SRAM static memoryIt is characterized by small capacity and high price. The advantage is that it can be used directly after powering on without software initialization.
DRAM dynamic memoryThe characteristics are large capacity and low price. The disadvantage is that it cannot be used directly after power-on, and can only be used after software initialization.
In single-chip microcomputer: the demand for memory is small, and the development is expected to be as simple as possible, which is suitable for all SRAM
embedded systems: the demand for memory is large, and there is no bootable medium such as NorFlash
PC: the demand for memory is large, and the software is complex, and does not care about the initialization of DRAM Overhead, suitable for all DRAM
external memory:
NorFlash: It is characterized by small capacity and high price. The advantage is that it can be directly connected to the CPU by bus, and can be read directly after the CPU is powered on, so it is generally used as a boot medium.
NandFlash (same as hard disk): It is characterized by large capacity and low price. The disadvantage is that it cannot be accessed by bus, that is to say, it cannot be directly read by the CPU when it is powered on. The CPU needs to run some initialization software first, and then read and write through the timing interface.
General PCs are: Small-capacity BIOS (NorFlash) + large-capacity hard disk (similar to NandFlash) + large-capacity DRAM
General microcontroller: small capacity NorFlash + small capacity SRAM
General embedded system: Because NorFlash is very expensive, many embedded systems now tend not to use NorFlash, but directly use: external large-capacity Nand + external large-capacity DRAM + SoC built-in SRAM, the startup method used by S5PV210 is: external large-capacity Nand + external high-capacity DRAM + SoC built-in SRAM
SDRAM: Synchronized Dynamic Ramdam Access Memory, synchronous dynamic random access memory
DDR: DDR is DDR SDRAM, which is an upgraded version of SDRAM. (DDR: double rate, double-speed SDRAM)
DDR has many generations: DDR1 DDR2 DDR3 DDR4 LPDDR
SDRAM features (large capacity, low price, power-down volatility, random read and write, bus access)
Both SDRAM and DDR belong to dynamic memory (relative to static memory SRAM), and they all need to run a piece of initialization code to initialize before they can be used, unlike SRAM, which can run directly after power-on. Similar to the difference between SDRAM and SRAM, there are NorFlash and NandFlash (hard disk). It is precisely because of the limitations of the hardware itself that the startup code is weird and complicated. And we study bare metal in order to study uboot, in uboot, we make full use of various features of hardware and deal with the complexity of hardware. All we have is uboot