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PC system layers

Understanding hardware and software system layers in a PC, if your troubleshooting a problem in a PC you need to figure out if it's hardware or software related, if it's a software problem you would reload the software, changing the drivers or the configurations of the software, if it's a hardware related problem you would replace the hardware that's defected, a motherboard, a disk drive, a video card, cables etc.

There are four layers in a PC the hardware, the BIOS, the operating system and the application, the application program communicates with the operating system through a standard interface known as the API (Application program interface) this could be a program such as Microsoft word which then translates the data in the word program via the operating system to the BIOS (Basic input output system) layer which in turn translates the data to the systems hardware where the word document can be saved to the hard drive in its binary data form.

Exploring the BIOS and drivers in a PC, If you think of BIOS as being all of the drivers for all of the hardware in a system, you have keyboard drivers, hard disk driver's, video drivers, drivers for your I/O ports so all of the drivers make up the BIOS, the BIOS is the interface between the hardware and the operating system, although the hardware may be different from system to system you need to have the correct drivers for each individual piece of hardware to communicate properly with the operating system so you need to make sure the drivers are the correct ones for the particular operating system in use on the system for example the drivers would be different for Windows 2000 as for operating systems such as Windows XP or Vista even though they're all versions of Windows operating systems.

The ROM BIOS, there are three major BIOS manufacturers AMI, Phoenix and Award, the main difference between RAM and ROM is that RAM is volatile so when the power is cut from the motherboard you lose the data and with ROM which is non-volatile the data remains in the memory. The fundamental difference between BIOS and CMOS is that BIOS is a set of instructions or code used to communicate with system devices and CMOS is an inventory list of the configurations and settings for the system devices.


There are four things in the motherboard ROM chip

  • The POST or power on self test which tests all the main components on the motherboard before it boots up when the system as completed the POST the system beeps.
  • The bootstrap loader which locates the master boot sector and loads it into RAM then the master boot sector loads another sector which loads the kernel files or system files of the operating system.
  • After the operating system is loaded, drivers for the keyboard, floppy disk, hard disk, serial port, parallel port are loaded, these drivers are only used with 16 bit operating systems and are replaced by files from the kernel for the 32 bit operating system driver files to RAM (There are three places driver files can be located the motherboard ROM, RAM and adapter cards which have their own ROM chips such as video, SCSI host adapters and network cards).
  • The BIOS setup program or CMOS (Complimentary metal-oxide semiconductor) is where all the BIOS settings can be configured and saved, you access this screen when you boot up the computer by pressing a key on the keyboard normally Delete or F2 or a combination of keys designated by the BIOS manufacturer.

BIOS setup

The motherboard ROM chip stores the BIOS program but the settings that can be saved in the BIOS is stored on the NVRAM/RTC (Non-volatile RAM/ real time clock) chip which uses the CMOS battery to give power to the NVRAM/RTC chip, it's recommended that you make a note of all the configurations in the BIOS settings especially the hard drive settings in case of the CMOS battery failure (An indication that the battery might need changing is when the clock starts losing time) you might need to reconfigure these settings in order to boot up the computer to the hard drive.

Masked ROMs these chips have the BIOS data etched into the chip using a photolithographic process and are not used anymore.

PROM programmable ROM the BIOS data is burnt into the chip by blowing a fuse; the chips are all ones or are holding a charge before the burning process which then creates the zeros on the chip when the fuse is blown.

EPROM erasable programmable ROM these chips can be erased and reprogrammed by using ultra violet light which is exposed to the chip through a quartz crystal window on the chip.

EEPROM electrical erasable programmable ROM or flash ROM the BIOS data can be downloaded from the manufacturer's web site and the BIOS update can then be performed through the computer.

BIOS is firmware that is stored on a ROM chip in the same process that data is stored in RAM but it differs from RAM in that it is non-volatile it can't be erased when the power is switched off the data remains in the chip, the old ROM chips where non-reprogrammable and the only way to change the BIOS would be to change the chip, but modern BIOS is now reprogrammable and known as flash ROM and can be altered electronically. once you've entered the CMOS setup program you can access various screens to alter and change configurations, the main screen shows processor information, the time and date, language settings, the advanced settings menu allows you to change floppy drives, peripheral configurations, the security settings menu allows you to set a password to enter the CMOS, the boot options menu allows you to change the boot order. Many expansion cards have their own BIOS built into them to assist the system BIOS how to communicate with then this is known as an option ROM chip. But most hardware comes with software on a CD-ROM called an installation disc that loads the device drivers which are the BIOS updates for a particular device be it the video card, sound card, network card or external devices such as the printer, cameras etc.

The system ROM contains the POST or power on self test a program that checks out the computer every time it boots up.

On most modern motherboards there are jumpers that can be set with a shunt to clear the CMOS, errors are indicated by beep codes, and text errors can appear on the monitor giving you an indication of the problem, a diagnostic card called a Post card can be used when checking for problems.

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