How a Computer Wakes Up

by Ron White

WHEN you hit your PC’s On switch, nothing much seems to happen for several seconds.

Actually, your computer is going through a complex set of operations to make sure all its components are working properly and to warn you if something is a miss. This operation is the first step in an even more complicated process called the boot-up, or simply, the boot.

The term comes from the idea of lifting yourself up by your own bootstraps. In a PC, bootstrapping is necessary because the PC must have some way of bringing all its components to life long enough to load an operating system. The operating system then takes on more complicated tasks that the boot code alone can’t manage, such as making the PC’s hardware interact with software.

But before your PC can even attempt to load an operating system, it has to make sure that all the hardware components are running and that the CPU (central processing unit) and memory are functioning properly. This is the job of the power-on self-test, or POST.

The POST is the first task your PC performs when you turn it on, and it’s your first warning of trouble with any of the components. When the POST detects an error from the display, memory, keyboard, or other basic components, it produces an error warning in the form of a message on your display and—in case your display is part of the problem—in the form of a series of beeps.

Usually neither the beeps nor the on screen message is specific enough to tell you exactly what is wrong. All they’re intended to do is to point you in the general direction of the component that has a problem.

A single beep combined with a display of the normal DOS prompt means that all components have passed the POST. But any other combination of short beeps and long beeps usually means trouble. Even no beep at all indicates a problem.

If no error message appears or beeps occur, however, that doesn’t mean all the hardware components of your system are functioning as they should. The POST is capable of detecting only the most general types of errors. It can tell if a hard drive that’s supposed to be installed isn’t there, but it can’t tell if there is trouble with the drive’s formatting.

All in all, the POST does not appear to be extremely helpful. That’s because most PCs function so reliably that only occasionally something triggers a POST alarm. The POST’s benefits are subtle but fundamental. Without it, you could never be sure of the PC’s ability to carry out its tasks accurately and reliably.

How the Power-On Self-Test Works

1. When you turn on your PC, a process called the POST (power-on self-test) begins with an electrical signal following a permanently programmed path to the CPU, or microprocessor. There, the electrical signal clears leftover data from the chip’s internal memory registers.

The signal also resets a CPU register called the program counter to a specific number. In the case of ATs and later computers, the hexadecimal number is F000. The number in the program counter tells the CPU the address of the next instruction that needs processing.

In this case, the address is the beginning of a boot program stored permanently at the address F000 in a set of read-only memory (ROM) chips that contain the PC’s basic input/output system (BIOS).

2. The CPU uses the address to find and invoke the ROM BIOS boot program, which in turn invokes a series of system checks. The CPU first checks itself and the POST program by reading code loaded into various locations and checking it against identical records stored permanently in the BIOS chip set.

3. The CPU sends signals over the system bus—the circuits that connect all the components with each other—to make sure that they are all functioning.

4. The CPU also checks the system’s timer, or clock, which is responsible for pacing signals to make sure all the PC’s operations function in a synchronized, orderly fashion.

5. The POST tests the memory contained on the display adapter and the video signals that control the display. It then makes the adapter’s BIOS code a part of the system’s overall BIOS and memory configuration. It’s at this point that you’ll first see something appear on your PC’s monitor.

6. The POST runs a series of tests to ensure that the RAM chips are functioning properly. The tests write data to each chip, then read it and compare what they read with the data sent to the chips in the first place. On some PCs at this point, you’ll see on the monitor a running account of the amount of memory that’s been checked.

7. The CPU verifies that the keyboard is attached properly and determines whether any keys have been pressed.

8. The POST sends signals over specific paths on the bus to the floppy and hard disk drives and listens for a response to determine which drives are available.

9. The results of the POST tests are compared with data in a specific CMOS chip that is the official record of which components are installed. CMOS is a type of memory chip that retains its data when power is turned off, as long as it receives a trickle of electricity from a battery. Any changes to the basic system configuration must be recorded in the CMOS setup. If the tests detect new hardware, you’re given a chance to update the configuration on the setup screen.

10. Some systems’ components, such as a SCSI controller card, contain a BIOS which interprets commands from the processor to control that hardware. Those components’ BIOS codes are incorporated as part of the system’s own overall BIOS. Sometimes these BIOS codes are copied from the slow CMOS BIOS chip to the PC’s faster RAM. Newer PCs may also run a Plug and Play operation to distribute system resources among different components. (See Chapter 5.) The PC is now ready to take the next step in the boot process: loading an operating system from disk.