Hardware Reference
In-Depth Information
Some of these, including FPM DRAM, EDO DRAM, and SDRAM, were also used for main memory
in PCs. All of the others were specifically designed for use in graphics subsystems.
p.
325
.
VRAM and WRAM
VRAM and WRAM are dual-ported memory types that can read from one port and write data through
the other port. This improves performance by reducing wait times for accessing the VRAM compared
to FPM DRAM and EDO DRAM.
SGRAM
SGRAM was designed to be a high-end solution for fast video adapter designs. SGRAM is similar to
SDRAM in its capability to be synchronized to high-speed buses up to 200MHz, but it differs from
SDRAM by including circuitry to perform block-writes to increase the speed of graphics fill and 3D
Z-buffer operations.
DDR SGRAM
DDR SGRAM is designed to transfer data at speeds twice that of conventional SGRAM by
transferring data on both the rising and falling parts of the processing clock cycle.
GDDR2 SGRAM
There have been several variations of what has been called GDDR2. The first was based on standard
2.5V DDR SDRAM with some enhancements, whereas the second was actually based on 1.8V DDR2
SDRAM, and with much higher performance and cooler operation.
GDDR3 SGRAM
GDDR3 SGRAM is based on DDR2 memory, but with two major differences:
• GDDR3 separates reads and writes with a single-ended unidirectional strobe, whereas DDR2
uses differential bidirectional strobes. This method enables higher data transfer rates.
• GDDR3 uses an interface technique known as pseudo-open drain, which uses voltage instead of
current. This method makes GDDR3 memory compatible with GPUs designed to use DDR,
GDDR2, or DDR2 memory. To determine the type of memory used on a particular video card,
check the video card manufacturer's specification sheet.
GDDR4 SGRAM
