Identifying PC133 Memory Modules!

What you should know..

Before we show you how to distinguish PC133 memory from all others, some background information might be appropriate. Elsewhere in our Performance Center we have discussed How to Visually Identify Memory Types, How to Verify PC100 Compliant Memory and what the PC100 Standard is all about.

With the ever present push for performance, processors, motherboard buses and other computer system components have seen dramatic speed increases. With that, memory technology has been making some leaps of its own. In 1995 memory speed jumped from 33MHz to 66MHz with the release of EDO memory. In 1997 we saw SDRAM speeds jump to 66MHz with the release of PC66 SDRAM, and shortly thereafter the engineers moved the speed lever again, this time to 100MHz with the release of PC100 in 1998. Engineers, though, were not ready to quit just yet, as barely a year later, in 1999, we saw the release of PC133 SDRAM, with memory speeds running at 133MHz and higher.

During 1999 and 2000 we saw dramatic changes in the engineering and development of memory technology with the release of RDRAM (Rambus®) memory running at 800MHz and DDR SDRAM running at 266MHz. On the surface it doesn't seem to make much sense to develop DDR SDRAM at 266MHz when you have Rambus® running at 800MHz, but in truth, they are direct competitors when it comes to memory speed, as DDR SDRAM is an entirely new design that reduces DRAM latencies and substantially increases memory bandwidth. Follow these links for a brief Introduction to DDR SDRAM, as well as a Comparison of DDR SDRAM and Rambus ® memory.

In spite of these new advances though, PC100 and PC133 SDRAM, is not dead by a long shot. Now that you have some background, let's discuss briefly what PC133 SDRAM is, and how to correctly identify it when you purchase it.

As was the case in our review of PC100, PC133 SDRAM must be manufactured to meet the specific standards set by Intel. And like PC100, beware, as there are unscrupulous suppliers selling PC133 modules that do not meet the Standard and this is reflected in their performance. If you have Adobe Acrobat installed, click this link to review Intel's PC133 Validation Specifications for PC 133 Modules.


Synchronous dynamic random access memory (SDRAM) delivers bursts of data at very high speeds using an interface that is synchronized to the CPU clock. SDRAM emerged in 1996, and represented a big step forward from EDO technology. When manufactured, PC133 SDRAM, must meet Intel's requirements (the PC133 Standard) for use with motherboards having a 133MHz FSB (front side bus). PC133 compliant SDRAM is almost always a requirement in Pentium III, AMD Athlon and Power Mac G4 based systems. This link will provide you with a review about Memory Speed.

While PC133 SDRAM can be used with motherboards having a 100MHz front side bus, your memory will only operate as fast as the slowest "link" in your system, in this case the motherboards 100MHz front side bus. As an example, if you were to install a PC133 module in a system with a 100MHz FSB, or in a system already containing a 100MHz module, the PC133 module will operate only at 100MHz. PC133 SDRAM is available only in the form of a 168-pin DIMM (as it pertains to personal computers).

Lets review what makes a PC133 module different from its predecessors. Keep in mind, that when suppliers sell memory modules, they often provide technical specifications and descriptions of the modules performance. If your supplier isn't providing this information, beware! Below are some of the terms you may see in those descriptions. You will also find additional definitions in our Memory Glossary.

Clocks and Latency (CL=2 - CL=3)

"CL=2" (also written as "CL2" or "CAS=2") and "CL=3" (also written as "CL2" and "CAS=2") refers to a module's CAS latency. CAS latency is the amount of time it takes for your memory to respond to a command. It only affects the initial burst of data. Once data starts flowing, latency is no longer significant. Following this link will take you to a more in depth discussion of Memory Latencies.

Latency is measured in terms of clock cycles. A CL=2 chip requires two clock cycles to respond, and a CL=3 chip requires three clock cycles, therefore CL=2 chips complete the initial data access a little more quickly than CL=3 chips. Keep in mind though, a clock cycle for a system with a 100MHz front side bus is only 10 nanoseconds (10 billionths of a second), therefore don't be too surprised if you're unable to tell the difference between a CL=2 and a CL=3 chip. While most systems will accept memory modules having either a CL=2 or CL=3 chip, there are some systems that require one or the other. Generally your motherboard's manufacturer will advise you of their requirements, however should you not be able to determine this, just let us know and we will help you select the right module.

