The memory access time of your computer decides how fast the system can read from or write to its random access memory (RAM). The shorter the access time, the faster is the system able to carry out its instructions - resulting in better performance and less "lag". Most modern RAM modules are capable of very short memory access times, hence producing very high memory bandwidth (for example, a DDR2 RAM module is theoretically capable of 6.400 GB/s bandwidth).
What is memory bandwidth?
Peak memory bandwidth = (Memory I/O clock frequency) * (Transfer per clock) * (Memory bus width)
However, in real life, actual bandwidth is much more complicated than this.
Memory Types
With so many different types of memory (RAM) in the market, even looking for the right kind of RAM module for your motherboard can prove challenging - let alone choosing from the many different specs for the same type of RAM! Here are the 4 most common types of RAM found today:
EDO RAM
Extended Data Out RAM is a type of dynamic random access memory that allows a new access cycle to be started while the data of the previous cycle has not yet been accessed by the CPU. It offers improvement over the earlier Fast Page Module RAM (not mentioned here) as it eliminates wait states by keeping the output buffer active until the next cycle began. EDO RAM is subsequently superceded by SDRAM.
SDRAM
SDRAM stands for Synchronous RAM (Dynamic Random Access Memory). It is a solid state computer memory that waits for the rising or falling edge of the clock signal to synchronize data transfer (see diagram below). SDRAM DIMMs are rated according to its maximum clock rate and they transfer 8 bytes per clock cycle. They are available in 168-pin DIMM module (used in desktop computers) and 144-pin SODIMM nodule (used in most laptops and embedded hardware devices). Physically, a 168-pin DIMM SDRAM has 2 "notches" (as opposed to 1 on a 184-pin DIMM DDR RAM).
DDR RAM
DDR RAM stands for double data rate RAM. It achieves greater bandwidth that the SDRAM because data exchange synchronization with the CPU is achieved at the rising and falling edge of the clock signal (see diagram below). Thus a DDR RAM has double the bandwidth of an equivalent SDRAM operating at the same bus speed. DDR RAM DIMM modules have 184-pin and one notch.
DDR2 RAM
The advantage of DDR2 RAM over DDR RAM is its ability for much higher clock speed and that DDR2 has a doubled bus frequency for the same physical clock rate, thus doubling the effective data rate another time. Hence, a DDR2 RAM module rated at 100MHz will have an I/O clock of 200MHz. DDR2 RAM DIMM module has 240 pins and is not backward compatible with DDR DIMMs.
This table displays the theoretical bandwidth of SDRAM, DDR RAM and DDR2 RAM DIMM modules:
| Type | Chip | Bus speed | I/O clock | Module | Theoretical bandwidth |
| SDRAM | 66 MHz | 66 MHz | PC-66 | 533 MByte/s | |
| 100 MHz | 100 MHz | PC-100 | 800 MBytes/s | ||
| 133 MHz | 133 MHz | PC-133 | 1064 MBytes | ||
| DDR (or DDR1) | DDR-200 | 100MHz | 100MHz | PC-1600 | 1.60 GByte/s |
| DDR-266 | 133MHz | 133MHz | PC-2100 | 2.13 GByte/s | |
| DDR-333 | 166MHz | 166MHz | PC-2700 | 2.67 GByte/s | |
| DDR-400 | 200MHz | 200MHz | PC-3200 | 3.20 GByte/s | |
| DDR2 | DDR2-400 | 100MHz | 200 MHz | PC2-3200 | 3.20 GByte/s |
| DDR2-533 | 133MHz | 266 MHz | PC2-4200 | 4.26 GByte/s | |
| DDR2-667 | 166MHz | 333 Mhz | PC2-5300 | 5.33 GByte/s | |
| DDR2-800 | 200MHz | 400 MHz | PC2-6400 | 6.40 GByte/s |
Testing your memory speed
PassMark's Performance Test provides user with an easy to use memory test suite that exercises your RAM and calculating the rate of data transfer of your memory module. These are some of the submitted baseline results taken from Performance Test users:
From the above results, the memory speed of the systems are between 1152 MBytes/s to 1669 MBytes/s.
Performance Test also has an Advanced Memory Test that allows the user to configure the memory test variable for more interesting results!