THE FOLLOWING COMPTIA A+ ESSENTIALS EXAM OBJECTIVES ARE COVERED IN THIS CHAPTER:
1.6 Compare and contrast memory types, characteristics and their purpose * Types
* DRAM
* SRAM
* SDRAM
* DDR / DDR2 / DDR3
* RAMBUS
* Parity vs. Non-parity
* ECC vs. non-ECC
* Single sided vs. double sided
* Single channel vs. dual channel
* Speed
* PC100
* PC133
* PC2700
* PC3200
* DDR3-1600
* DDR2-667
A personal computer (PC) is a computing device made up of many distinct electronic components that all function together in order to accomplish some useful task (such as adding up the numbers in a spreadsheet or helping you write a letter). Note that this definition describes a computer as having many distinct parts that work together. Most computers today are modular. That is, they have components that can be removed and replaced with a component of similar function in order to improve performance. Each component has a specific function. In this chapter, you will learn about the components that make up a typical PC, what their functions are, and how they work together inside the PC.
NOTE Unless specifically mentioned otherwise, throughout this book the terms PC and computer can be used interchangeably.
In this chapter, you will learn how to identify system components common to most personal computers, including the following:
* Motherboards
* Processors
* Memory
* Cooling systems
Identifying Components of Motherboards
The spine of the computer is the motherboard, otherwise known as the system board (and less commonly referred to as the planar board). This is the olive green or brown circuit board that lines the bottom of the computer. It is the most important component in the computer because it connects all the other components of a PC together. Figure 1.1 shows a typical PC system board, as seen from above. All other components are attached to this circuit board. On the system board, you will find the central processing unit (CPU), underlying circuitry, expansion slots, video components, random access memory (RAM) slots, and a variety of other chips.
Types of System Boards
There are two major types of system boards:
Nonintegrated system board Each major assembly is installed in the computer as an expansion card. The major assemblies we''re talking about are items like the video circuitry, disk controllers, and accessories. Nonintegrated system boards can be easily identified because each expansion slot is usually occupied by one of these components.
It is difficult to find nonintegrated motherboards these days. Many of what would normally be called nonintegrated system boards now incorporate the most commonly used circuitry (such as IDE and floppy controllers, serial controllers, and sound cards) onto the motherboard itself. In the early 1990s, these components had to be added to the motherboard using expansion slots.
Integrated system board Most of the components that would otherwise be installed as expansion cards are integrated into the motherboard circuitry. Integrated system boards were designed for simplicity. Of course, there''s a drawback to this simplicity: when one component breaks, you can''t just replace the component that''s broken; the whole motherboard must be replaced. Although these boards are cheaper to produce, they are more expensive to repair.
With integrated system boards, there is a way around having to replace the whole motherboard when a single component breaks. On some motherboards, you can disable the malfunctioning onboard component (for example, the sound circuitry) and simply add an expansion card to replace its functions.
System Board Form Factors
System boards are also classified by their form factor (design): ATX, micro ATX, BTX, or NLX (and variants of these). Exercise care and vigilance when acquiring a motherboard and case separately. Some cases are less flexible than others and might not accommodate the motherboard you choose.
Advanced Technology Extended (ATX)
The ATX motherboard has the processor and memory slots at right angles to the expansion cards. This arrangement puts the processor and memory in line with the fan output of the power supply, allowing the processor to run cooler. And because those components are not in line with the expansion cards, you can install full-length expansion cards in an ATX motherboard machine. ATX (and its derivatives) are the primary motherboards in use today.
Micro ATX
One form factor that is designed to work in standard ATX cases, as well as its own smaller cases, is known as micro ATX (also referred to as ?ATX). Micro ATX follows the same principle of component placement for enhanced cooling over pre-ATX designs but with a smaller footprint. With this smaller form come trade-offs. For the compact use of space, you must give up quantity: quantity of memory modules, quantity of motherboard headers, quantity of expansion slots, quantity of integrated components, even quantity of micro ATX chassis bays, although the same small-scale motherboard can fit into much larger cases, if your original peripherals are still a requirement.
Be aware, however, that micro ATX systems tend to be designed with power supplies of lower wattage, in order to help keep down power consumption and heat production, which is generally acceptable with the standard micro ATX suite of components. As more off- board USB ports are added and larger cases are used with additional in-case peripherals, larger power supplies might be required.
New Low-Profile Extended (NLX)
An alternative motherboard form factor, known as New Low-Profile Extended (NLX), is used in some low-profile case types. NLX continues the trend of the technology it succeeded, Low Profile Extended (LPX), placing the expansion slots (ISA, PCI, and so on) sideways on a special riser card to use the reduced vertical space optimally. Adapter cards, or daughter-boards, that normally plug into expansion slots vertically in ATX motherboards, for example, plug in parallel to the motherboard, so their most demanding dimension does not affect case height. Figure 1.2 shows a low-profile motherboard with its riser card attached.
LPX, a technology that lacked formal standardization and whose riser card interfaces varied from vendor to vendor, enjoyed great success in the 1990s until the advent of the Pentium II processor and the Accelerated Graphics Port (AGP). These two technologies placed a spotlight on how inadequate LPX was at cooling and accommodating high pin counts. NLX, an official standard from Intel, IBM, and DEC, was designed to fix the variability and other shortcomings of LPX, but NLX never quite caught on the way LPX did. Newer technologies, such as micro ATX, and proprietary solutions have been more successful and have taken even more market share from NLX.
