The motherboard provides a common interface for all the components of the PC. Motherboards come in two basic form factors: AT and ATX. The AT form factor was used with the original IBM-compatible PCs. The ATX form factor was designed later to provide better case layout. Both AT and ATX motherboards come in several different sizes. In general, the AT form factor places the keyboard connector about 4 inches from a corner of the motherboard and the ATX places it near a corner. The most current ATX motherboards have a different power connector than previous versions of the board that work with the ATX power supply. Instead of having a separate P8 and P9 connector, the ATX motherboard has one double-row power connector.
Typically, the largest IC chip or socket for an IC chip is for the CPU. Almost all new motherboards use a Zero Insertion Force (ZIF) socket for the CPU chip. The ZIF socket was invented to make it easier to insert and remove CPU chips when 486 CPU chips were developed. A lever on the side of the ZIF socket is used to secure the CPU chip after installation.
Different types of sockets are for different CPU chips and the sockets may have anywhere from 169 pins to more than 350 pins. The sockets types used on different boards range from a Socket 1 socket to a Socket 8 socket; the most common types are the Socket 3 (for 486 and Pentium Overdrive CPUs) and Socket 7 (for Pentium 75 to Pentium 200 CPUs) sockets. The previous photo shows the Socket 3 and Socket 7 sockets. Note the difference in the pin layout.
Important: The Socket 3 socket is for 486 CPUs and the Socket 7 socket is for Pentium 75 to Pentium 200 CPUs.
The motherboard also has specific slots for the RAM. Different types of RAM are available for PCs, so there are different types of slots on the motherboard for RAM. (We cover RAM in more detail later in this chapter.)
The CMOS chip, the battery for the CMOS chip, and the BIOS chip are also located on the system board. A common data bus connects the I/O cards to the motherboard and provides a control and a data path to the RAM and CPU chip. On most newer boards, several different data buses may be available to support the different types of I/O cards.
CMOS
If you wanted a battery-backed real-time clock on an original IBM-compatible PC, you had to install an I/O card with a clock on it. The system did not have the capacity to hold configuration information while the system was powered down. For this reason, the CMOS was designed into the newer 286 motherboards, all 386 motherboards, and 486 and newer motherboards.
The CMOS is a battery-backed IC chip that stores configuration information while PCs are powered down. Power is first applied to the system components as they are initialized during the POST. The information retrieved during the POST is compared to the configuration information stored in CMOS. Newer technology allows the CMOS battery (shown in Figure 2.3) to last for three to five years.
A connector on the motherboard connects to an external battery that provides additional battery backup power to the CMOS. When the CMOS battery is dead, the system loses configuration information while the system is powered down and a CMOS error is generated during the boot process.
Important: The battery-powered CMOS stores the date, time, and hardware configuration information on systems that are powered down. If a CMOS error occurs after the system has been powered down for an extended period of time, the probable cause is a dead CMOS battery.
BIOS
The BIOS is a part of the operating system that handles communications between a PC and its peripherals. Normally, it is a Read-Only Memory (ROM) chip located on the motherboard. The BIOS chip also holds the POST.
Some of the most common BIOS chips are the AMI BIOS, Award BIOS, and Phoenix BIOS. The BIOS chip is easily recognizable, because a shiny sticker on top of the chip is labeled BIOS. Each manufacturer has a set of beep codes that provide information about failures in the POST. We cover these error codes in Chapter 5.
Processor Chip
Processor Chip
The CPU chip, also called the processor chip, is the brain of the PC and performs all the mathematical computations that occur on the computer. Table 2.1 shows some of the Intel processors that have been or are presently used in IBM-compatible PCs.
At present, Intel has released a Pentium II processor that is packaged in a Single Edge Contact (SEC) cartridge. It provides greater speed and capability than the Pentium Pro. The Pentium II processor has two built-in L1 caches and a built-in L2 cache. For current Pentium II processors, the L2 cache is 512 bytes. Two new versions of the Pentium II include
The Celeron — a lower-cost Pentium II with no built-in L2 cache
The Xeon — a higher-cost Pentium II with 2 MB of built-in L2 cache
Intel is also developing the IA-64/Merced Processor. Information about these processor chips is available at the Intel Web site at http://developer.intel.com. AMD also has a new K7 chip that is about to be introduced. More information about the K7 chip is available at http://www.amd.com.
The photo on the previous page shows examples of 286–Pentium CPU chips, the cooling fan used on the 486 chip, and Pentium fans for additional cooling.
Two items that have a great impact on the overall performance and speed of the system are the processor speed and the width of the data path. These items can be compared to the speed limit on a highway and the number of available lanes. The faster the speed limit, the quicker the vehicles can get from Point A to Point B. The more lanes available, the more throughput the road can support. Each of the major milestones in the Intel processor chip line provides a significant boost to the width of the bus, addressable memory, and/or top speed the of the processor.
When a program is run, the executable instruction set or code for that program is placed in RAM. Each line of code asks the processor chip to manipulate data that is stored in RAM and place the solution back in RAM. Basically, all the processor does is add numbers — very quickly. The operating system and application use this information to provide a useful service to the user.
All processors, from the 486 CPU to the Pentium class processors, have a Floating Point Unit (FPU) or math coprocessor built into the processor chip. The FPU lets the CPU not only add very quickly, but also multiply numbers. This capability speeds up calculations tremendously, especially with spreadsheet applications and graphic-intensive applications. In chips before the 486, a math coprocessor was a separate chip.
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