Mac's next memory offers improvements

From...

by David Read

(IDG) -- When users consider Mac performance, they tend to focus on the PowerPC processor and prospects for speedier G4 chips. But many other components affect system speed, and one of these--memory--is poised for advances that could lead to at least modestly improved performance in future generations of Apple hardware.

Macs currently use stock PC100 RAM, but high-end PCs are employing a new memory called PC133 that increases bandwidth from 100MHz to 133MHz. In addition, some high-end models from Dell and other manufacturers use Rambus RAM (RDRAM), a new proprietary memory from Rambus that's up to 60 percent faster than PC133. Slated to appear at year's end is Double Data Rate RAM (DDR RAM), which will at least double PC100 bandwidth.

ATI's Radeon graphics controller, available in retail and build-to-order versions for the Mac, already uses DDR memory. It's especially suitable for use in graphics cards because manufacturers can reduce the number of memory chips and get the same bandwidth, said Mueez Deen, director of DRAM and graphics memory marketing at Samsung. "Fewer I/O pins and less board space can be cheaper, and create fewer FCC interference issues."

Apple will not discuss future products, so it's unclear which memory system will appear on the upcoming Unified Motherboard Architecture-2 (UMA-2). However, DDR would appear to be the logical choice.

"DDR RAM is very similar to existing SDRAM," said one hardware developer who requested anonymity. "The chipsets are available and could easily be worked into the Mac's UniNorth controller." As for Rambus: "It would make little sense to pay for a complex proprietary memory design that has not tested well and that Apple is unfamiliar with," he said. "DDR RAM is the low-hanging fruit and would be an inexpensive performance boost for Apple's next systems."

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Modest gains?

Although faster RAM provides speed benefits, they are not necessarily as dramatic as the specs would suggest. Because the CPU generally runs much faster than memory, hardware designers use an L2 cache, a small (256KB to 1MB) block of memory, to store frequently used data. The cache runs on its own bus at half the speed of the CPU, much faster than the main memory bus. A good caching scheme usually results in 90 percent of CPU data requests being cache hits--data is in the cache and is served up quickly--with the remainder being misses--the data needs to be retrieved from main memory.

PowerPC chips usually have larger caches than their PC brethren to minimize performance hits, and in both environments, the benefits of faster RAM are generally small. For example, using PC133 to increase memory performance in a Mac would produce an actual performance gain of only about four percent. But doubling memory bandwidth with DDR would lead to more-significant performance gains.

Faster RAM could help RISC processors, such as the PowerPC, more than a CISC processor, said Mike Siebert, strategic marketing manager for DRAM at memory vendor Micron. Code written for RISC processors is generally larger than CISC code, increasing the possibility of cache misses. That's one reason why PowerPC caches tend to be larger.

Complicating the scenario is the emerging development of DDR and Quad-Data Rate SRAM, the memory type used in high-speed caches. It's unclear if these will be used with PowerPC, since both Motorola and IBM have indicated that they are moving to on-die caches that run at the same speed as the CPU. IBM's latest G3, the 750CX--used on the new iBooks--has a 256KB on-die cache, but no support for further cache. Motorola's Apollo G4 design features a 256KB on-die cache and support for an off-chip L3 cache.

A little history

Apple began using off-the-shelf PC100 RAM with the Blue and White G3, and continues implementing it in all Power Mac G4s, PowerBooks, and even iMacs, although sometimes in different form factors. With a 64-bit datapath running at 100MHz, PC100's memory bandwidth works out to 800MB/second. PC133 tops out at a theoretical maximum of 1.06GB/second.

Both PC100 and PC133 come with varying Column Address Strobe (CAS) ratings, a measure of how quickly the first bit of data is retrieved from the memory. Macs ship with CAS 3 RAM--identified as "PC100 3-2-2"--meaning the CAS latency is 3 nanoseconds (ns). You can also buy more-expensive PC100 2-2-2 where the CAS latency is 2ns. Generally, CAS 2 RAM provides only a small performance boost, but this benefit increases as more calls are made to main memory. Either version will work in a Mac.

