Enhanced SDRAM (ESDRAM) ESDRAM (pronounced “ehs-dee-ram”) is
actually SDRAM with a small built-in SRAM cache that is used to increase
memory transfer times. It works with data bus speeds of up to 200MHz.
Direct Rambus DRAM (DRDRAM) DRDRAM (pronounced
“dee-are-dee-ram” or “Doctor DRAM”) is a proprietary DRAM technology
developed by Rambus, Inc. (www.rambus.com) and Intel. DRDRAM, along
with a similar approach, SLDRAM (SyncLink DRAM), is capable of supporting
memory speeds of up to 800MHz.
FRAM (ferroelectric RAM) FRAM (pronounced “fram”) has features of
both DRAM and SRAM, which means it can store data even after its power
source is removed.Video RAM
Back when PC monitors were all monochrome (black and white), the PC could easily set
aside 2K of memory to support the needs of the display. However, today’s multicolor
monitors require significantly more memory to generate their graphical displays. The
monochrome monitor was fine using primary memory for its support, but today’s moni-
tors need a memory source much closer and faster than standard RAM. To provide the
video system with the RAM it needs, memory has been added to the video adapter card,
which places it much closer to the video controller and the monitor itself. This memory is
calledvideo memoryorvideo RAM (VRAM).DRAM as Video RAM
The first type of video memory used was standard DRAM. This didn’t work out, primarily
because it had to be continually refreshed, which meant that while it was being refreshed
it couldn’t be accessed by the video system. In addition, DRAM was unable to support
the extremely fast clock speeds of video systems. DRAM is asingle-portedmemory. This
means that it can only support access from one source at a time. In a video system situa-
tion, only the CPU or video controller could be accessing it, not both. These problems and
others lead to the development of memory technologies specifically designed for the
video system.VRAM
To provide the support and speeds required by the video system, VRAM must be
dual-ported, which allows it to accept data from the CPU at the same time it is providing
data to the video controller. This means that while it is receiving data about new displays,
itcanbesupplyingthevideosystemwiththedataitneedstorefreshthedisplay’simage.
Whenanimageisdisplayedonthemonitor,theimagedataistransferredfromprimary
RAM to the video RAM. TheRAM digital-to-analog converter (RAMDAC)reads the data
from VRAM and converts it into analog signals, which are used by the monitor’s display
device, such as a CRT (cathode ray tube), to create the image desired. More information is
available on the RAMDAC and the video system in Chapter 12.(^146) PC Hardware: A Beginner’s Guide