Editorial: March 1, 1996

One of the latest buzz technologies for PCs today is the unified memory architecture (UMA). The idea behind this technology is simple: Use some of the PC's main memory to serve as the video buffer and save the cost, board space, and complexity of a separate video memory. This isn't a new idea. Some of the earliest PCs, such as the Apple II, the Processor Technology Sol, and all of the early Commodore µPs, employed this architecture. That was in an era when processors were a lot slower than DRAM, so you could easily multiplex the µP's and video hardware's access to RAM with no performance loss.

The reason for UMA's return is not because DRAM access times have suddenly outpaced processor clock rates. Rather, there is a worldwide DRAM shortage. UMA cuts the number of DRAMs in a system, and, if your design uses fewer parts, you have a proportionately better chance of shipping the product. The DRAM shortage probably won't abate until 1997, so, for the next year or so, you need to focus on part reduction in your designs and ensuring that the parts you do select will be available.

(Come to think of it, reducing the number of parts used is always a good idea. So now is the right time to develop the proper design habits.)

Back to UMA. The UMA restores a bottleneck that had been broken when PC vendors decided to use separate video memories because video performance was really suffering. High video-refresh rates and megapixel color bit maps are gluttons for memory bandwidth. Any design that devotes some main-memory bandwidth to video is necessarily robbing the PC's µP of precious memory-access cycles. On the surface, this looks like a bad idea. However, if your µP doesn't really need all of the main memory's bandwidth, then using UMA may be an excellent design approach. Slow processors (are there any left?), overzealous caching, or relaxed overall performance specs may permit you to use UMA.

The point of all this is that you cannot afford a knee-jerk reaction to any design technique; that's not engineering. To be on top of your design game, you need to evaluate every possibility, no matter what your emotional reaction is to it. So, if your next design requires a video buffer, consider UMA for its technical merits. As we go backward into the future.


Steven H. Leibson
Editor In Chief