Understanding Computer Memory - Part 2

Learn About Random Access Memory Technologies

Some DDR RAM cartridges are also rated in speed by their maximum throughput, these cartridges were given a PC-4200, PC-2700 designation, etc. PC designations were actually a multiple of the memory clock or an equivalent of the system bus for which they were used. Usually they were multiples of eight. You may see RAM modules with speed ratings of PC-2700 or 333MHz/PC-2700, meaning 333MHz multiplied by 8. PC-3200 would be 3200 divided by 8, or 400MHz and so on. Either way, you will find both forms of speed rating designations on these types of RAM.

DDR2: Effectively doubles the data transfer rate again. DDR2 transfers data 4 times for each clock cycle, twice as the frequency rises and then twice again as it falls. These cartridges consisted of a 240-Pin configuration. DDR2 is measured by its maximum throughput and by its internal memory clock speed. For instance, You may see designations of DDR2-800 or PC2-6400, they both mean the same thing. DDR2 RAM speed is always written as twice its actual speed. Remember it doubles the data rate twice! So, DDR2-800 RAM has an actual clock speed of 400MHz, this is multiplied by 2 to get the 800MHz designation as stated on the cartridge. Likewise, DDR2-667 which means 667 MHz, is actually 333 MHz in terms of real speed. The PC2 designation always means DDR2. PC2 like the PC designation is a measure of the RAM modules theoretical maximum throughput. So, PC2-6400 is DDR2-800 multiplied by 8, or 8 X 8 = 64. So, the maximum throughput of PC2-6400 RAM measured in MB/sec would be 6400MB/s. When we state maximum throughput, we are nearly referring to the maximum data rate of data transfer possible within a given peroid of time. DDR2-667 is PC2-5300, meaning 667MHz X 8 = 5336MB/s (These numbers always given as an approximate value on RAM cartridges, never an exact value as we 've just shown).

DDR2 has several memory speeds you should become familiar with:

(DDR2-533 or PC2-4200), (DDR2-400 or PC2-3200), (DDR2-667 or PC2-5300), (DDR2-800 or PC2-6400), and (DDR2-1066MHz or PC2-8500). Just remember to multiply the DDR2-MHz factor by 8 in order to get the PC2 equivalent of maximum throughput.

Memory module types based on computer architecture:

There are 3 general types of RAM cartridges that are used in computers today. As we stated above, desktop PC’s generally use DIMM’s (Dual Inline Memory Modules), while laptops use SO-DIMM’s (Single Outline Dual Inline Memory Modules). SO-DIMM’s are much smaller but carry with them the same architecture. For instance, there are DDR2 DIMM’s and SO-DIMM’s for both desktop motherboards and for laptops, same with DDR3, etc.

Continue To Part 3 (RAM)