Dissecting a PC-100 compliant memory module.
When a memory module is assembled, in addition to the precision manufactured SDRAM chips, there must also be a correctly programmed SPD (Serial Presence Detect) EEPROM. The SPD EEPROM is a small little black chip found near the edge of the module. It is normally located on the right hand corner of the module, near pin 84 if you count from left to right.
The SPD EEPROM must contain precise information about the memory module, in order that the system can identify the memory correctly during the initialization and boot process. In essence, computers use the SPD to identify the characteristics of the memory modules. Computer manufacturers identify modules through SPD signatures written on the DIMM Module. Some computers cannot work with memory modules having a mis-match SPD. Below is a “short list” example of the type of information provided by the SPD EEPROM. At this point this information is meaningless to you, however it is vital to the smooth operation of the PC system. You’ll understand more as we continue our review.
- Type of memory used
- Number of Row and Columns
- Number of DRAM chips on the PC Board
- The data width of the module
- The Voltage of the module
- The Speed of the device(s)
Here is an example of the data obtained from an SPD EEPROM on a 64MB, PC-100 qualified module. The SPD data was read using a Simmtester test equipment.
|Byte 0||Number of Bytes Used||128|
|Byte 1||SPD Size||8|
|Byte 2||Memory Type||4|
|Byte 3||Row Address||11|
|Byte 4||Column Address||9|
|Byte 5||Number of Banks||1|
|Byte 6||Data Width (L)||64|
|Byte 7||Data Width (H)||0|
|Byte 8||Interface Level||1|
|Byte 9||Cycle Time||128|
|Byte 10||Clock Access Time||96|
|Byte 11||DIMM Configuration Type||0|
|Byte 12||Refresh Rate/Type||128|
|Byte 13||Primary SDRAM Width||8|
There are a total of 256 bytes in the EEPROM and most manufacturers will only use the first 128 bytes to contain specific data. A PC-100 compliant module has the following bytes programmed exactly with this data:
- Byte 9 – is programmed with a 8ns Cycle time
- Byte 10 – is programmed with a 6ns access time
- Byte 18 – is programmed with a CAS latency of 3
*Unfortunately, you are not able to read this data from the module’s SPD unless you own a memory tester or a SPD reader.
Write Protect Feature
As part of its specifications, Intel requires that all PC-100 modules include a write-protect pin (module pin 81) to be connected to pin 7 of the SPD EEPROM. According to that ‘specification’ any module that does not include this write-protect pin cannot be consider as being PC-100 compliant. The specifications for PC-66 memory modules do not require any connection on the write-protect pin.
How to Identify a PC-100 Module quickly.
The simplest and most convenient way to identify a PC-100 compliant ship (or non-compliant one for that matter) is to read the chip markings. A PC-100 compliant memory chip includes a label affixed to it which identifies the module as being “PC-100 compliant”. The chip marking should indicate “-8” or even “7.5” after the string of manufacturer part numbers. As an example, a Samsung chip will bear this part number, KMM374S803AT-G8, and as the illustration below shows, a Micron part “MT48LC8M8A2 – 8C”. This may not be entirely accurate, but it is the simplest and most convenient method to identify memory modules without having to use sophisticated memory testers. Usually a PC-66 memory will indicate “-10” after the part numbers, and PC-133 memory modules will indicate “-7.5”, however nothing is set in stone, and you should be aware of it.
What happens if my memory is not PC-100 compliant?
It means you may experience system errors in a 100mhz system because the memory’s performance cannot keep up with the system requirement. The system will operate at the speed of the slowest component. For example, installing 66MHz SDRAM memory in a PC-100 system will cause the bus to operate at 66MHz, rather than the speed it was designed to operate at.
Who should you buy memory modules from?
Obviously it is our desire that you purchase the memory modules you need from us, however there are important factors that you need to know. The deciding factor in where you purchase the memory modules you need is the manufacturer’s ability to achieve PC-100 (or PC-133) compliance. Only highly experienced and well-capitalized memory module manufacturers possess the design and application engineering capabilities to meet the difficult timing parameters of the PC-100 specifications. Micron, Samsung, IBM, Toshiba, NEC, Hyundai, for example, are among the few memory manufacturers able to produce both PC-100 DRAM chips and PC100 memory modules. The difficulty in meeting the specifications and standards for memory technology results in a premium product price. This however, as we mention above, is also an enticement for some third party module manufacturers in the industry to participate in the distribution of non-compliant memory products that in no manner will meet the specification set forth by Intel.
In recent months there has been an increasing influx of non-compliant memory modules in Europe, Australia, the America’s and other heavy technology areas from memory vendors located in Asian countries that are using 100MHz SDRAM chips to build modules labeled as PC-100 compliant. Although the 100MHz SDRAM peaks at a bus speed of 100MHz, it cannot sustain the speed required in the PC-100 specification. An SDRAM that internally clocks at 125MHz or greater is necessary to sustain the 100MHz local bus speed.
Things to remember, “Caveat Emptor”, buyer beware. If it sounds too good to be true, it probably is. There are no replacements for branded memory that uses branded SDRAM chips. Is it a little more expensive than that special memory deal?.. more than likely it is. But consider what your time is worth, especially when you may spend several days trying to sort out your computer only to discover that the problem all along was your “new” memory.
We only sell branded and fully qualified memory modules. In addition to the manufacturers testing, we check every module ourselves before shipment. It is important for you to closely analyze SDRAM offerings to ensure PC-100 compliance. Since you, the consumer, is unable to verify PC-100 or PC-133 compliance, you are advised to purchase SDRAM modules from reputable vendors who thoroughly understand the importance of meeting module specifications.
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 Identify PC-133 Memory Modules
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: 11/01/2000