Ups and Downs: Memory Timings Put to the Test

How SDRAM Works

State-of-the-art RAM modules generally transfer data in 64 bit chunks. They contain DRAM chips that send data synchronously with the clock pulse signal and generally use the double-data-rate method (DDR). The difference between DDR and SDR-SDRAMs is that the DDR modules transfer data during both the rising and the falling edges of the clock pulse. That means that DDR400 RAM really only sends data at 200 MHz using the DDR method.

A better measure of memory speed is the module's cycle time, which is the amount of time needed to complete one clock cycle. A cycle time of 10 ns means that 100 million cycles are possible per second, and the chips run at up to 100 MHz. To reach 133 MHz, you need 7.5 ns; for 166 MHz, 6.0 ns.

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Cycle Time TMax. Frequency fBandwidth SDR*Bandwidth DDR*
10 ns100 MHz800 MB/s (PC100)1,600 MB/s (DDR200)
7.5 ns133 MHz1,064 MB/s (PC133)2,100 MB/s (DDR266)
6 ns166 MHz-2,700 MB/s (DDR333)
5 ns200 MHz-3,200 MB/s (DDR400)

* Here's how to calculate bandwidth: frequency x interface width (64 bits are 8 Bytes). DDR RAM offers twice the transfer rate of SDR RAM.

Nomenclature: RAM Names

The name game was a lot easier with conventional SDR-SDRAM, which was simply named for the clock speed (PC100, PC133 SDRAM). The rules changed with the advent of DDR RAM. The modules are now titled using the maximum bandwidth (in MB/s). So PC2100 is DDR266, PC2700 is DDR 333, etc. This sea change was based on the nomenclature used for Rambus DRAM (RDRAM), whose names - PC800 or PC1066 - were also derived from their frequency. The following table provides more information.

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NameTypeName:Effective Clock SpeedData BusBandwidth
PC66SDRAM66 MHz64 Bit0.5 GB/s
PC100SDRAM100 MHz64 Bit0.8 GB/s
PC133SDRAM133 MHz64 Bit1.06 GB/s
PC1600DDR200100 MHz64 Bit1.6 GB/s
PC1600Dual-DDR200100 MHz2 x 64 Bit3.2 GB/s
PC2100DDR266133 MHz64 Bit2.1 GB/s
PC2100Dual-DDR266133 MHz2 x 64 Bit4.2 GB/s
PC2700DDR333166 MHz64 Bit2.7 GB/s
PC2700Dual-DDR333166 MHz2 x 64 Bit5.4 GB/s
PC3200DDR400200 MHz64 Bit3.2 GB/s
PC3200Dual-DDR400200 MHz2x 64 Bit6.4 GB/s
PC4200DDR533266 MHz64 Bit4.2 GB/s
PC4200Dual-DDR533266 MHz2 x 64 Bit8.4 GB/s
PC800RDRAM Dual400 MHz2 x 16 Bit3.2 GB/s
PC1066RDRAM Dual533 MHz2 x 16 Bit4.2 GB/s
PC1200RDRAM Dual600 MHz2 x 16 Bit4.8 GB/s
PC800RDRAM Dual400 MHz2 x 32 Bit6.4 GB/s
PC1066RDRAM Dual533 MHz2 x 32 Bit8.4 GB/s