Differences between Rom, ram, and Flash

ROM: read-only memory, which has beenFlash
Substitution

Ram
: Random Access to memory, divided into SRAM and drama, with fast speed

Flash
: Divided into norFlash
And NandFlash
The former can execute code locally, and the latter cannot.

 

Ram
The full name of random access memory is random access memory, which is equivalent to mobile storage on a PC, used to store and store data. It can be read and written at any time,Ram
It is usually used as a temporary storage medium (or system memory) for the operating system or other running programs ).

However, when the power supply is offRam
Data cannot be retained. To save the data, you must write it to a long-term storage (such as a hard disk ). Because of this, sometimesRam
It is called "variable memory ".Ram
Memory can be further divided into staticRam
(SRAM) and dynamic memory (Dram. DRAM has a low unit capacity price, so it is widely used as the primary memory of the system.

Rom (read only
The full name of memory is read-only memory, which is equivalent to a hard disk on a PC and used to store and store data. Rom data cannot be updated at will, but can be read at any time. Even if it is disconnected
Power, ROM can also retain data. However, data 1 can only be modified using special methods or cannot be changed after being written. Therefore, Rom is often used in an embedded system to store the operating system. Mainstream
The PDA Rom size is 64 MB and 128 MB.

Ram
The biggest difference between ROM and Rom isRam
After a power failure, the data stored in the above will automatically disappear, and the Rom will not.
The hard-to-change feature of Rom makes it quite troublesome to update materials.Flash
The development of memory,Flash
Memory has the advantage that Rom does not need power to maintain data, and can change the data as needed. However, the unit price is higher than that of general Rom.

The speed of SRAM is very fast. It is currently the fastest reading and writing storage device, but it is also very expensive, so it is only used in demanding places, such as the first-level buffer of CPU and second-level buffer. DynamicRam
(DynamicRam
/Dram) data is retained for a short period of time, and the speed is also slower than SRAM. However, it is faster than any Rom, but DRAM is much cheaper than SRAM in terms of price, the computer memory is dram.
DRAM is divided into many types, including fpram/fastpage, edoram, SDRAM, and DDR.Ram
, RDRAM, sgram, and wram. Here we will introduce a DDRRam
.
DDRRam
(Date-RateRam
) Also known as ddr sdram, this improvedRam
It is basically the same as SDRAM. The difference is that it can read and write data twice in a clock, which doubles the data transmission speed. This is the most widely used memory in computers, and it has a cost advantage. In fact, it beat Intel's other memory standard-Rambus DRAM. High-speed DDR is also available on many high-end video cards.Ram
To increase the bandwidth, which can greatly improve the pixel rendering capability of the 3D accelerator card.

Memory Working principle: memory is used to store the data and programs currently in use (that is, execution). The memory we usually refer to as dynamic memory (that is, Dram ), the so-called"
Dynamic "means that after we write data into dram, data will be lost after a period of time, so we need a memory refresh operation by using a peripheral circuit.

The specific working process is as follows: whether a dram storage unit stores 0 or 1 depends on whether the capacitor has a charge. If there is a charge, it indicates 1, and if there is no charge, it indicates 0. However, for a long time, the capacitance of 1 will discharge,
It indicates that the capacitor of 0 will absorb the charge, which is the cause of data loss. The refresh operation regularly checks the capacitor. If the power is greater than 1/2 of the full power, it indicates 1, and the capacitor is fully charged; if the power is less
1/2, it is regarded as 0, and the capacitor is discharged to maintain data continuity.

There are also many types of Rom. The difference between the prom and the EPROM is that the prom is one-time, that is, after the software is filled in, it cannot be repaired.
This is an early product, but it is no longer usable. The EPROM is a general-purpose memory, which is used to erase the original program through ultraviolet radiation. Another type of EEPROM is through the electronic wipe
Output, the price is very high, the write time is very long, the write is very slow.
For example, the mobile phone software is usually placed in the EEPROM. When we call, some of the last dialing numbers are temporarily stored in the SRAM, rather than writing the pass record immediately (the call record is stored in the EEPROM ), at that time, there was a lot of important work (CALLS) to do. If you write data, a long wait would be unbearable for users.

Flash
Memory, also known as flash memory, combines ROM andRam
Advantages: not only can the electronic Erasable Programmable (EEPROM) performance, but also can quickly read data without power loss (NVRAM advantage ), this memory is used in USB flash drives and MP3 files. In the past 20 years, embedded systems have been using ROM (EPROM) as their storage devices. However, in recent yearsFlash
It fully replaces the position of RoM (EPROM) in the embedded system, and is used to store bootloader, operating system or program code, or directly used as the hard disk (USB flash disk ).
CurrentlyFlash
There are two types of norFlash
And nadnFlash
. NorFlash
The reading is the same as the reading of our common SDRAM. You can directly runFlash
This can reduce the capacity of the SRAM and save the cost. NandFlash
The random read Technology of memory is not used. It reads 512 bytes at a time, usually in the form of one read.Flash
Relatively cheap. Users cannot directly run NandFlash
Therefore, Nand is widely used.Flash
In addition to using NAND flah, the Development Board also has a small norFlash
To run the startup code.
Nor is usually used for small capacityFlash
Because of its fast reading speed, it is used to store important information such as the operating system, while the large-capacity NandFlash
, The most common NandFlash
Applications are embedded systems using DOC (Disk On Chip) And we usually use flash disks, which can be erased online. Currently availableFlash
Mainly from Intel, AMD, Fujitsu and mxic, while producing NandFlash
The main manufacturers include Samsung, Toshiba and Hynix.