CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [92]
Figure 6-10 A 168-pin DIMM above a 144-pin SO-DIMM
SDRAM tied to the system clock, so its clock speed matched the frontside bus. Five clock speeds were commonly used on the early SDRAM systems: 66, 75, 83, 100, and 133 MHz. The RAM speed had to match or exceed the system speed or the computer would be unstable or wouldn’t work at all. These speeds were prefixed with a “PC” in the front, based on a standard forwarded by Intel, so SDRAM speeds were PC66 through PC133. For a Pentium III computer with a 100-MHz frontside bus, you needed to buy SDRAM DIMMs rated to handle it, such as PC100 or PC133.
RDRAM
When Intel was developing the Pentium 4, they knew that regular SDRAM just wasn’t going to be fast enough to handle the quad-pumped 400-MHz frontside bus. Intel announced plans to replace SDRAM with a very fast, new type of RAM developed by Rambus, Inc., called Rambus DRAM, or simply RDRAM (Figure 6-11). Hailed by Intel as the next great leap in DRAM technology, RDRAM could handle speeds up to 800 MHz, which gave Intel plenty of room to improve the Pentium 4.
Figure 6-11 RDRAM
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NOTE The 400-MHz frontside bus speed wasn’t achieved by making the system clock faster, it was done by making CPUs and MCCs capable of sending 64 bits of data two or four times for every clock cycle, effectively doubling or quadrupling the system bus speed.
RDRAM was greatly anticipated by the industry for years, but industry support for RDRAM proved less than enthusiastic due to significant delays in development and a price many times that of SDRAM. Despite this grudging support, almost all major PC makers sold systems that used RDRAM for a while. From a tech’s standpoint, RDRAM shares almost all of the characteristics of SDRAM. A stick of RDRAM is called a RIMM. In this case, however, the letters don’t actually stand for anything; they just rhyme: SIMMs, DIMMs, and now RIMMs, get it?
RDRAM RIMMs came in two sizes: a 184-pin for desktops and a 160-pin SO-RIMM for laptops. RIMMs were keyed differently from DIMMs to ensure that even though they are the same basic size, you couldn’t accidentally install a RIMM in a DIMM slot or vice versa. RDRAM also had a speed rating: 600 MHz, 700 MHz, 800 MHz, or 1066 MHz. RDRAM employed an interesting dual-channel architecture. Each RIMM was 64 bits wide, but the Rambus MCC alternated between two sticks to increase the speed of data retrieval. You were required to install RIMMs in pairs to use this dual-channel architecture.
RDRAM motherboards also required that all RIMM slots be populated. Unused pairs of slots needed a passive device called a continuity RIMM (CRIMM) installed in each slot to enable the RDRAM system to terminate properly. Figure 6-12 shows a CRIMM.
Figure 6-12 CRIMM
RDRAM offered dramatic possibilities for high-speed PCs, but ran into three roadblocks that Betamaxed it. First, the technology was owned wholly by Rambus; if you wanted to make it, you had to pay the licensing fees they charged. That led directly to the second problem, expense. RDRAM cost substantially more than SDRAM. Third, Rambus and Intel made a completely closed deal for the technology. RDRAM worked only on Pentium 4 systems using Intel-made MCCs. AMD was out of luck. Clearly, the rest of the industry had to look for another high-speed RAM solution.
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NOTE Betamaxed is slang for “made it obsolete because no one bought it, even though it was a superior technology to the winner in the marketplace.” Refers to the VHS versus Betamax wars in the old days of video cassette recorders.
DDR SDRAM
AMD and many major system and memory makers threw their support behind double data rate SDRAM (DDR SDRAM). DDR SDRAM basically copied Rambus, doubling the throughput of SDRAM by making two processes