The next-generation RAM standard is a matter of strong debate within the PC community, with Rambus contending against DDR.

Rambus, founded in 1990, is not a manufacturer. Instead, its profits come from licensing fees charged to vendors such as Intel and Sony that implement its technology. Rambus memory uses a relatively small 16-bit datapath running at an astonishing 400MHz in its fastest PC800 modules, which boast a 1.6GB of memory bandwidth.

Normal SDRAM moves data only on the rising part of the clock frequency cycle. Rambus and DDR are both double-clocked, transferring data at the rising and falling edges of the clock signal. The low-end Rambus PC600 runs at 266MHz and has a bandwidth of a little more than 1GB per second--no higher than PC133. And the chip's larger die sizes, complexity and low manufacturing yields make it expensive. The PC800, arguably the only one that a manufacturer would want to use, carries a 60 percent price premium over PC133 CAS2 RAM. Additionally, RIMMs (Rambus In-line Memory Modules) require their own heat sinks due to the high clock frequency, adding to the costs. Memory manufacturers recommend adding a fan to further dissipate heat--another reason why Apple may frown upon the technology.

RDRAM is taking a beating in the press and the marketplace. Numerous independent tests--Tom's Hardware Guide's Rambus Requiem is the most convincing--have shown that Rambus' performance is disappointing. This, coupled with its high price tag and the proprietary nature of the technology, have slowed its acceptance by the PC industry.

Rambus has been closely allied with Intel, which is a significant investor in the company. However, most PC and RAM manufacturers have lined up behind DDR, an open standard that offers high performance, low cost and simple implemention because it is only slightly different from today's PC100 RAM. Even Intel has announced that it will support the technology in addition to Rambus, but with a different chipset.

DDR benefits

DDR RAM, which began development in 1996, works almost exactly like existing PC100 or PC133 DRAM, the difference being that it's double clocked. Because it can move twice the data per clock cycle on the same 64-bit datapath, it effectively doubles bandwidth. Unfortunately, DDR is not backward-compatible with existing PC100 or PC133 RAM.

Implementing DDR is relatively easy. "There are no royalties with DDR," Siebert said. The standard is maintained by JEDEC, the semiconductor engineering standardization body of the Electronic Industries Alliance (EIA).

Currently, there are two varieties of DDR RAM: PC1600 and PC2100. The numbers indicating the memory bandwidth: The PC1600 runs at 100MHz while PC2100 runs at 133MHz. Siebert told MacWEEK that developers are trying to get it to 200MHz, and that it will likely go even faster in the near future.

DDR is not yet widely available. The first DDR controllers were incorporated into the Northbridge controllers used on Wintel machines about a year ago. Memory vendors began offering the first non-graphics DDR samples about a month or two ago. DDR mainboards and whole systems will arrive by the end of this year, Siebert said. He added that production will kick into high gear next year, with strong support from AMD, which will be using DDR with its Athlon, Athlon Thunderbird, and Duron CPUs.

Currently, DDR carries a 20 percent price premium over PC133 SDRAM. But Siebert believes that prices will come into parity over time. In its favor, DDR requires 2.5v as compared to regular DRAM's 3.3v specification. "Manufacturers are reporting a power savings of about 40 percent when using DDR RAM as opposed to PC133," Siebert said.

Not dead yet

Manufacturers are reluctant to dismiss Rambus entirely. "We see Rambus and DDR RAM coexisting for the time being," said Echo Sarlya, a Micron spokesperson. "The market is segmenting, but SDRAM (PC100 and PC133) will still make up the majority of the RAM market next year. Micron is definitely working on Rambus products, but none are currently in production."

Samsung and Hyundai are also developing Rambus products. However, each of the top three RAM manufacturers has privately complained about the difficulty of getting tolerable yields of RDRAM. Micron, Samsung and Hyundai are all working on DDR RAM products.

Intel is supporting Rambus by including it on some of its high-end motherboards. Rambus is also included in Sony's new PlayStation 2. However, Nintendo backed out of an earlier deal to use RDRAM on the Dolphin, its next generation console.

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Rambus
Double Data Rate RAM
Samsung
Micron
Hyundai

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