Memory Speed


The speed and performance issue with memory is confusing to some because of all the different ways to express the speeds of memory and processors. Memory speed was originally expressed in nanoseconds (ns), whereas the speeds of newer forms of memory are usually expressed in MHz (megahertz) and MBps (megabytes per second) instead. Processor speed was originally expressed in MHz ( megahertz ), whereas most current processor speeds are expressed in GHz (gigahertz). Although all these different speed units may seem confusing, fortunately, it is possible to translate from one to the other.

A nanosecond is defined as one billionth of a second ”a very short piece of time indeed. To put some perspective on just how small a nanosecond really is, consider that the speed of light is 186,282 miles (299,792 kilometers) per second in a vacuum . In one billionth of a second (one nanosecond), a beam of light travels a mere 11.80 inches or 29.98 centimeters ”slightly less than the length of a typical ruler!

Memory speeds have often been expressed in terms of their cycle times (or how long it takes for one cycle), whereas processor speeds have almost always been expressed in terms of their cycle speeds (number of cycles per second). Cycle time and cycle speed are really just different ways of saying the same thing ”that is, you could quote chip speeds in cycles per second, or seconds per cycle, and mean the same thing.

As an analogy, we could express the speed of a bicycle using the same relative terms. For example, in the United States we normally express vehicle speeds in miles per hour. If you were riding a bicycle at 5 miles per hour (mph), it would take 0.2 hours per mile (hpm). At 10 mph, it would take 0.1 hpm. In other words, you could give the speed as either 0.2 hpm or 5 mph, and it would mean exactly the same thing.

Because it is confusing to speak in these different terms for chip speeds, I thought it would be interesting to see exactly how they compare. Table 6.1 shows the relationship between nanosecond (ns) cycle times and the megahertz (MHz) speeds they represent.

Table 6.1. The Relationship Between Memory Cycle Times in Nanoseconds (ns) and Clock Speeds in Megahertz (MHz)

Cycle Time (ns)

Clock Speed (MHz)

60.0

16.67

15.0

66.67

10.0

100.00

7.5

133.33

6.0

166.67

5.0

200.00

3.8

266.67

3.0

333.33

2.5

400.00

1.9

533.33

As you can see, as cycle time decreases, clock speed increases proportionately.

If you examine Table 6.1, you can clearly see that the 60ns DRAM memory used in older systems for many years is totally inadequate when compared to processor speeds of 400MHz and higher. Up until 1998, most DRAM memory used in PCs had been rated at an access time of 60ns or higher, which works out to be 16.67MHz or slower! The dominant standard in the year 2000 was to have 100MHz and even 133MHz memory, called PC100 and PC133, respectively. Starting in early 2001, double data rate (DDR) memory of 200MHz and 266MHz become popular. In 2002 we had 333MHz memory, and in 2003 400MHz DDR memory became available. Note that these memory speed milestones are dated for desktop systems; laptops and notebooks are somewhat behind the curve on memory speeds. For example, in late 2003 most laptops used 266MHz DDR memory, whereas desktops of the same vintage were using 400MHz memory instead.

System memory timing is a little more involved than simply converting nanoseconds to megahertz. The transistors for each bit in a memory chip are most efficiently arranged in a grid, using a row and column scheme to access each transistor . All memory accesses involve selecting a row address and then a column address, and then transferring the data. The initial setup for a memory transfer where the row and column addresses are selected is a necessary overhead normally referred to as latency . The access time for memory is the cycle time plus latency for selecting the row and column addresses. For example, SDRAM memory rated at 133MHz (7.5ns) typically takes five cycles to set up and complete the first transfer (5 x 7.5ns = 37.5ns) and then perform three additional transfers with no additional setup. Thus, four transfers take a total eight cycles, or an average of about two cycles per transfer.

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. Table 6.2 shows the progress and relationship between system board (motherboard) speeds in PCs and the various types and speeds of main memory or RAM used and how these changes have affected total bandwidth.

