Standard DRAM is accessed through a technique called paging. The following sections look at these memory types in more detail.
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Table 6.3 lists the primary types and performance levels of PC memory. In 2009, DDR3 memory became the most popular memory type in new systems, with standard speeds of up to 1,600MHz. By 2007, DDR2 memory was available at speeds of up to 1,066MHz, and DDR3 came on the market at 1,066MHz and faster. DDR2 memory continued to match processor bus speed increases in PCs during 20, rising to 667MHz and 800MHz, respectively, during that time. During 2004, we saw the introduction of DDR2, first at 400MHz and then at 533MHz.
In 2002, DDR memory increased to 333MHz in 2003, the speeds increased further to 400MHz. Starting in early 2001, double data rate (DDR) SDRAM memory of 200MHz and 266MHz become popular. From that point forward, memory has largely evolved in step with the processor bus, with newer and faster types coming out to match any increases in processor bus speeds.īy the year 2000, the dominant processor bus and memory speeds had increased to 100MHz and even 133MHz (called PC100 and PC133 SDRAM, respectively). However, starting in 1998 the industry shifted to faster SDRAM memory, which was able to match the 66MHz speed of the processor bus at the time. When the speed of the memory bus equals the speed of the processor bus, main memory performance is optimum for that system.įor example, using the information in Table 6.2, you can see that the 60ns DRAM memory used in the original Pentium and Pentium II PCs up until 1998 works out to be an extremely slow 16.7MHz! This slow 16.7MHz memory was installed in systems running processors up to 300MHz or faster on a processor bus speed of 66MHz, resulting in a large mismatch between processor bus and main memory performance. More recently, however, systems using DDR, DDR2, and DDR3 SDRAM have memory bus performance equaling that of the processor bus. Over the development life of the PC, memory has had a difficult time keeping up with the processor, requiring several levels of high-speed cache memory to intercept processor requests for the slower main memory. The Relationship Between Megahertz (MHz) and Cycle Times in Nanoseconds (ns)Īs you can see from Table 6.2, as clock speeds increase, cycle time decreases proportionately. Table 6.2 shows the relationship between common nanosecond (ns) and megahertz (MHz) speeds associated with PCs from yesterday to today and tomorrow. Earlier in this chapter I listed formulas you could use to mathematically convert these values. Today's processors run in the 2GHz–4GHz range with most performance improvements coming from changes in CPU design (such as multiple cores) rather than pure clock speed increases.īecause it is confusing to speak in these different terms for speeds, I thought it would be interesting to see how they compare. In one billionth of a second, a beam of light travels a mere 11.80 inches or 29.98 centimeters-less than the length of a typical ruler!Ĭhip and system speeds have often been expressed in megahertz (MHz), which is millions of cycles per second, or gigahertz (GHz), which is billions of cycles per second. To put some perspective on that, the speed of light is 186,282 miles (299,792 kilometers) per second in a vacuum. Fortunately, you can easily translate MHz/GHz to ns, and vice versa.Ī nanosecond is defined as one billionth of a second-a very short time indeed. Newer and faster types of memory usually have speeds expressed in MHz, thus adding to the confusion. The speed and performance issue with memory is confusing to some because memory speed is sometimes expressed in nanoseconds (ns) and processor speed has always been expressed in megahertz (MHz) or gigahertz (GHz). Learn More Buy RAM Types and Performance Upgrading and Repairing PCs, 19th Edition
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