Nowadays, both computers and mobile phones are inseparable from storage devices.

According to von Neumann structure, storage device and CPU are separated.

In order to balance the storage space, read-write speed, price and other factors, most devices use 2 to 3 levels of different storage devices.

The first level of storage needs to have the fastest speed, but the capacity is very small. Cache as the CPU cache, using the fastest SRAM circuit, by copying the data in the main memory to the cache, improve the access speed of the cup. The key technology in cache is address mapping. There are three methods: direct mapping, fully connected mapping and group associated mapping.

The second level storage needs fast speed and a certain capacity. This is what we often call main memory (memory module). DRAM circuit is usually used to store and read data through capacitance. The biggest problem of DRAM is that there is not enough space in a cell to produce suitable capacitance with the reduction of transistor size. In addition, because of the discharge characteristics of the capacitor, DRAM needs additional refresh operation to maintain the power on the capacitor, and the refresh time is generally Ms.

The third level storage does not need fast speed, but it needs a large capacity. Generally, there are two kinds of storage: HDD (hard disk drive) and SSD (solid static disk). HDD is mechanical hard disk, SSD is solid state hard disk. SSD can be divided into nor and NAND. NAND adopts floating gate technology, that is, a layer is added under the gate of NOMs. By adding a large voltage to VGS to charge floating gate and change Vth, the current in the process of reading data is changed, and this process is reversible. Floating gate can be restored. Floating gate was invented by Dr. f. MASUOKA of Toshiba in 1986.

SRAM and DRAM are both volatile memory, and they lose data when power down. SSD and HDD are nonvolatile memory, which can also store data when power down, such as small movies on hard disk.

Later, if you have time, I will introduce these three common storage technologies in detail.

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