RAM Types

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Modern system RAM is implemented as a type called Double Data Rate Synchronous Dynamic Random Access Memory (DDR SDRAM).

• Unpacking that name reveals a history of PC system memory implementations:
  • Dynamic RAM stores each data bit as an electrical charge within a single bit cell
    • A bit cell consists of:
      • a capacitor to hold a charge (the cell represents 1 if there is a charge and 0 if there is not)
      • a transistor to read the contents of the capacitor
  • Synchronous DRAM (SDRAM) is so-called because its speed is synchronized to the motherboard system clock
  • Double Data Rate SDRAM (DDR SDRAM) makes two data transfers per clock cycle
• DDR memory modules are labeled using the maximum theoretical bandwidth
  • E.g., PC1600, PC2100, etc.

Example

Example of how the maximum theoretical bandwidth is derived:

• The internal memory device clock speed and memory bus speed (between the memory devices and memory controller) are both 100 MHz.
• The data rate is double this as there are two operations per clock “tick.”
  • expressed in units called megatransfers per second (200 MT/s)
  • This gives the DDR-200 designation
• The peak transfer rate is 1600 MBps (200 MT/s multiplied by 8 bytes (64 bits) per transfer).
  • This gives the “PC-1600” designation
  • 1600 MBps is equivalent to 1.6 GBps

• Subsequent generations of DDR technology—DDR2, DDR3, DDR4, and DDR5—increase bandwidth by multiplying the bus speed, as opposed to the speed at which the actual memory devices work
  • This produces scalable speed improvements without making the memory modules too unreliable or too hot
• Design improvements also increase the maximum possible capacity of each memory module

RAM Type	Data Rate	Transfer Rate	Maximum Size
DDR3	800 to 2133 MT/s	6.4 to 17.066 GB/s	8 GB
DDR4	1600 to 3200 MT/s	12.8 to 25.6 GB/s	32 GB
DDR5	4800 to 6400 MT/s	38.4 to 51.2 GB/s	128 GB

Info

• The transfer rate is the speed at which data can be moved by the memory controller.
• Memory modules also have internal timing characteristics, expressed as values, such as 14-15-15-35 CAS 14.
• These timings can be used to differentiate performance of RAM modules that are an identical DDR type and speed. Lower values are better.