Random Access Memory (RAM)
Of the two flavors of memory (ROM and RAM), RAM is probably the more fundamental. ROM is permanent memory, or permanent storage of information. As the computer's primary working memory, RAM, or Random Access Memory, stores information temporarily. RAM is volatile, meaning that it needs constant electrical current to maintain the information that resides in its chips. If the electrical current is lost, the contents of RAM are erased. When the computer is powered off, all the contents of RAM are flushed out.
The following sections discuss the different types of RAM.
Dynamic RAM (DRAM) is probably the most popular type of memory today and the one that you are most often going to upgrade. When someone says to you, "I have 1024MB of Dynamic RAM," he or she is talking about DRAM.
Dynamic RAM gets its name from the fact that the information stored in DRAM needs to be constantly refreshed. Refreshing involves reading the bits of data stored in DRAM and then rewriting the same information back. DRAM is single ported - meaning that you can read and write to the memory but not at the same time.
Older implementations of RAM measured the memory's performance based on the time it took the CPU to access that data. The measurement used to determine the speed of memory is nanoseconds (ns) - one nanosecond equals a billionth of a second. If you have memory that is 50 ns, and your best friend has memory that is 70 ns, your memory is presumably faster. Your CPU receives the information from memory after waiting only 50 billionths of a second, whereas your best friend's CPU waits 70 billionths of a second.
Remember The lower the number of nanoseconds, the better the performance.
The speed of older DRAM ranges from 60 ns to 80 ns. Today's implementations of DRAM measure the speed of memory in megahertz (MHz) - typically matching the motherboard speed. For example, my Pentium II system uses 100 MHz memory because it runs on a 100 MHz motherboard. For more information on the types of DRAM, see the section, "Identifying the Types of DRAM," later in this tutorial.
Static RAM (SRAM) - so-called because the information held in its memory cells doesn't need to be refreshed - requires less overhead than DRAM to maintain the information stored in memory.
With speeds running from 10 ns to 20 ns, SRAM is much faster than DRAM. Because SRAM is faster memory than DRAM it is also more expensive, which is why people add DRAM to their systems more often than they add SRAM.
SRAM is typically used for cache memory. Cache memory stores frequently used data and program code after it is read from slower DRAM. Think of cache memory as a bucket that sits beside the CPU and stores frequently used information. After the system has searched through DRAM once for specific information, it can store that information in the bucket for easy access later. The next time the data is requested, it is read from cache instead of from system memory.
Because cache memory is much faster than DRAM, the CPU first tries to retrieve the information from cache, specifically L1 cache first and then L2 cache. If the information is not located in cache, the system then tries to retrieve the information from memory. If the information is not located in system memory, it then is retrieved from disk. Attempting to retrieve the requested information from cache first reduces wait time if the information actually resides there because of how fast cache is compared to DRAM.
Cache memory (SRAM) stores frequently used data and program code. Because cache memory is faster than DRAM, retrieving information from cache means that the processor does not have to wait for the slower DRAM, thus enhancing system performance.
In this tutorial:
- Understand Memory
- Understanding the Types of Memory
- Read-Only Memory (ROM)
- Random Access Memory (RAM)
- CMOS RAM
- Shadow RAM
- Identifying the Types of DRAM
- Extended data output
- Rambus DRAM
- Memory Packages
- Understanding Error-Checking Memory
- Working with Cache Memory
- Installing or Upgrading Memory
- Installing memory on desktop PCs