GDDR4 SDRAM (Graphics Double Data Rate Synchronous Dynamic Random Access Memory, version 4) is a type of graphics card memory specified by the JEDEC Semiconductor Memory Standard. It is a rival medium to Rambus's XDR DRAM. GDDR4 is based on DDR3 SDRAM technology and was intended to replace the DDR2-based GDDR3, but it ended up being replaced by GDDR5 within a year.
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GDDR4 SDRAM introduced DBI (Data Bus Inversion) and Multi-Preamble to reduce data transmission delay. Prefetch was increased from 4 to 8 bits. The maximum number of memory banks for GDDR4 has been increased to 8. To achieve the same bandwidth as GDDR3 SDRAM, the GDDR4 core runs at half the performance of a GDDR3 core of the same raw bandwidth. Core voltage was decreased to 1.5 V.
Data Bus Inversion adds an additional active-low DBI# pin to the address/command bus and each byte of data. If there are at least four 0 bits in the data byte, the byte is inverted and the DBI# signal transmitted low. In this way, the number of 0 bits across all 9 pins is limited to 4. This reduces power consumption and ground bounce.
On the signaling front, GDDR4 expands the chip I/O buffer to 8 bits per two cycles, allowing for greater sustained bandwidth during burst transmission, but at the expense of significantly increased CAS latency (CL), determined mainly by the double reduced count of the address/command pins and half-clocked DRAM cells, compared to GDDR3. The number of addressing pins was reduced to half that of the GDDR3 core, and were used for power and ground, which also increases latency. Another advantage of GDDR4 is power efficiency: running at 2.4 Gbit/s, it uses 45% less power when compared to GDDR3 chips running at 2.0 Gbit/s.
In Samsung's GDDR4 SDRAM datasheet, it was referred as 'GDDR4 SGRAM', or 'Graphics Double Data Rate version 4 Synchronous Graphics RAM'. However, the essential block write feature is not available, so it is not classified as SGRAM.
The memory became available with ATI Technologies' Radeon X1950 XTX and Radeon HD 2900 XT and 2600 XT video cards. GDDR4 is intended to achieve clock rates as high as 1.4 GHz (2.8 GBit/s). However, Samsung had aimed to increase GDDR4 to effective clock rates as high as 1.6 GHz (3.2 GBit/s, at higher voltage) and was rumoured to have implemented this improvement into some of the Radeon HD 2900 XT cards .
Graphics cards incorporating GDDR4 memory are now available for purchase with a clock rate of around 1.0 GHz to 1.1 GHz. Samsung was quoted saying they would have 1.6 GHz GDDR4 ready for market as early as July, 2006. NVIDIA was also rumored to have had plans of utilizing the memory on newer revisions of their current-generation GeForce 8-Series GPUs, but instead NVIDIA has used GDDR3 in all of the GeForce 8 cards. Adoption of GDDR4 has been minimal compared to GDDR3 which is still extremely prevalent in most mainstream Graphics cards. Some graphics vendors are adopting DDR3 instead of moving to GDDR4 or 5.
The video memory manufacturer Qimonda (formerly Infineon Memory Products division) has stated it will "skip" the development of GDDR4, and move directly to GDDR5.[6]
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