Unified Memory Architecture

A new design trend in PCs, particularly lower cost models, is Unified Memory Architecture, sometimes abbreviated as UMA. This design unifies the memory in a PC by moving the video frame buffer from its privileged position, connected through a bus to the video controller, to part of the main memory of the PC. In other words, part of the memory of the system gets allocated for storing video images.

The chief attraction of this design is that it cuts the cost of a PC. Manufacturers no longer need pay for video board memory. It can also improve the performance of a PC because the frame buffer, as part of main memory, operates at the same speed as main memory rather than at the lower rate associated with the expansion bus or local bus.

The UMA design can be misleading. Because some of the memory in a UMA system must be given over to the video buffer, it is not available to your applications. Hence a UMA system acts as if its has less memory than the specification sheet implies.

To the digital logic of your PC's microprocessor, the UMA design appears no different than a conventional video system. The frame buffer appears in the same logical location as always. If it did not, your applications would not be able to access it. On the other hand, the hardware implementation of UMA is radically different and, if poorly carried out, can have dramatic effects on the performance of the overall PC.

The conventional memory design makes video memory a self-contained subsystem. It handles its own housekeeping, and provides its own, decidated, access channel for the video controller, which operates independently of the host microprocessor. The UMA design makes video memory part of the main memory system, where it is treated as ordinary DRAM. The graphics chip, should the system have one, must share access to the frame buffer with the rest of the system. Or the microprocessor may manage video operations itself, taking over the function of the graphics chip (which again, helps to trim costs). Either way, performance suffers. The multi-tasking aspect of having a graphics accelerator is lost. Moreover, because video memory in UMA designs is drawn from the PC's main memory, it cannot benefit from high speed memory designs (such as Video RAM) without forcing the entire system to use the more costly technology.