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Waiting For Windows: The Boot-Up Process

Waiting For Windows: The Boot-Up Process

Most users never think about what happens inside their computer. For most of us, as long as the Windows Desktop appears when we turn it on, we’re content to forge ahead with our computing tasks -- and that’s the way it should be. Behind the screens, however, a computer is a busy place from the moment the power is turned on.

The Windows operating system resides on your system’s hard drive. It’s stored there because hard-drive storage is inexpensive, plentiful, and operating systems are space hogs. In order to contain costs, PCs are designed to use a combination of storage/memory devices known as ROM, DRAM, and hard drives. In this four-part series, we’ll take a brief look at each of these components and what happens when you press the power switch, then twiddle your thumbs while Windows springs or slogs to life. (Caution: Of necessity, portions of this series will require an occasional lapse into geekspeak, but I’ll do my best to keep it to a minimum.)

When you press the power button, you initiate the boot-up process. Many people believe that the term "boot up" derives from the Latin bootus discus. That's not true. It actually derives from the word "bootstrap," which is a loop (sometimes two loops) of leather sewn at the top of a boot to help in pulling it on. Used within the context of computers, it refers to a process that is self-initiating, as in starting-up ("booting") a computer.

The boot-up process begins with a surge or electricity flowing through all the chips, dips, and circuits hidden within your system. Most catastrophic computer failures occur during this phase. Hot electricity hitting cold, delicate computer parts can be problematic. That’s why I leave my desktop computers on all the time and restart them (via Start > Turn Off Computer > Restart) once a week to clear out memory and otherwise reset the system. (This has been discussed in several previous issues, so I won’t revisit the topic at this point.)

Once electricity jump-starts your PC, instructions for what the system should do next are found in the Read Only Memory, Basic Input/Output System or ROM BIOS. ROM, as its name implies, is read-only memory that contains instructions that are permanently burned into the chip. It is non-volatile, which means that its instructions will not be lost or destroyed when the power is turned off.

ROM BIOS, or BIOS for short, begins barking commands as soon as it receives power. The BIOS is, in effect, a computer program written into the chip itself, that oversees and manages the boot-up process. Without the BIOS, the computer would sit there like an expensive dust collector.

The first task that BIOS completes is to make sure that all of your hardware components are present and working properly, such as your disk drives, external ports, mouse, printer, scanner, etc. This is called the Power On Self Test or POST, to the delight of acronym enthusiasts everywhere.

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After the POST is complete -- or “post POST,” if one has a fondness for cheesy word play -- the BIOS activates other chips that reside on various cards (such as video and audio) installed in slots within the computer.

Next, the BIOS passes the digital baton to the Central Processing Unit or CPU. The CPU is a one-chip processor or microprocessor that has two primary functions: It executes all the mathematical operations, and it has the ability to intelligently manage the flow of instructions and data going into and out of its various circuits. (Put your hands together for the CPU!)

The final instruction that BIOS issues to the CPU is where to go -- but in a nice way. It provides an address where the CPU should look for its next instruction in order to continue the boot-up process.

Unlike a street address on an envelope that a disgruntled postal worker then loses, computers use numerical addresses to keep track of information. The larger the number in an address, the more locations it can find. Most current computers use a 32-bit address space for memory (as opposed to emerging 64-bit), which doesn’t sound all that impressive, but a 32-bit address has the capacity for more than 4 billion separate locations to hold information.

The instructions that the BIOS provides to the CPU are transmitted through a chip on a bus (a set of wires) to an address specified. The data bus carries information into and out of the chip within the CPU. The information is not available within the CPU so it has to look elsewhere. The CPU then sends the address on another bus called an address bus. When the CPU does this, it is called a fetch. The address bus "fetches" information from elsewhere within the computer, namely from the computer’s memory.

Memory is a special type of silicon chip that can hold instructions or data. This type of memory is called Dynamic Random Access Memory or DRAM (pronounced DEE-ram), and unlike ROM (which we discussed in Part 1), DRAM is volatile. That means once the power is turned off, DRAM loses its memory or information. Since DRAM is basically a blank slate, the CPU has within it a set of instructions that tells it where to look for required information during the boot-up sequence.

Before the address bus can get to memory, it has to pass through a set of chips called a chipset. The chipset facilitates communication between the primary components of a computer, the CPU, memory, graphics, and I/O (Input/Output) system.

The I/O bus connects the chipset to other places where information is stored, such as the hard drive. The hard drive allows the CPU to read from it and write to it. The hard drive is non-volatile so it retains its data or information once the power is turned off, which is a good thing since all your saved data resides on the hard drive. Today, hard drives are 40, 60, 80GB (gigabytes) or larger. In a word, they’re huge! It takes longer to retrieve data from a hard drive than it does from memory, but memory is more expensive.

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Some day, hard drives as we know them today, will be a thing of the past, and all data will be saved on flash memory or other forms of non-volatile, but virtually instantaneous memory. Turning on a computer will then be much like turning on a calculator or hand-held PDA, where it will be “instant on.” Remember the old days of radio and television when they had to “warm up”? Relatively speaking, that’s the era we’re in today when it comes to computers, but that will be changing as technologies continue to evolve.

Once the hard drive receives an address (via the I/O bus and chipset), it retrieves the information it has been requested to provide, sends it back through the chipset, then puts it on the address bus heading back into the CPU. So the chipset functions as a bridge over troubled buses: the I/O bus and the address bus.

The CPU uses a four-step sequence: Fetch, Decode, Execute, and Store, or FDES, not that anybody refers to it as such. Since the CPU does not retain its memory, it has to obtain information or fetch information from elsewhere within the computer.

Once the information has been fetched, it has to be decoded. During this phase, the CPU has to decide which circuits or pathways are most appropriate to use for executing the instructions. Once that decision has been made, the CPU begins to execute the instructions. The part of the CPU where the actual execution of instructions takes place is called the Arithmetic Logical Unit or (ALU). Attempting to explain the ALU almost put me in the ICU, so trust me, it’s nothing any of us need to know. The final responsibility of the CPU is to store the information it has received. This step takes place after the ALU steps in, which we already agreed that we don’t care about.

The CPU also has an internal clock it uses to keep track of the timing of the flow of information and multiple processes of the computer. This clock is critically important for the synchronization of all of the processes of the computer.

The end result of all this technological gobbledygook is that the CPU locates the operating system, which then loads into memory (RAM). At this point, the operating system (Windows) takes over, and the computer is ready to be used for checking email, visiting Web sites, playing games, or shutting it down and taking a nap.

So the next time you start up your computer and begin to get annoyed because Windows is taking too long to launch, try to have a little patience because there’s a heck of a lot happening just below the surface, and what you’re seeing on screen is really just the tip of the digital iceberg.

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