Memory Classification - Type of RAM

RAM

Random Access Memory — random-access storage, with nearly uniform access time to any region. Compared with a hard disk, the former is like a phone call—any number takes roughly the same time to connect; the latter is like walking down a street, visiting house #1 on Street A and house #1 on Street B, where the starting point on Street A or B has a significant effect on the time spent. This is also why the read/write speeds advertised by hard disk vendors are hard to achieve in practice. The advertised sequential read/write speed can be understood as starting from house #1 on Street A and visiting in order, while in most real-world application scenarios, after visiting house #1 on Street A, the next destination is very likely house #21 on Street B—efficiency drops significantly.

SRAM & DRAM

• SRAM - Static RAM
  • Commonly used as CPU cache
• DRAM - Dynamic RAM
  • Data is stored in 1C1T cells
  • Cheaper than SRAM, also slightly slower than SRAM
  • Used for most replaceable storage (memory, main memory, DIMM — call it whatever you like)

ECC

Error Correcting Code

• DRAM with units for detecting and correcting random errors
• Commonly used as server memory; requires support from other components (CPU, motherboard)

SDRAM & DDR

SDRAM - Synchronous Dynamic RAM

In response to the increasing operating speed of other computer components, SDRAM was developed. Previously, memory was asynchronous, running independently of the CPU. SDRAM memory, however, synchronizes with the system bus. After DDR memory appeared, SDRAM was correspondingly also called SDR, Single Data Rate.

DDR - Double Data Rate

• Faster
• More power-efficient

Relatively speaking, the lower the memory voltage, the lower the power consumption, the less the heat, and the more power saved.

Various DDR memory voltages:

-

Voltage

DDR

2.5V

DDR2

1.8V

DDR3

1.5V

DDR4

1.2V

DDR5

1.1V

SDR vs DDR

• SDR — uses only one edge of the clock signal to transfer data[^1] (the term "edge" is hard to put into words; calling it the rising/falling edge of the high/low level feels no better than a machine translation. My mastery is shallow; I'll leave a hole to fill later.)
• DDR — transfers data on both the rising and falling edges of the clock signal

[^1]: translator's note pending