Ups and Downs: Memory Timings Put to the Test

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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 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.

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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