The Motherboard, Processor, and Memory - The Memory
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The system memory, or RAM (Random Access Memory), is where data is stored while the processor is working on it or after it has finished calculating. Memory performance is crucial to a speedy system. Slow memory can bottleneck the system and reduce the processor’s effectiveness. Two types of memory are used in a system: cache and main memory.
Cache The main memory isn’t the only place from which the processor gets data. It also uses its own banks of storage, called Level 1 (L1) and Level 2 (L2) cache. Since the cache is right on the processor’s die, the processor can access it faster than it can access the main memory.
Data is whisked from the main memory to the cache by the cache controller. The cache controller tries to predict what information the processor will ask for next. If the processor asks for that information, it’s immediately available and the processor goes to work on it. This is called a cache hit. When the processor asks for information that’s not loaded into cache, it’s called a cache miss.Al-though RAM’s getting faster, it still doesn’t run nearly as fast as the processor. This is why cache hits are crucial to the speed of the system.
L2 cache differs from L1 cache in several ways. First, it’s larger: current processors have 256K or 512K of L2 cache, and only 64K of L1 cache. Second, it takes a few nanoseconds longer to reach L2 cache. L2 cache is on the die with current processors, but it didn’t used to be; in older PCs, it’s on the motherboard, separate from main memory, and instead of running at the processor core speed like L1 and current L2 cache, it runs at the speed of the FSB.
Cache hits are vital to the speedy performance of a processor. It’s always better to have the needed data right on the die than it is to have to fetch it across the FSB from main memory. No matter how fast the FSB or the memory of the computer, it can’t beat the near instantaneous fulfillment of a cache hit.
What About Upgrading the Cache? -- You can’t upgrade your processor’s cache, because the cache is an integral part of the chip itself. In earlier times, cache came in the form of socketed chips on the motherboard and could be added onto as needed. Cache has since been moved to the processor die.
Main Memory Main memory is used for data storage. Programs drop data there that the processor needs to calculate, and the processor returns the results to memory for the program to use. In a nutshell, that’s how the processor runs programs.
Memory comes in the form of modules called SIMMs, DIMMs, RIMMs, or something similar. The IMM portion stands for Inline Memory Module, and the three precursors are Single, Double, and Rambus, respectively. Memory is measured in megabytes or even gigabytes. Figure 2-13 shows a DDR memory module.
It’s easy to get confused about what’s in memory and what’s in storage, the latter of which refers to hard disks, removable media, and optical drives. Memory and storage are different animals, however. Their characteristics are as follows:
| Memory | Storage |
| Is volatile, meaning that it doesn’t store data when powered down. | Is nonvolatile. It keeps data stored when powered down. |
| Comes in the form of chips on rectangular printed circuit boards. | Comes in the form of hard disk platters,floppy disks, CD-ROMs, DVD-ROMs, and other physical media. |
| Is used by the processor to run programs. | Is used to store data for the long term. |
| Is accessed much faster than storage. | Is accessed more slowly than memory. |
As you can see, memory is quite different from storage. The hard drive is the primary storage device in a system. It contains metal platters, and data is written to it with a magnetic head.
System memory is also referred to as RAM. When you see the specifications of a computer, the list will include an amount of memory written as, say, “512MB RAM.” Although many devices are randomly accessed, including most storage devices, the term RAM usually refers to system memory.
 This chapter is from Build Your Own High Performance Gamers' Mod PC, by Chen and Durham (McGraw-Hill/Osborne, 2004, ISBN: 0072229012). Check it out at your favorite bookstore today. Buy this book now. |
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