Memory Primer

Definition of RAM

A computer is made up of many components.  One of these components is RAM which is short for Random Access Memory.  RAM is also known simply as memory.  Think of RAM as a desk.  Each application you open is stored in memory.   Many operating system files also reside in memory.  The processor only works with data stored in memory.  If needed data is not found in memory, it is retrieved from the hard drive and put in memory for the processor to use.  It takes a very long time to get data from the hard drive but memory is much quicker.  Most of the data the computer works with is accessed over and over again.  Because of this, the data stored in memory will remain there until the application is closed or when the memory is completely filled with data.

Each time the computer needs to work with data, it will grab it from RAM.  The computer can perform tasks in less time if it can get this data from RAM quicker.  Choosing the best RAM is important because different types of RAM operate at different speeds.  Choosing the best RAM is not as easy as picking out the fastest RAM because not all types of RAM will work with all computers.  A savvy computer shopper should be aware of the three types of RAM available today.  These types are SDRAM, DDR RAM, and RDRAM.  Shoppers should know the difference between ECC and non-ECC RAM.  


The first type of available RAM is SDRAM.  SDRAM stands for Synchronous Dynamic RAM.  SDRAM is synchronous because it operates at the same speed as the system clock.  This speed is either 100MHz or 133MHz.  SDRAM running at 100MHz is called PC100 SDRAM and SDRAM running at 133MHz is called PC133 SDRAM.  The computer can retrieve eight bits from memory per clock cycle.  If the system is running at 100MHz with PC100 SDRAM, the computer will be able to retrieve eight bits from memory 100 million times per second.  SDRAM is called dynamic because the data stored on each chip must be recreated every so often.  This is called refreshing the RAM.  SDRAM can be used with the Celeron, Pentium III, Duron, Athlon, and Athlon Thunderbird processors.  One of the biggest advantages of DDR RAM is the price.  DDR RAM is only a few dollars more than SDRAM of the same size.


The second type of available RAM is DDR.  DDR stands for Double Data Rate.  DDR memory can be accessed twice per clock cycle.  This is known as clock doubling.  DDR memory can operate at 100MHz, 133MHz, 166MHz, and 200MHz.  However, DDR memory that runs at 133MHz will not be listed as 133MHz DDR RAM.  Rather, 133MHz DDR RAM will be listed as 266MHz DDR RAM because RAM manufacturers factor in the clock doubling ability of DDR when computing the speed.  See figure 2 for a comparison of listed speed to actual speed.  DDR RAM can transfer 16 bits per clock cycle.  DDR RAM can be used with the Celeron, Duron, Pentium 4, Athlon, Athlon Thunderbird, and Athlon XP processors.


The third type of available RAM is RDRAM.  RDRAM stands for Rambus Dynamic RAM and is sometimes called Rambus memory.  Rambus memory operates at speeds from 600MHz to 800MHz.  Rambus memory can also be accessed twice per clock cycle.  The high clock speeds of Rambus memory are deceiving because Rambus memory transfers only four bits per cycle.  Rambus memory is very expensive.  A comparable stick of RDRAM will cost approximately three times as much as DDR memory.

Comparing “Apples to Apples”

Each type of memory must be evaluated using a common variable in order to properly compare each type of memory.  The common variable is the number of bytes transferred per second.  One megahertz (MHz) is one million hertz.  A hertz is equivalent to one second.  Thus, one megahertz is one million times per second.  The chart below computes the number of bytes transferred per second with each type of memory at the speeds available today.

SDRAM100 MHzx8 bytes/hertz=800 million bytes per sec
133 MHzx8 bytes/hertz=1.064 billion bytes per sec
DDR Memory266 MHzx8 bytes/hertz=2.128 billion bytes per sec
333 MHzx8 bytes/hertz=2.664 billion bytes per sec
400 MHzx8 bytes/hertz=3.2 billion bytes per sec
Rambus Memory600 MHzx4 bytes/hertz=2.4 billion bytes per sec
800 MHzx4 bytes/hertz=3.2 billion bytes per sec

Error Correction Code

Error Correction Code (ECC) is an option for error checking on memory.  ECC is available for all three types of memory.  Memory problems are a rare occurrence.  These problems, often called “hiccups”, result in a loss of data.  ECC was created to fix most of these problems.  ECC memory is more expensive than non-ECC memory.  ECC memory also requires an ECC compliant motherboard in order for it to work.  ECC memory should be avoided in most home and office systems.


SDRAM should be avoided because of its inherent slow speed.  Pentium III systems and all systems older than the Celeron do not have the option of using faster DDR or RDRAM.  To get the best performance, use DDR RAM with Athlon systems.  Pentium 4 systems can use DDR RAM or RDRAM.  DDR RAM is still the best choice because it offers similar performance for a much lower price.    However, you should note that Rambus is working on a new type of RDRAM which will operate at 1066 MHz.  Until then, DDR is the best RAM choice.



Figure 1 – The type of RAM usable on each platform

TypeUsable on
SDRAMCeleron, Duron, Pentium III, Athlon, & Athlon Thunderbird
DDR RAMCeleron, Duron, Pentium 4, Athlon, Athlon Thunderbird, & Athlon XP
RDRAMPentium 4

Figure 2

TypePC RatingActual SpeedListed Speed
SDRAMPC100100 MHz100 MHz
PC133133 MHz133 MHz
DDRPC1600100 MHz200 MHz
PC2100133 MHz266 MHz
PC2700166 MHz333 MHz
PC3300200 MHz400 MHz
RambusPC600600 MHz600 MHz
PC700700 MHz700 MHz
PC800800 MHz800 MHz

Works Cited

Meyers, Michael. All in One A+ Certification Exam Guide Third Edition. Berkely: Osborne/McGraw Hill, 2001.

“RDRAM/DDR: Part 1, Technical Aspects.” Retrieved 20 July 2002 <>

“Ram Guide.” Tom’s Hardware Guide. Retrieved 20 July 2002 <>

“What does MHz have to do with how fast my computer runs?” Pathways Development Group, Inc. Retrieved 20 July 2002 <>

“Rambus RDRAM RIMM 4200 White Paper.” Rambus. Retrieved 20 July 2002 <>

About The Author

Eric Vanderburg

Eric Vanderburg is an author, thought leader, and consultant. He serves as the Vice President of Cybersecurity at TCDI and Vice Chairman of the board at TechMin. He is best known for his insight on cybersecurity, privacy, data protection, and storage. Eric is a continual learner who has earned over 40 technology and security certifications. He has a strong desire to share technology insights with the community. Eric is the author of several books and he frequently writes articles for magazines, journals, and other publications.

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