A Description of Common Memory

Contents

SDRAM
SDRAM (Synchronous Dynamic Random Access Memory) is the most commonly used memory on the market at the present time. It has a 168-pin configuration and is powered by 3.3 volts, and is made by many manufactures in memory sizes of 32 to 512 MB single-chip solutions. All SDRAM are DIMMS (Dual In-line Memory Modules). The DIMM uses a 64-bit data path, or a 72-bit data path with parity bits. The SDRAM memory chips are based on a 64-bit data channel. Therefore, if using SDRAM memory chips, only one DIMM would fill the data channel. As for the older SIMM modules, they required the memory to be used in pairs, making it so you had to use two at a time.

RDRAM
RDRAM (Rambus Dynamic Random Access Memory) is a new memory technology that promises a much faster data transfer rate compared to today's SDRAM. The Rambus RAM will be capable of a data transfer rates of 1.6GB per second, delivering the data on an 800MHz data bus. The Rambus memory chips are placed on a small circuit board that is referred to as the Rambus In-line Memory Module (RIMM).

DDR
DDR is a common term for Double Data Rate. This is a type of memory that has been used in different applications, but more frequently in the video card market. DDR as referred to here is a type of SDRAM that supports data transfers on both edges of each clock cycle, effectively doubling the memory chip's data throughput. The first chipsets that will be supporting the DDR memory technology appeared in graphics subsystem early in 1999, and system core logic chipsets will be coming in early 2001.

Flash ROM
Flash ROM is a special type of ROM (Read Only Memory). The flash memory uses a special type of EEPROM (Electronically Erasable Programmable Read Only Memory) that can be erased and reprogrammed in blocks instead of one byte at a time. The ROM memory retains its data content even when the power is turned off. DRAM, instead, requires the supply of a constant electrical current in order to maintain the data within each respective cell. In previous technologies, the ROM could only be programmed in special proprietary devices. Today, with the flash ROM technology as advanced as it is, the data can be altered and programmed with the electronic device itself. This is commonly used to store the flash BIOS programs on motherboards and video cards.

Cache

Cache is commonly referred to as a high speed data storage system with the main use of storing the most frequently accessed data in a buffer, so that these data can be accessed and processed without delay. Because cache needs to be much faster than normal storage devices, it is usually smaller in capacity and more expensive to purchase.

The memory cache is a part of the memory subsystem that stores the most frequently accessed data. It is made of high-speed static RAM (SRAM), instead of the slower and cheaper dynamic RAM (DRAM) used for main memory. Because the SRAM memory chips are more expensive and much larger in size, the cache memory size is always limited to within a few KB. Memory caching is effective because most programs access the same data or instructions over and over. By keeping as much of this information as possible in SRAM, the computer avoids accessing the slower DRAM.

Sometimes the cache memory is built into the microprocessor, as with most x86 processors today. Currently, the cache memory is usually categorized in different levels. The level 1 (L1) cache is the fastest and smallest in capacity, normally between 16KB and 32KB, the level 2 (L2) cache is the second fastest and a bit larger in capacity, normally between 128KB to 512KB for PC usage, and as high as 2MB for server applications.