Bus Interface

One memory speed issue arises not from semiconductor technology but from system design. Where memory is located and how it is connected to the system microprocessor can have a dramatic effect on memory performance.

Memory is usually connected to the microprocessor's local bus-that means it runs at the microprocessor's clock speed and connects through a bus as wide as the microprocessor's data bus. Many motherboards lack sufficient space for memory expansion and consequently relegate expansion memory to daughterboards. These daughterboards can link to the motherboard either with a proprietary connection that operates at full microprocessor speed and bus width, or through a standard expansion bus like ISA, EISA, or Micro Channel.

The first kind of memory boards, which are often termed proprietary memory boards, generally impose no performance penalty on expansion memory. The latter boards can impose severe penalties in all systems more powerful than 8 MHz AT-compatibles.

Higher speed machines (those with microprocessors operating in excess of 8 megahertz) suffer severely when memory is added through expansion slots. The slots typically operate at 8 megahertz or so, notwithstanding the higher speed of the microprocessor. Accessing this add-in memory thus requires the system to slow down to the 8 MHz rate. A 66 MHz machine would be effectively slowed to one-eighth speed when accessing expansion board memory.

The memory in classic AT bus slots imposes an additional slowdown on 386DX, 486, and Pentium microprocessors because of the maximum 16-bit width of the bus. Because the memory bus would be only half or less the width of the microprocessor data bus, transferring data from memory to microprocessor automatically takes twice as long as it would with the full-width connection used by the motherboard and proprietary memory board RAM. The Pentium's 64-bit data bus makes the problem even more severe.

The simple rule is to keep memory out of normal expansion slots in any PC faster than an ancient 8 MHz AT if you want full performance. In some cases, however, sliding a few megabytes into an expansion slot makes eminent sense, such as when you have no other expansion, when a specialized application doesn't require the utmost in memory speed, or when you already have a memory expansion board.

Some PCs have limited motherboard expansion capabilities, and slotted memory may be the only kind that you can add. An all-too-common scenario is the system board that uses a proprietary memory expansion board, and you cannot acquire the needed proprietary board because the manufacturer is unreachable, out of business, or never bothered to make the boards in the first place.

If you just need a RAM disk to speed up program compiles or database sorts, expansion board memory may suffice. While the RAM drive would not be as fast as one created using motherboard memory, it would nevertheless far outrace any mechanical disk drive.

If you already have a populated memory expansion board, there's no reason to put it to pasture if your PC accepts it. As long as you configure the memory on the board to be addressed above the top of all system board memory, it will be the last to be used by your applications. Only when a program needs every last byte will it reach into slow down territory. Certainly performance will suffer at that point, but you'll be able to run software you wouldn't otherwise be able to.

Moreover, if you're using an environment like Windows that takes advantage of virtual memory techniques, the slow slotted memory will still outperform the virtual memory that is emulated by a mechanical disk drive. While such slotted memory is not an ideal solution, it can be acceptable, affordable, and workable.