Memory

DRAM

DRAM = Dynamic Random Access Memory

SDRAM

SDRAM = Synchronous Dynamic RAM

SDRAM is a variant of DRAM in which the memory speed is synchronized with the clock pulse from the CPU.

This synchronization enables the SDRAM to pipeline read and write requests. Pipelining enables the SDRAM to accept commands at the same time as it is processing other commands.

Voltage: 3.3 V

• Registred/Unregistred
• ECC/Non-ECC

DDR SDRAM

DDR SDRAM = Double Data Rate SDRAM

DDR SDRAM is an improvement over regular SDRAM, also known as SDR SDRAM (Single Data Rate SDRAM).

DDR SDRAM doubles the bandwidth of SDR DRAM by transferring data twice per cycle on both edges of the clock signal, implementing burst mode data transfer.

Voltage: 2.5V

Note: ¹⁾ Standardised by JEDEC

• Registred/Unregistred
  • Registered modules are slightly slower than non-registered modules, because the registering process takes one clock cycle.
  • Registered modules always includes ECC.
• ECC/Non-ECC

DDR2 SDRAM

DDR2 SDRAM is the second generation of DDR SDRAM.

DDR2 SDRAM improves on DDR SDRAM by using differential signaling and lower voltages to support significant performance advantages over DDR SDRAM.

Differential signaling requires additional contacts, so the number of contacts on a DDR SDRAM DIMM module was raised from 184 to 240. The voltage of DDR SDRAM DIMM's was lowered from 2.5V to 1.8V. This improves power consumption and heat generation, as well as enabling more dense memory configurations for higher capacities.

Voltage: 1.8V

Note: ¹⁾ Standardised by JEDEC

• Registred/Unregistred
• ECC/Non-ECC
• Fully Buffered/Unbuffered
  • DDR2 modules with an integrated memory controller that helps send data in packets down the line to the CPU without any errors.
  • The FB-DIMM’s most noticeable feature is the Advanced Memory Buffer that separates the data into packets that are more easily handled and allow for a more error free data flow.
  • The FB-DIMMs have ECC or Error Checking and Correcting functionality as well.

DDR3 SDRAM

Same numer of pins as DDR2, but different pinout.

Voltage: 1.5V

Rambus DRAM

RIMM uses Direct Rambus DRAM memory technology.

Voltage: 2.5V or 1.8V

• C-RIMM = Continuity RIMM
  • Free memory slots has to be filled with C-RIMMs
• Mixing PC800 & PC1066 modules.
  • You can mix speeds in most systems. However the system will run at the lowest speed of RDRAM installed. In this case your system would only run at 800 Mhz.
• Can I install both ECC and non-ECC?
  • Mixing ECC and non-ECC is not recommended. If any of the modules do not support ECC, then the ECC functionality will be disabled and in some cases you may have difficulty booting your machine.

Time line

• 1987: FPM
• 1995: EDO
• 1997: PC66 SDRAM
• 1998: PC100 SDRAM
• 1999: RDRAM
• 1999: PC133 SRAM
• 2000: DDR SDRAM
• 2004: DDR2 SDRAM
• 2007: DDR3 SDRAM

FAQ

• PC1600/PC2100 vs PC100/PC133
  • PC1600 runs at same clock rate as PC100.
  • PC1600 is double datarate than PC100.
  • Each module is 8 bytes. Due to naming of Rambus modules, PC100*2*8 became PC1600.
  • PC133*2*8 = PC2100.
• Can I run PC3200 memory in a PC1600 computer?
  • Yes. Underclocking at PC1600 speed is alright.
• Can I run PC1600 memory in a PC3200 computer?
  • Maybe?

Single Rank vs Dual Rank

One rank is 64 bits (ECC: 72 bits) of DRAM. Use single rank to utilize the motherboard best. Dual rank requires switching between ranks with a CS (chip select) pin.

Mixing Single Rank and Dual Rank is generally allowed, but you might have to insert your Dual Rank modules in the first slots.

Links

• Crucial: Memory Module Identification
• www.interfacebus.com VLP-DIMM description
• Qimonda Product Information June 2006: Micro-DIMM
• Viking InterWorks' memory module capacities, mechanical dimensions and performance