Rambus Video Memory, a Case Study
QRSL Video Memory
Sander Sassen
However, Rambus has recently introduced QRSL, Quad Rambus Signaling Levels. QRSL is Rambus’ latest signaling technology, fundamentally different from Rambus Signaling Levels, RSL, as used in RIMMs. Almost all component interconnect technology today transfers data using 0’s and 1’s represented by voltage levels. Traditionally a ‘1 ‘was presented by 5 Volts and a ‘0’ by 0 Volts. RSL used a reference voltage of 1.4 Volts, wherein a '1’ was presented by 1.0 Volts and ‘0’ by 1.8 Volts. QRSL uses four voltage levels to represent two bits of information. Logic 10, 11, 01 and 00.
This multi-level signaling technology allows higher bandwidth, double that of RSL, without increasing clockspeed. In this case, a single Rambus device is capable of providing up to 3.2 GB/s of memory bandwidth, using only a 16-bit databus. If scaled upwards to a 64 or even a 128-bit databus as often used in today’s videocards, this comes to:
64-bit / 16-bit = 4 : 4 x 3.2 = 12.8 GB/s for a 64-bit databus
128-bit / 16-bit = 8 : 8 x 3.2 = 25.6 GB/s for a 128-bit databus
By looking at the incredible memory bandwidth offered by QRSL one can only wonder what would happen if this technology were used on a graphics card. However unlike RSL, as used in Rambus RDRAM, its theoretical application and technical feasibility has been proven, but has to be evaluated in order to see if it is indeed suited for use as video memory.
Another interesting possibility is the following; now that QRSL has been announced, there is an interesting technical advantage that can be explored. For example, some of today's videocards are capable of either using SDRAM or DDR SDRAM as video memory, so that two different market segments can be addressed with a single GPU. Because the protocols for SDRAM and DDR SDRAM are similar, and because the interfaces are similar, it is feasible from a design standpoint to support both types of memory. RSL and QRSL offer the same possibilities for videocards in the future. Video memory based on RSL, Rambus RDRAM, and QRSL technology will use the same protocol, will have compatible I/O voltages, and allow for pin compatibility. Besides offering very high bandwidths, multiple market segments can be addressed with a single GPU in the same manner that SDRAM and DDR SDRAM are used today.
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1 Introduction
2 Memory Bandwidth
3 Increasing Memory Bandwidth
4 Calculating Memory Bandwidth
5 Rambus Video Memory
6 QRSL Video Memory
7 Cost Analysis
8 Conclusion
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