As an example, a few systems built by Dell and Gateway require a particular type of CL=2 memory known as 2-clock memory. While this memory technology is no longer used in modern systems, Crucial, Samsung and a few other manufacturers continue to offer this unique type of module for upgrade customers. If you need 2-clock memory for your system, just let us know the make and model of your computer and we will provide you with the correct module.

How does a PC determine what CL value to use?

During the startup (Boot) process, the motherboards BIOS software reads the value for CL (CAS Latency), tRCD and tRP that is programmed into the Serial Presence Detect (SPD) EEPROM on the SDRAM DIMM memory module. The memory controller will then issue SDRAM commands to meet the memory device requirements.

Let's look at the differences between PC133 and other SDRAM Memory.

As you may have noted above, the PC133 SDRAM module was designed to improve the memory bandwidth of the personal computer from 100Mhz to 133Mhz. Typically, the 133 MHz SDRAM chip has a speed rating of 7.5 nanoseconds (7.5 billionths of a second) when running on a motherboard with a 133 MHz Front Side Bus.

This table, courtesy of Micron, shows the specification and speed differences between the most recent forms of SDRAM, PC66, PC100 and PC133.




TIME (ns)










-10 10 30 1


33 30
-10 10-15 30 2 2 66 15


-8A/B 15 24 2 3 100 10
-8C 15 20 2 2 100 10
-8E 15 20 2 2 100 10


-75 15 20 2 3 133 7.5
-7E 14 15 2 2 133 7.5

*As a general rule, personal computers use 2 clock memory

As you can see from the above table, memory speeds have accelerated from PC66 at 33MHz and 30 nanoseconds (30 billionths of a second) to PC133 at 133MHz and 7.5 nanoseconds (7.5 billionths of a second). We know what you're thinking, how can you really tell the difference between 30 billionths of a second and 7.5 billionths of a second? Simply put, you can't, but your computer can, and it makes a difference!

Dispelling the confusion between Front Side Bus Frequency and CPU Frequency

The Front Side Bus (FSB), the memory bus between the Processor and the Memory module, is the main information or data highway in the PC system. The faster the bus runs, the faster data can transfer between the processor and memory.

The speed of the FSB is not the same as processor speed (yet), but technology is quickly changing this, and very shortly you will see the processor and FSB running at the same speed. If you have a 600MHz Pentium processor with a 100MHz Front Side Bus, the information flowing within the processor will run at 600MHz, whenever the data is transferred outside the processor, the data will flow only at 100MHz. At present, one of the overall limiting factors in PC systems today is the bus speed. While you may have a processor running at 800 or 1,000 MHz, and memory capable of running at 800 MHZ, data transfers will never run faster than the Front Side Bus speed. Once developers conquer this limitation, personal computers will operate at speeds previously unheard of.

Now it's time to get down to the nitty gritty!

Most 133MHz SDRAM chips are actually designed to run at 150MHz and faster. These chips are often referred to as "-7.5" (7.5 nanosecond parts). You can identify the chips by reading “-7” in the last two digits on the chip part numbering found on most PC133 memory modules. The “-7” refers to the minimum operating clock cycle of the device.

How to determine the frequency of the module?

Again, the simplest way to determine if the module is PC66, PC100 or PC133, is by simply reading the last digit or two of the part number on the actual chip. Here are two examples, a PC100 module from Micron and a PC100 chip from Samsung.

As you can see from the red arrow, this Micron chip has a "-8" designation that identifies it as 100MHz bus and 10 nanosecond. As noted earlier, had this chip had a "-7" or "-7.5" designator, that would indicate it was a PC133 chip, and had it been "-10" it would be PC66. Even knowing this, should you be uncertain of the modules specifications, Micron makes it easy to verify them by providing a part number cross reference, which you can see by clicking this link.

Now let's look at the Samsung chip.

Although the above chip is made by Samsung, directly identifying the speed without a reference sheet that explains the codes is a little more difficult. The last pair of digits in the part number, “G8”, indicate that it is a 125Mhz device, or a PC100 memory chip. Recently, Samsung changed their part number scheme, and permanently removed their old reference sheets. The following links will provide you with Samsung's new SDRAM part number reference.

