Computer recovery
Mar 5, 2010 12:10 PM, By Russell Brown
What do you do when you hit the power button on your computer and nothing happens? If you’ve ever been in this situation, whether it’s a company machine or the one sitting at home, you just want it to work again. This article will cover several steps you can take to determine what happened and what you can do about it.
There are two basic scenarios in which this might occur: one is when nothing happens when the power button is hit and the other is when it seems to start up but does not finish.
When nothing happens
Figure 1: Outboard connections
Select figure to enlarge.
After you have checked the obvious, like the outlet and power cord, when nothing happens after you hit the power button, there are a couple of other steps to take. First, disconnect all USB and FireWire (IEEE 1394) connections to the computer. These ports provide power to the peripherals plugged into them. It’s a good idea to also disconnect any Ethernet connections as well. If a hub has gone bad, it can prevent the computer from powering up. If the computer boots after the USB and FireWire ports are cleared, then you can find the failed equipment and replace it. (See Figure 1.)
But if removing all the USB and FireWire peripherals doesn’t bring the computer back, it’s time to check the power supply. Power supplies are the most common failure point in today’s modern electronic equipment. Many modern computer servers have two removable power supplies, which makes it much easier to keep systems up and running; if one fails, an alarm goes off letting you know. If your station has several computers of the same type, it’s a good idea to have a few spare power supplies on hand. If you have some unique but important computers, you should keep a spare power supply on hand for them as well. This will save great deal of time in finding a replacement if it does fail. (See Figure 2.)
What if the computer powers up but doesn’t complete it’s boot process? We’ll cover that after the basics.
Computer boot basics
As a starting point, it’s a good idea to go over what happens when you hit the computer’s power button. Whether it’s a Windows, Macintosh or Lynx system, they all basically boot the same. Whenever a computer is turned on or reset, it is said to be booting (short for “bootstrap loading), which means that the computer is self-loading the main program that will run it.
Figure 2: The USB hub could be the culprit.
Select figure to enlarge.
As power is applied, the CPU is held in reset until the power stabilizes, at which point the power supply issues a “power good” command that allows the CPU to begin operating. The CPU then starts executing commands from a preprogrammed point in ROM, which is the basic input/output system (BIOS) program, also called firmware.
The memory range of a computer is addressed in hexadecimal, or base 16, counting. The first 640KB of memory range from 0 to 9C400 in hex, and beyond this, up to 1MB (or FFFFFhex), is where the ROM BIOS is stored as well as access to other parts of the computer system, which include video and disk controller card addresses. (See Figure 3.)
BIOS
As the CPU executes the BIOS program, it first runs a series of tests called power-on self-tests (POST). To communicate with the operator, the BIOS first looks for the video card and initializes its BIOS to start the video card and be able to address it. After the video card boots, it displays its own startup screen, which is usually the first thing seen on the computer as it boots.
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