Balanced Technology Extended (BTX)
In 2003, Intel announced its design for a new motherboard, slated to hit the market mid- to late-2004. When that time came, the new BTX motherboard was met with mixed reactions. (Let''s postpone accusations of acronym reverse-engineering until "CTX" is announced as the name of the next generation.) Intel and its consumers realized that the price for faster components that produced more heat would be a retooling of the now- classic (since mid-1990s) ATX design. The motherboard manufacturers saw research and development expense and potential profit loss simply to accommodate the next generation of hotter-running processors, processors manufactured by the same designers of the BTX technology. It was this resistance that caused the BTX form factor to gain very little ground over the next couple of years. Nevertheless, with the early support of Gateway, and later buy-in of Dell, the BTX design dug in and charted a path for future success.
Marketing aside, the BTX technology is well thought out and serves the purpose for which it was intended. By lining up all heat-producing components between air intake vents and the power supply''s exhaust fan, Intel found that the CPU and other components could be cooled properly by passive heat sinks. A heat sink is a block of aluminum or other metal, with veins throughout, that sits on top of the CPU, drawing its heat away. Fewer fans and a more efficient airflow path create a quieter configuration overall. While the BTX design benefits any modern onboard implementation, Intel''s recommitment to lower-power CPUs has at once lessened the need to rush to more expensive BTX systems and given the market a bit more time to assimilate this newer technology.
NOTE There are other motherboard designs, but these are the most popular and also the ones that are covered on the exam. Some manufacturers (such as Compaq and IBM) design and manufacture their own motherboards, which don''t conform to the standards. This style of motherboard is known as a motherboard of proprietary design.
System Board Components
Now that you understand the basic types of motherboards and their form factors, it''s time to look at the components found on the motherboard and their locations relative to each other. Figure 1.3 illustrates many of the following components found on a typical motherboard:
* Chipsets
* Expansion slots and buses
* Memory slots and external cache
* CPU and processor slots or sockets
* Power connectors
* Onboard disk drive connectors
* Keyboard connectors
* Peripheral ports and connectors
* BIOS
* CMOS battery
* Jumpers and DIP switches
* Firmware
In this subsection, you will learn about the most-used components of a motherboard, what they do, and where they are located on the motherboard. We''ll show what each component looks like so you can identify it on most any motherboard you run across. Note, however, that this is just a brief introduction to the internal structures of a computer. The details of the various devices in the computer and their impact on computer service practices will be covered in later chapters.
Bus Architecture
Many components of a computer system work on the basis of a bus. A bus, in this sense, is a common collection of signal pathways over which related devices communicate within the computer system. Expansion buses of various architectures, such as PCI and AGP, incorporate slots at certain points in the bus to allow insertion of external devices, or adapters, into the bus, usually with no regard to which adapters are inserted into which slots; insertion is generally arbitrary. Other buses exist within the system to allow communication between the CPU and other components with which data must be exchanged. Except for CPU slots and sockets and memory slots, there are no insertion points in such closed buses because no adapters exist for such an environment.
The term bus is also used in any parallel or bit-serial wiring implementation where multiple devices can be attached at the same time in parallel or in series (daisy-chained). Examples include Universal Serial Bus (USB), Small Computer System Interface (SCSI), and Ethernet.
Chipsets
A chipset is a collection of chips or circuits that perform interface and peripheral functions for the processor. This collection of chips is usually the circuitry that provides interfaces for memory, expansion cards, and onboard peripherals and generally dictates how a mother- board will communicate with the installed peripherals.
Chipsets are usually given a name and model number by the original manufacturer. For example, if you see that a motherboard has a VIA KT7 chipset, you would know that the circuitry for controlling peripherals was designed by VIA and was given the designation KT7. Typically, the manufacturer and model also tell you that your particular chipset has a certain set of features (for example, onboard video of a certain type/brand, onboard audio of a particular type, and so on).
Chipsets can be made up of one or several integrated circuit chips. Intel-based motherboards typically use two chips, whereas the SiS chipsets typically use one. To know for sure, you must check the manufacturer''s documentation.
The functions of chipsets can be divided into two major functional groups, called Northbridge and Southbridge. Let''s take a brief look at these groups and the functions they perform.
Northbridge
The Northbridge subset of a motherboard''s chipset is the set of circuitry or chips that performs one very important function: management of high-speed peripheral communications. The Northbridge subset is responsible primarily for communications with integrated video using AGP and PCI Express, for instance, and processor-to-memory communications. Therefore, it can be said that much of the true performance of a PC relies on the specifications of the Northbridge component and its communications capability with the peripherals it controls.
NOTE When we use the term Northbridge, we are referring to the set of chips and circuits that make up a particular subset of a motherboard''s chipset. There isn''t actually a Northbridge brand of chipset.
The communications between the CPU and memory occur over what is known as the frontside bus (FSB), which is just a set of signal pathways between the CPU and main memory. The clock signal that drives the FSB is used to drive communications by certain other devices, such as AGP and PCI Express slots, making them local-bus technologies. The backside bus (BSB), if present, is a set of signal pathways between the CPU and Level 2 or 3 cache memory. The BSB uses the same clock signal that drives the FSB. If no backside bus exists, cache is placed on the frontside bus with the CPU and main memory.
The Northbridge is directly connected to the Southbridge (discussed next) and helps to manage the communications between the Southbridge and the rest of the computer.
Southbridge
The Southbridge subset of the chipset, as mentioned earlier, is responsible for providing support to the myriad onboard slower peripherals (PS/2, Parallel, IDE, and so on), managing their communications with the rest of the computer and the resources given to them. These components do not need to keep up with the external clock of the CPU and do not represent a bottleneck in the overall performance of the system. Any component that would impose such a restriction on the system should eventually be developed for FSB attachment.
(Continues...)
Excerpted from CompTIA A+ Complete Deluxe Study Guideby Quentin Docter Emmett Dulaney Toby Skandier Copyright © 2009 by John Wiley & Sons, Ltd. Excerpted by permission.
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