Briefly, here is what the part number represents. We have broken it down based upon Samsung's reference sheets:

1 2 3 4 5 6 7 8 9 10 11 12 13
KM 4 8 S 8 0 3 0 B T   - G 8
1  KM indicates that the chip is a Samsung part.
2  4 = Indicates that this a DRAM part
3  8 = Indicates the chip organization (x8)
4  S = Indicates the ship is specifically SDRAM
5  8 = Indicates the density of this chip as 8M
6  0 = Indicates the Refresh, in this case 4K
7  3 = Indicates the number of chip banks (4 banks)
8  0 = Indicates type and mount of voltage (LVTTL (3.3V)
9  B = Indicates the Revision number (3rd generation)
10  T = Packaging type. In this case TSOP II (400mil)
11  - No value
12  G = Indicates Power - Auto & Self-Refresh (3.3V)
13  8 = Indicates Min. cycle time 8ns(125MHz@CL=3)

Before we close this subject, we feel it is necessary to have you understand that all too often, consumers do not read the entire part number when they inspect the memory modules they receive. All of the numbers and letters making up the part number of a memory module are important in correctly identifying its specifications. Here's a data sheet from Samsung.

Normally the five red arrows that you see above would not be present when you review a data sheet. If you look closely at each of the five part numbers, you will see that the only difference are the last two digits. Those two digits alone determine whether the module is PC66, PC100, PC125 or PC133 as you can see by the chart.

Kingston Technology uses a similar method to indicate the parameters and specifications of their memory modules, and this link will take you to the Kingston Technology Reference.

As long as your memory modules, and the chips on them, are manufactured by a major supplier, such as Micron/Crucial, Kingston, Samsung, IBM, Hyundai, NEC, Toshiba, Hitachi, you can usually rest assured that you are receiving quality memory. Just be careful to make sure that you purchase your modules from reputable resellers. It is also important to note that many manufacturers have both premium as well as inexpensive versions of their memory products. You get what you pay for! Also important, as mentioned elsewhere on this Website, some modules arriving from certain Asian countries have been re-marked to change the part information. Beware!

The Bottom Line, what makes a good PC133 Module?

When Intel introduced the PC100 SDRAM specification, a list of standards and specifications were compiled to insure the uniformity of manufacture of memory chips and modules. These standards and specifications had to be met by both semiconductor and module manufacturers to not only insure uniformity of fabrication, but also to insure the accuracy of data handling at higher DRAM speeds. As SDRAM speed changed, increasing from 100MHz to 133MHz (and above), the specifications for the new SDRAM changed as well. The PC133 Intel/JEDEC Standard still includes the following:

The Jedec/Intel specification goes to great length to detail each of the above issues, dictating what the manufacturer must do in order to meet the standard. In theory, as long as manufacturers meet or exceed these specifications, all memory modules produced by all manufacturers will be identical and rarely produce problems. This common uniformity standard was designed to insure that all SDRAM memory module should be created equal and there shouldn’t be any major variations between any two module made by different companies. Unfortunately though, in the real world, you will find that SDRAM modules with identical SDRAM chips, can sometimes reach entirely different frequencies for no other reason than differences in the manufacturing of their printed circuit boards and the trace layouts on them. For this reason alone, always try and purchase all of the memory you need at the same time and from the same supplier.


  1. Simply put, usually the last digit or two of the part number on the memory chip will indicate the memory type. PC66 memory chips will be "12", PC100 will be either "8" or "10", and PC133 will be either "-65", "-7" or "-75", representing 6.5, 7 and 7.5 nanoseconds respectively.

    Every day we field questions from people who want to upgrade their PC's by adding more memory, and one of the most Frequently Asked Questions involves that of compatibility. We constantly field questions as to whether PC100 and PC133 memory can be mixed on the same motherboard, or whether replacing PC66 memory with PC133 will make someone's system faster.

  2. While there are many cases where PC100 modules, and even the older PC66 SDRAM modules, have worked together on the same motherboard at 133MHz bus speeds, however those situations are extremely rare and ill advised. In an emergency, anything is worth a try. Just remember that the purpose of your computer is that of dealing with data, regardless of whether you're dealing with games or physics calculations. It is pointless to mix memory types when the end result will almost certainly result in corrupted data.

  3. When purchasing memory for a new system, make it a point to purchase all of the memory you need at the same time from the same supplier. If you are upgrading, try and match as closely as possible the memory modules you already have.

If you would like to review more about memory related issues, you may want to follow these links:

Memory, Evolution or a Revolution?

How Memory Speeds Are Determined

How to Identifying Different Memory Types

Does your memory meet the Standard?

Frequently Asked Questions About Memory

Troubleshooting Memory Problems

Megabyte (MB) vs. Megabit (Mb)

Memory Trends in 2001

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This page updated: 1/27/2001