Remembrance of things future

by Alan Zisman (c) 1997. First published in Canadian Computer Wholesaler, May 1997

In this series, we?re looking at changes in the hardware?how new features in PC-design promise to continue to provide users with machines that will do more?not only faster but easier. We?ve looked at Intel?s new family of MMX processors, now available, and at Universal Serial Bus, Firewire, and the Accelerated Graphics Port?all somewhere in between promise and reality.

But none of these advances will work without memory?the chips the computer uses to remember what it?s just done and what it?s about to do.

Many users still get confused between storage?their hard drive, for example, and memory?the computer?s RAM. Perhaps because both can be measured in Megabytes. The difference is simple?imagine looking up a recipe in a cookbook, and then trying to cook it. The cookbook is like your hard drive?information in long-term storage. But in order to make dinner, you need to read the recipe, and remember it?at least long enough to cook the meal. Your brain, in this case, is acting like the computer?s RAM? remembering just enough of the recipe to get you from the cookbook to the stove.

And more ram is always better. In our recipe example, more RAM would mean being able to remember several of the cookbook?s instructions at once, instead of having to go back and read each step, one at a time. As a result, adding more ram makes any computer run faster and more efficiently.

But while our computers sport ever-faster CPUs, the speed of the RAM hasn?t kept up. The original IBM PC featured an Intel 8088 processor, running at a blazing 4.77 mhz, and came standard with between 16 and 64 kb of 200 nanosecond (nsec) RAM. Your 200 mhz Pentium processor is 40 times faster, and about 400 times as powerful as that original processor. And with 16 megs or more RAM, it?s got 1,000 times as much memory. But that memory is probably running at 60 nsec?not even four times as fast as that on the 1981 PC. Designers have had to build in a series of tricks to keep the processor and the RAM working together:

? Wait States. Standard DRAM (Dynamic RAM) needs to be refreshed?electrically recharged with its data. Designers add ?wait states??periods where the CPU sits idle, waiting for the DRAM to be refreshed. Faster RAM means fewer wait states.
? RAM caches. Most often, CPUs access the same instructions over and over again. (?Peel a carrot. Slice it. Place in frying pan. Stir. Peel a carrot??) As a result, a little bit of very fast RAM can go a long way towards meeting the CPU?s needs, letting the CPU access the slower main RAM only when something isn?t in the cache. Typically, modern computers have several levels of cache-ram. A tiny bit of very fast ram is built right into the CPU itself (much of the performance increase of the new MMX CPUs comes from doubling the size of this on-board cache). Next, there?s the so-called Level 2 (L2) cache on the motherboard? typically 256-512kb of fast but expensive SRAM (Static RAM, which doesn?t need to be constantly refreshed). Much of the power of the Pentium-Pro design comes from it including the L-2 cache right on the chip.
? Newer and faster RAM designs. Over the past couple of years, old standard DRAM was replaced with more efficient Fast Page Mode (FPM) RAM. That in turn was replaced with today?s standard?Extended Data Out (EDO)  RAM. By letting the CPU read and write at the same time as the refresh cycle, this speeds up access and eliminates wait states. But even EDO is reaching its limits, as processors get faster and faster.

Replacing EDO is Synchronous DRAM (SDRAM); because it can run in higher speed systems, it?s poised to become the new standard for 1997-98. But increases in processor speed that are already in the works limit the potential of SDRAM much beyond that.

Coming right at us is Rambus DRAM (RDRAM).  Rambus is a hot company, with a recent, wildly successful stock offering, based on owning the next generation of RAM technology. Early versions of its RAM are available in high-end Silicon Graphics workstations, and in $300 Nintendo-64 game machines, as well as on some PC graphics cards.

While more standard RAM communicates with the CPU in 16 or 32 bit pieces, Rambus-RAM uses a humble 8-bit interface. It overcomes this seeming-limitation because the interface is extremely fast?currently at 250 MHz, speeds of 600 MHz have been demonstrated, with even faster versions on the horizon.

To make use of this super-fast memory design, motherboard companies will need to license the custom Rambus Channel?the special chip set that allows the CPU to interact with the RAM. CPU and motherboard giant Intel has thrown its support behind Rambus?the two companies have announced plans to cooperate to design the next generation, being called nDRAM. They?re hoping to reach speeds up to 1.6 GBps (Gigabytes per second!) by 1999, just in time for Intel?s 64-bit P7 Merced processor.

Until that becomes reality, there are other shifts in how RAM is being sold. 386 and many 486 generation computers typically used so-called 30-pin SIMMs. These were 8-bit pieces (ironically, like the futuristing RDRAM), and had to be added in pairs for 16-bit 386SX busses, and in sets of four for 32-bit 386DX and 486 systems.

Pentium systems, however, use a 64-bit memory bus? to use 30-pin SIMMs, users would have to install these eight at a time, an impractical arrangement. So virtually all such systems were built to use newer, 32-bit 72-pin SIMMs. These could be used singly in a 486, or in matched pairs on a Pentium. New Pentium and Pentium-Pro motherboards have, however, stolen an idea from recent Macintosh designs, and now include slots for 64-bit DIMMs (dual in-line memory modules). A single DIMM can replace a pair of SIMMs?an advantage as users try and cram more and more memory into their case.

But that?s not the only Mac design showing up on PCs in the near future.

Currently, PCs end up with RAM in several places. There?s the main system RAM, of course. But there?s also RAM on the video card?typically 2 ? 4 megs worth. There may even be RAM on a wavetable sound card, for storing sound samples, cache ram on a high-end disk controller, and other bits and pieces around the system. For several years, Macintoshes have been designed using a Unified Memory Architecture (UMA)? a simpler design in which the main system RAM is shared around as needed.

When RAM was expensive, this made sense? you might have 4 meg of RAM on your video card, but only be using 1 meg or so, depending on the number of colours and screen resolution?the rest of the video RAM would be going to waste. Implementing UMA could, as a result, lower system prices.

UMA, however, has been a mixed benefit. Performance of UMA systems has tended to be lower than systems with dedicated graphics RAM; this is especially true when running at high video resolution and colour depth, which requires a large amount of RAM?under UMA, this deprives the CPU access to a significant amount of RAM. As well, constantly deciding how to share the system RAM around is just one more task to dump onto the CPU.

Operating systems have to be designed to support UMA?this support is not currently written into either Windows 95 or NT, but is expected to show up in future generations.

That?s because the performance hit associated with UMA will become less of an issue in the future?with more powerful CPUs accessing large pools of system RAM. As a result, expect to see this becoming a standard feature on low-mid-range systems over the next few years.

For now, make sure your systems support DIMMs; these will be needed to support the large amounts of RAM users will be requiring. EDO RAM is on the way out?SDRAM will be the standard by late 1997, through 1998. After that, look for the results of the Intel-Rambus cooperation to take us through the turn of the century.

Avoid Unified Memory Architecture models for now?but expect this to become common on entry-level systems in a year or so.

Now if only I could get this computer to make dinner for me!

Next month?making PCs simpler: the NetPC and the Zero-Administration initiative.

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Alan Zisman is a Vancouver educator, writer, and computer specialist. He can be reached at E-mail Alan