Chapter 7: Computer Memory^137
Each of the different types of RAM has a specific purpose to which it is best suited: Static RAM (SRAM), a.k.a.Flash RAM Used for cache memory and PCMCIA
(Portable Computer Memory Card Industry Association) memory cards.
Dynamic RAM (DRAM) Most commonly used for primary or main memory
on a PC. It is commonly referred to as simply RAM.
Parameter RAM (PRAM) Used on Macintosh computers to store internal
information, such as the computer’s date and time and other configuration
data that must remain in memory after the computer powers down.
Pseudo-Static RAM (PSRAM) Specifically made for use in portable
computers.
Video RAM (VRAM) Used on video adapter cards for buffering between
the PC system and the video display.Todownloadanexcellenttutorialonmemorysystems,visitKingstonTechnologies’Web
site atwww.kingston.com/tools/umg/and download their “Ultimate Memory Guide.”Static RAM
The primary difference between SRAM and DRAM is that SRAM (pronounced “ess-ram”)
does not require the constant refreshing required of DRAM (pronounced “dee-ram”). DRAM
must be electrically refreshed about every two milliseconds, but SRAM is only refreshed
when data is written to it. SRAM is also faster than DRAM, but it is much more expensive and
requires a much larger physical space to store the same amount of data as DRAM. Because of
these characteristic differences, SRAM is most commonly used for cache memory (see
Chapter 8) and DRAM for common system memory, a.k.a. RAM.Name Usage
SRAM (static RAM) Also called Flash RAM, used in cache
memory and in PCMCIA memory cards
DRAM (dynamic RAM) Personal computers
PRAM (parameter RAM) The equivalent of CMOS on a Macintosh
computer
PSRAM (pseudo-static RAM) Notebooks and other portable PCs
VRAM (video RAM) Frame buffer for video and color
graphics supportTable 7-3. RAM Types