Section 2 - The Motherboard

The Motherboard

You will commonly hear the motherboard referred to as the systemboard. A motherboard is usually designed to be one of the following types:

Non-integrated - The traditional style of motherboard that requires the circuitry for major subsystems, such as video, disk controllers, audio, etc.
Integrated - This style incorporates most of the circuitry that would normally be added through expansion cards: video, disk controllers, etc.

The shape, packaging, and to a certain extent, the function of a motherboard is called its form factor. Three standard forms of motherboards exist:

AT - A motherboard patterned after the original IBM AT motherboard.
Baby AT - A smaller motherboard used in IBM clones.
ATX - Smaller like an Baby AT board, but installed with a 90 degree rotation. This rotation allows for easier installation of full-length expansion cards and cables, it is also easier to cool.

The Motherboard Components

Microprocessors

     A microprocessor is an integrated circuit that contains millions of transistors interconnected with very small aluminum wires. The exact functions performed by the microprocessor are dictated by software. Since 1981, microprocessors have been packaged in a form factor called the Pin Grid Array (PGA), which arranges its transistors into a square shape.

     The first IBM PC had the 8088 CPU manufactured by the Intel corporation was manufacturing CPUs before the first PC.

     The PGA microprocessor installs on the motherboard in a special socket called a ZIF (Zero Insertion Force). Newer ZIP sockets, such as ZIF Socket 7, also come with a lever (ZIF handle) to lock and release the chip.

Packaging Abbreviations:

DIP - Dual Inline Packaging. The most common package for all types of integrated circuits.
PGA - Pin Grid Array. There are various sizes of PGA packages and mountings. For instance, the 486 uses a 169-pin package; the pentium mounts in either 273 or 296-pin socket; and the pentium pro uses a 387-pin socket.
SEC - Single Edge Contact. This new type of package mounts the processor as a single module into a specifically designed slot on the motherboard.

Microprocessor Characteristics
CPU	Package	Speed (MHz)	Data Bus (bits)	Memory (MB)	Co-processor?	Cache (KB)
8086	DIP	8	16	1	No	0
80286	PGA	20	16	16	No	0
386SX	PGA	25	16	16	No	0
386DX	PGA	40	32	4096	No	0
486SX	PGA	20,25,33	32	4096	No	8
486DX	PGA	25,33,50	32	4096	Yes	8
486DX2	PGA	40,50,66	32	4096	Yes	8
486DX4	PGA	75,100	32	4096	Yes	16
Pentium	PGA	100,133,150,166	64	4096	Yes	16
Pentium MMX	PGA	150,166,200	64	4096	Yes	32
Pentium Pro	PGA	166,200	64	4096	Yes	16
Pentium II	SEC	166,180,200,266,300+	64	4096	Yes	64

     The 486DX was the first CPU with an integrated math coprocessor; the Pentium Pro offers 64-bit data bus; and the Pentium and Pentium II processors offer MMX. The Pentium processor is basically a combination of two 486DX chips that create a parallel processing environment that Intel calls superscalar.

     Until the 486DX came along, it was beneficial to add a math coprocessor to the motherboard to help speed up the system. The coprocessor installed in a socket located adjacent to the CPU on 808x, 286, 386DX and SX, and 486SX motherboards. Coprocessors were numbered relative to the system each was designed to support. The coprocessor for the 80286 was the 80287; the 386DX used the 80387DX; and the 486SX used the 80487SX.

     The Pentium processor operates at 185 degrees Fahrenheit. Thermal grease improves the heat conductivity between the processor and its heat sink. It eliminates the gaps between the two, allowing the CPU's heat to transfer to the heat sink more efficiently.

     A chipset is a set of microchips that work as a unit to set or control the amount of memory and the type of RAM chips that can be used, and provide support for certain peripherals. The term chipset is commonly used to refer to the basic functions of a motherboard.

The System BUS

The CPU moves data values and signals around the computer on a network of very small wires that interconnect it to all the other components on the motherboard. This network is called the bus.

The lines used to move data around inside the computer is the internal bus. The lines used to communicate with peripherals and other devices attached to the motherboard is the external bus. The number of wires is a bus controls the number of bits that can be transferred over the bus. A 32-bit bus will have 32 wires. You can find four primary types of bus structures on most motherboards:

Address: - The component on the motherboard pass memory addresses to another over the address bus.
Control: - Used by the CPU to send out signals to coordinate and manage the activities of the motherboard components.
Data: - Because the primary job of the computer is to process data, logically the data must be transferred between peripherals, memory, and the CPU.
Power: - The power bus provides electrical power to the motherboard's components.

Memory Chips

Memory chips come in three handy package sizes, with one right for every system:

DIP - Dual inline package. Small byte-sized pieces installed individually.
SIMM - Single inline memory module. Large packages that combine multiple DIPS on a single module.
DIMM - Dual inline memory module. High capacity modules that have memory on each side of the module board that connects through a single connector.

Up through the 80286, computers used the DIP type of memory chips. Prior to the 286, systems used either 64-kilobit or 256-kilobit chips. Because the 286 can address more memory, a 1-megabit chip can be used on that system. For those systems that can use them, DIP chips are installed in banks of up to 16 chips each. The 1-megabit chip is two pins longer than the other chips, but some motherboards can take both sizes.

The BIOS

     One of the central functions of the motherboard is the BIOS. The BIOS or Basic Input and Output System contains the programming used to control the keyboard, disk drives, monitor display, and data communications, as well as a few miscellaneous hardware-related actions. The BIOS servers to interface between the PC's hardware and the operating system; and monitors the hardware settings, and controls the boot sequence of the PC.

     The BIOS code is installed on the motherboard in a ROM chip. All microprocessors look in the same physical location for their first instruction. This location is the first location of the BIOS. That first instruction is called the jump instruction. The jump instruction of the BIOS program is located at address FFFF0h of conventional memory.

     Because RAM is faster than ROM, some manufacturers design their systems to perform shadowing, a procedure that copies the BIOS code from ROM into RAM when the computer boot sequence begins. On newer systems, the BIOS system is installed on the motherboard on an electrically erasable, programmable, read-only memory (EEPROM) chip. These chips can be updated under software control.

     The BIOS ROM is installed in a motherboard socket and is easily found because of its shiny foil label that identifies its version, date of manufacture, and the name of the manufacturer. If the BIOS ROM chip is updatable with software procedures called flashing, the version reported on the chip may be lower than the actual version in the chip.

POST Hardware Diagnostic Messsage Groups
Code	Description
1xx	Systemboard errors
2xx	Main memory errors
3xx	Keyboard errors
5xx	Color monitor error
6xx	Floppy disk controller error
14xx	Printer errors
17xx	Hard disk controller errors
86xx	Mouse error

Storing the PC's Configuration

Older computers stored their setup data through a series of DIP switches located on the motherboard. Since the introduction of the 80286 processor, PCs have stored their system configuration information on CMOS (complementary metal oxide semiconductor) memory chips. CMOS is one of the technologies used to manufacture integrated circuits (ICs). This material was chosen in the early days because of its special properties, which include low power requirements and long data retention. The CMOS used to hold the BIOS setup is a special memory type that's powered by a low-voltage battery. As long as the battery has power, the CMOS retains its contents.