| In this chapter When it's time for the CPU to process something, RAM (random access memory ) is the workspace it uses. RAM is one of two types of memory found in your computer; the other type of memory is ROM (read-only memory). What's the difference? RAM's contents can be changed at any time, whereas ROM's contents require special procedures to change. Think of RAM as a blank sheet of paper and a pencil: You can write on it, erase what you've done, and keep making changes. On the other hand, ROM is like a newspaper. If you want to change what's printed on the newspaper, you must recycle it so it can be reprocessed back into newsprint and sent through the newspaper's printing presses again. RAM is used for programs and data, and by the operating system for disk caching (to hold recently accessed disk sectors). Thus, installing more RAM improves transfers between the CPU and both RAM and disk drives . If your computer runs short of RAM, Windows can also use the hard disk as a very slow substitute for RAM. The swapfile (Windows 9x/Me) or paging file (Windows NT/2000/XP) is a file on the hard disk used to hold part of the contents of memory if the installed RAM on the system isn't large enough for the tasks currently being performed.  | The CD included with this book contains important Study Lab material for this chapter, as well as Chapters 2 “22 in this book. The Study Lab for each chapter contains terms to study, exercises, and practice tests ”all in printable PDF format (Adobe Acrobat Reader is included on the CD, too). These Study Lab materials will help you gear up for the A+ Exam. Also, the CD includes an industry-leading test engine from PrepLogic, which simulates the actual A+ test so that you can be sure that you're ready when test day arrives. Don't let the A+ test intimidate you. If you've read the chapters, worked through the Study Lab, and passed the practice tests from PrepLogic, you should be well prepared to ace the test! Also, you'll notice that some words throughout each chapter are in bold format. These are study terms that are defined in the Study Lab. Be sure to consult the Study Lab when you are finished with this chapter to test what you've learned. |
Although the hard disk can substitute for RAM in a pinch , don't confuse RAM with magnetic storage devices such as hard disks. Although the contents of RAM and magnetic storage can be changed freely , RAM loses its contents as soon as you shut down the computer, whereas magnetic storage can hold data for years . Although RAM's contents are temporary, RAM is much faster than magnetic storage: RAM speed is measured in nanoseconds (millionths of a second), while magnetic storage is measured in milliseconds (thousandths of a second). Even though every computer ever made is shipped with RAM, you will probably need to add more RAM to a computer as time passes . Ever-increasing amounts of RAM are needed as operating systems and applications get more powerful and add more features. Because RAM is one of the most popular upgrades to add to any system during its lifespan, you need to understand how RAM works, what types of RAM exist, and how to add it to provide the biggest performance boost to the systems you maintain. When you must specify memory for a given system, there are several variables you need to know: -
Memory module type (184-pin DIMM, 168-pin DIMM, and so on ) ” The module type your system can use has a great deal to do with the memory upgrade options you have with any given system. Although a few systems can use more than one memory module type, in most cases if you want to change to a faster type of memory module, such as from 168-pin DIMM (used by SDRAM) to 184-pin DIMM (used by DDR SDRAM ), you need to upgrade the motherboard first. -
Memory chip type used on the module (SDRAM, DDR SDRAM, RDRAM, and so on ) ” Today, a particular memory module type uses only one type of memory. However, older memory module types such as 72-pin SIMM and early 168-pin DIMMs were available with different types of memory chips. You need to specify the right memory chip type in such cases to avoid conflicts with onboard memory and provide stable performance. -
Memory module speed (60ns, PC-133, PC800, PC2700, and so on ) ” There are three ways to specify the speed of a memory module: the actual speed in ns (nanoseconds ) of the chips on the module (60ns), the clock speed of the data bus (PC-133 is 133MHz; PC800 is 800MHz), or the throughput (in MBps) of the memory (PC2700 is 2,700MBps or 2.7GBps). -
Error checking (parity, non-parity, ECC ) ” Most systems don't perform parity checking (to verify the contents of memory) or correct errors, but some motherboards and systems support these functions. Although parity-checked memory mainly slows down the system, ECC memory can correct memory errors as well as detect them. If a system is performing very critical work (high-level mathematics or financial functions, departmental or enterprise-level server tasks), ECC support in the motherboard and ECC memory are worthwhile options to specify. -
Allowable module sizes and combinations ” Some motherboards insist you use the same speeds and sometimes the same sizes of memory in each memory socket, whereas others are more flexible. To find out which is true about a particular system, check the motherboard or system documentation before you install memory or add more memory. -
The number of modules needed per bank of memory ” Systems address memory in banks, and the number of modules per bank varies with the processor and the memory module type installed. If you need more than one module per bank, as with SIMM memory on a Pentium-class system, and only one module is installed, the system will ignore it. Systems that require multiple modules per bank require that modules be the same size and speed. -
Whether the system requires or supports dual-channel memory (two identical memory modules instead of one at a time ) ” Dual-channel memory treats two matched modules as a single unit, similar to the way that older systems use two or more modules per bank. However, dual-channel memory is faster than single-channel memory, and is becoming common on more and more systems. -
The total number of modules that can be installed ” The number of sockets on the motherboard determines the number of modules that can be installed. Very small-footprint systems (such as those that use Micro-ATX, Flex-ATX, or Mini-ITX motherboards) often support only one or two modules, but systems that use full-size ATX motherboards often support three or four modules. Although very old systems dating back to the late 1980s and earlier used standard or proprietary memory boards in expansion slots for additional memory, all memory expansion in more recent systems is done on the motherboard. If some of these factors aren't familiar to you yet, relax! My job is to make sure you understand them, and by the time you finish this chapter, you'll be well on the road to being a memory expert. |
