Types of RAM

Types of RAM

RAM is not all the same. Over time, RAM technology has improved, changed form, and been used for specialized components. The most common types of RAM are discussed here.

-SRAM

Static RAM (SRAM) was the first type of RAM available. SRAM can be accessed at approximately 10 nanoseconds (ns), meaning that it takes about 10ns for the processor to receive requested information from SRAM. The structure of SRAM chips limits them to a maximum data capacity of 256KB. Although SRAM is very fast compared with DRAM, it is also very expensive. For this reason, SRAM is typically used only for system cache.

-DRAM

Dynamic RAM (DRAM) chips provide much slower access than SRAM chips but can store several megabytes of data on a single chip (hundreds of megabytes if they are packaged together on a module). Every “cell” in a DRAM chip contains one transistor and one capacitor to store a single bit of information. This design makes it necessary for the DRAM chip to receive a constant power refresh from the computer, preventing the capacitors from losing their charge. This constant refresh can make access even slower and causes the DRAM chip to draw more power from the computer than an SRAM chip. Because of its low cost and high capacity, DRAM is used as “main” memory in the computer.

The first DRAM chips were very slow (~80–90ns), so faster variants have been developed. The list is quite large and includes fast-paged RAM, EDO RAM, SDRAM, RDRAM, SDLRAM, and BEDO RAM. As computer systems improve, the list of DRAM technologies continues to grow. However, EDO, SDRAM, and RDRAM are currently the most common, so they are described here.

-EDO RAM

Extended data out (EDO) RAM improves on traditional DRAM by performing more than one task at a time. While that piece of data is being transferred, the EDO RAM is looking for the next piece to retrieve for the processor. This process enables the chip’s data to be accessed at about 60ns. EDO RAM chips can be used only in a computer system whose processor and motherboard support its use.

-SDRAM

Synchronous dynamic RAM, or SDRAM, is about twice as fast as EDO RAM because it is able to run at the speed of the system bus (up to 100–133MHz). However, as faster system bus speeds are developed, SDRAM is being replaced with other, faster types of DRAM, such as RDRAM. Like EDO RAM, SDRAM can be used only in systems that support it.

-RDRAM

RDRAM is an acronym for Rambus Dynamic RAM and gets its name from the company that developed it, Rambus, Inc. RDRAM uses a special Rambus channel that has a data transfer rate of 800MHz. The channel width can be doubled, resulting in a 1.6GHz data transfer! RDRAM can be used only in computers with special RDRAM channels and slots. RDRAM is fairly new, so you shouldn’t expect to see it in computers that were manufactured before 1999.

-VRAM

Video RAM (VRAM) is a specialized type of memory that is used only with video adapters. The video adapter is one of the computer’s busiest components, so, to keep up with video requirements, many adapters have an on-board miniprocessor and special video RAM. The adapter can process requests independently of the CPU, then store its results in the VRAM until the CPU retrieves it. VRAM is much faster than EDO RAM and is capable of being read from and written to at the same time. The result is better and faster video performance. Because VRAM includes more circuitry than regular DRAM.

The term Video RAM refers to both a specific type of memory and a generic term for all RAM used by the video adapter (much like the term DRAM, which is often used to denote all types of memory that are dynamic). Faster versions of video memory have been introduced, including WRAM.

-WRAM

Window RAM (WRAM) is another type of video RAM but provides faster access than VRAM. It uses the same dual-ported technology that allows devices to read and write data to the video memory at the same time. The term “window” refers to its ability to retrieve large blocks (windows) of data at one time.