memory RAM is installed on the system.
- The CPU then searches for the CMOS chip on the motherboard along with any necessary switches or jumpers necessary to assign system resources for this session. These can vary depending on a combination of the motherboard, chips set thereon, and the OS. Different systems will perform their own diagnostic tests.
- The system now tries to load the hardware devices that match these diagnostics. For example, in a PC it will search for the bootup device (either the old floppy drive or more likely the hard drive; we will see in latter articles that you can configure these priorities).
- The system, as directed by the CPU, finds the boot code instructions for this bootup device (Master Boot Record for PCs). It copies these instructions into the RAM, and runs them.
- After running this code successfully, the system searches for the boot partition(s) on the hard drive. These are the sections on the hard drive reserved for booting the computer alone. They must be distinguished from other partitions on the drive for OS to load correctly. A bootup menu displays on the screen offering a choice of more than one OS whenever an additional OS has been installed.
This last step (see the Boot Loader process in the table and chart above)
overlaps with the IPL.
- System files continue to operate and pass on information from the IML. The system begins searching for then loading the kernels and registry information of the OS.
- The OS collects and configures all devices that have been installed.
- The OS searches for any hardware profiles and loads the software drives necessary to control these.
- The OS loads the log on and security files that prompt you for verification such as a password.
Next time: If you truly understood all of the above – more power to you! This information will help tremendously as we look, both for choice parts for your computer as well as get into the types of configurations required to put it all together.