Table 6.2. Portable System DRAM Memory Module and Bus Standards/Bandwidth (Past, Current, and Future)

Module Standard

Module Format

Chip Type

Clock Speed(MHz)

Cycles per Clock

Bus Speed (MTps)

Bus Width (Bytes)

Transfer Rate (MBps)

-----------------------------------------------------------------------------------------------------------------------------------------------------

FPM

72/144-pin SO-DIMM

60ns

22

1

22

8

177

EDO

72/144-pin SO-DIMM

60ns

33

1

33

8

266

-----------------------------------------------------------------------------------------------------------------------------------------------------

PC66

144-pin SO-DIMM

10ns

66

1

66

8

533

PC100

144-pin SO-DIMM

8ns

100

1

100

8

800

PC133

144-pin SO-DIMM

7/7.5ns

133

1

133

8

1,066

-----------------------------------------------------------------------------------------------------------------------------------------------------

PC1600

200-pin SO-DIMM

DDR200

100

2

200

8

1,600

PC2100

200-pin SO-DIMM

DDR266

133

2

266

8

2,133

PC2400

200-pin SO-DIMM

DDR300

150

2

300

8

2,400

PC2700

200-pin SO-DIMM

DDR333

166

2

333

8

2,667

PC3000

200-pin SO-DIMM

DDR366

183

2

366

8

2,933

PC3200

200-pin SO-DIMM

DDR400

200

2

400

8

3,200

PC3500

200-pin SO-DIMM

DDR433

216

2

433

8

3,466

PC3700

200-pin SO-DIMM

DDR466

233

2

466

8

3,733

PC4000

200-pin SO-DIMM

DDR500

250

2

500

8

4,000

PC4300

200-pin SO-DIMM

DDR533

266

2

533

8

4,266

Some DDR standards listed here are proposed or future standards not yet available.

MTps = megatransfers per second

MBps = megabytes per second

ns = nanoseconds (billionths of a second)

FPM = Fast Page Mode

EDO = extended data out

SO-DIMM = small outline dual inline memory module

DDR = double data rate

Generally, things work best when the throughput of the memory bus matches the throughput of the processor bus. Compare the memory bus transfer speeds (bandwidth) to the speeds of the processor bus as shown in Table 6.3, and you'll see that some of the memory bus rates match that of some of the processor bus rates. In most cases, the type of memory that matches the CPU bus transfer rate is the best type of memory for systems with that type of processor.

Table 6.3. Processor Bus Bandwidth

CPU Bus Type

Clock Speed (MHz)

Cycles per Clock

Bus Speed(MTps)

Bus Width (bytes)

Bandwidth (MBps)

33MHz 486 FSB

33

1

33

4

133

66MHz Pentium I/II/III FSB

66

1

66

8

533

100MHz Pentium I/II/III FSB

100

1

100

8

800

133MHz Pentium I/II/III FSB

133

1

133

8

1,066

200MHz Athlon FSB

100

2

200

8

1,600

266MHz Athlon FSB

133

2

266

8

2,133

333MHz Athlon FSB

166

2

333

8

2,667

400MHz Athlon FSB

200

2

400

8

3,200

400MHz Pentium 4/M FSB

100

4

400

8

3,200

533MHz Pentium 4/M FSB

133

4

533

8

4,266

800MHz Pentium 4/M FSB

200

4

800

8

6,400

FSB = front side bus
MTps = megatransfers per second
MBps = megabytes per second

Because the L1 and L2 cache fairly well insulate the processor from directly dealing with main memory, memory performance has often lagged behind the performance of the processor bus. More recently, however, systems using SDRAM and DDR SDRAM have memory bus performance equaling that of the processor bus. When the speed of the memory bus equals the speed of the processor bus, memory performance is optimum for that system.



Upgrading and Repairing Laptops
Scott Muellers Upgrading and Repairing Laptops, Second Edition
ISBN: 0789733765
EAN: 2147483647
Year: 2003
Pages: 182
Authors: Scott Mueller

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