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How SDRAM Works

Ups and Downs: Memory Timings Put to the Test
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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.

Cycle Time T Max. Frequency f Bandwidth SDR* Bandwidth DDR*
10 ns 100 MHz 800 MB/s (PC100) 1,600 MB/s (DDR200)
7.5 ns 133 MHz 1,064 MB/s (PC133) 2,100 MB/s (DDR266)
6 ns 166 MHz - 2,700 MB/s (DDR333)
5 ns 200 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.

Name TypeName: Effective Clock Speed Data Bus Bandwidth
PC66 SDRAM 66 MHz 64 Bit 0.5 GB/s
PC100 SDRAM 100 MHz 64 Bit 0.8 GB/s
PC133 SDRAM 133 MHz 64 Bit 1.06 GB/s
PC1600 DDR200 100 MHz 64 Bit 1.6 GB/s
PC1600 Dual-DDR200 100 MHz 2 x 64 Bit 3.2 GB/s
PC2100 DDR266 133 MHz 64 Bit 2.1 GB/s
PC2100 Dual-DDR266 133 MHz 2 x 64 Bit 4.2 GB/s
PC2700 DDR333 166 MHz 64 Bit 2.7 GB/s
PC2700 Dual-DDR333 166 MHz 2 x 64 Bit 5.4 GB/s
PC3200 DDR400 200 MHz 64 Bit 3.2 GB/s
PC3200 Dual-DDR400 200 MHz 2x 64 Bit 6.4 GB/s
PC4200 DDR533 266 MHz 64 Bit 4.2 GB/s
PC4200 Dual-DDR533 266 MHz 2 x 64 Bit 8.4 GB/s
PC800 RDRAM Dual 400 MHz 2 x 16 Bit 3.2 GB/s
PC1066 RDRAM Dual 533 MHz 2 x 16 Bit 4.2 GB/s
PC1200 RDRAM Dual 600 MHz 2 x 16 Bit 4.8 GB/s
PC800 RDRAM Dual 400 MHz 2 x 32 Bit 6.4 GB/s
PC1066 RDRAM Dual 533 MHz 2 x 32 Bit 8.4 GB/s
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