What is the function of external memory, and what is not included in common external memory

Due to technical and technical reasons, computer memory has limited capacity and cannot accommodate all software, so the computer system must be equipped with external memory. External memory, also known as auxiliary memory, is generally large in capacity, and most of it can be moved to facilitate the exchange of information between different computers. External memory is used to store programs, data, and various software resources.

The CPU cannot directly access the external memory in the same way as it accesses the memory, and must pass through the memory to exchange information with the external memory. On microcomputers, the commonly used external memory includes disks, optical disks, and tapes, and disks can be divided into soft disks and hard disks.

A floppy disk is a round thin film with a magnetic coating on both sides. At present, the floppy disks commonly used on microcomputers are divided into inch disks and inch disks according to size. Inches vary from inch to inch in size, format, and sleeve material, but the principle is the same.

Each side of a floppy disk contains many concentric circles called tracks. The tracks are numbered sequentially from the outside to the inside, with the outermost track being 0 and the innermost track being the last track. For the convenience of recording information, each track is divided into several sections, called sectors.

When a disk reads and writes, the basic unit is the sector, and each sector stores the same amount of information. When using a new floppy disk, the floppy disk is generally "formatted", and when formatted, the system will automatically divide the disk into tracks and sectors, and also store some necessary initialization information. Only a formatted floppy disk can be used to store information.

If you format a disk that already has information stored, all of the information in it will be "lost".

Hard disks are assembled from several metal platters. Hard drives are stored in a similar format to floppy disks, but they have a long lifespan, large storage capacity, and fast access speeds. The hard drive must also be formatted first when it is used for the first time.

The storage medium of optical discs is different from disks and belongs to a different type of storage. Optical disk storage consists of an optical disc and an optical drive. Depending on how they work, optical discs are divided into three main categories:

Read-only, write-once, and rewritable discs. Optical disc memory is a large-capacity information storage device with rapid development in recent years, especially the development of multi-** technology, optical drive has become the basic configuration of microcomputer.

Refers to external memory, and external memory refers to the memory of the computer in addition to.

and storage other than the CPU cache, which can still hold data even after a power failure. Common external storage devices include hard disks, floppy disks, optical discs, and USB flash drives.

Wait. The external memory unit is low**, the capacity is large, the speed is slow, and the data will not be lost after power failure.

Most of the external memory used in current microcomputers does not include memory

1. Hard disk, external memory is like a diary, you can record anything on it, and the information will not be lost after shutting down. The magnetic disc of the hard drive is made of a hard material and has a high degree of precision. The hard disk is enclosed in a case along with the drive, and its capacity is much larger than that of floppy disks and optical disks, and the read and write speeds are much faster than that of floppy disks and optical disks.

Since the hard disk is fully enclosed and fixedly installed, the hard disk drive is also fixed together and installed in the main chassis, which is not visible from the outside of the main chassis.

2. Floppy disk. A floppy disk is a disk that can be removed, inserted, and taken with you at any time. Due to its small storage capacity and slow access speed, floppy disks have now been phased out.

There are two types of floppy disks: inch and inch floppy disks. Floppy disk drives are also installed inside the main chassis, but there is an interface on the front of the main unit and a small door for plugging and locking floppy disks.

3. CD-ROMs. Optical discs have a large capacity, about 700 megabytes to 50 gigabytes, which are easy to carry and inexpensive, and their capacity is equivalent to the capacity of hundreds of floppy disks or USB disks combined. Discs are mainly divided into five layers:

Substrates, recording layers, reflective layers, protective layers, and printing layers. The disc is made of plastic, the data surface is coated with a layer of aluminum, and the data is recorded on uneven grooves of different heights, and the data is read by the laser head of the disc driver.

The essential difference between memory and external memory is that one is the function of internal operation to provide caching and processing, which can also be understood as a channel for collaborative processing; The external memory is mainly for the storage of documents, **, **, text and other information carriers, which can also be understood as storage space.

Memory is one of the most important components of a computer, and it is the bridge that communicates with the CPU. The running of all programs in the computer takes place in memory, so the performance of the memory has a very large impact on the computer. Memory, also known as internal memory, is used to temporarily store computing data in the CPU and exchange data with external memory such as hard disks.

As long as the computer is running, the CPU will transfer the data that needs to be calculated to the memory for calculation, and when the operation is completed, the CPU will send the result out, and the operation of the memory also determines the stable operation of the computer. Memory is made up of parts such as memory chips, circuit boards, gold fingers, etc.

In order to expand the storage capacity, the computer is also equipped with disk memory and optical disk memory with a larger storage capacity, which are called external memory, referred to as external memory. This type of memory can generally hold data even after the power is off. Common external memory includes hard disks, floppy disks, optical discs, U disks, etc.

Internal memory is the direct place where the CPU exchanges data with external devices, and the internal memory speed is second to the CPU speed, but it is also considered a high-speed storage device, which includes RAM (what we often call memory), video memory, and some cache. External storage mainly includes disk storage, tape storage, and optical disk storage.

External memory is an external storage device, which is much slower than memory, but can save data for a long time, and disk is the most commonly used external memory, usually it is divided into two categories: floppy disk and hard disk. There are also CD-ROMs, flash drives, and more.

What is the difference between memory and external memory?

Because the role of internal and external memory is different. The internal memory is a temporary storage area when the program is executed, and all data is lost after power failure; The external memory is used to store the original data and calculation results, which needs to be stored for a long time, and the data will not be lost after power failure.

The most prominent feature of internal memory is that the access speed is fast, but the capacity is small and expensive; External memory is characterized by large capacity and low capacity, but slow access speed. Internal memory is used to store programs and data that are immediately needed; External memory is used to store programs and data that are not in use for the time being. There is often a frequent exchange of information between the inner and outer memory.

It should be pointed out that external memory is also an input and output device, which can only exchange information with internal memory and cannot be directly accessed by other parts of the computer system.

Memory reads and stores quickly, matches the frequency of the CPU, and stores data that is always used and needs to be processed. External memory is used to store some data that is not used temporarily and needs to be stored for a long time.

Memory is what helps the system run... Generally smaller... But the storage speed is fast...

When you run the program... Save some ... Temporary files...

Hard disk... Generally larger... Save some ...

Large files ...

The memory is used to store the programs to be executed and the corresponding data, and the external memory is used as the backup device of the memory, which stores the programs and corresponding data that are not executed temporarily and will be executed in the future; Without the external memory input of the program and data, all kinds of information can not be stored for a long time (that is to say, the data in the memory will be lost after shutdown), and the next time you use the program, you have to re-enter.

The most commonly used memory is: Single Data Rate SDRAM (SDRAM-SDRAM), abbreviated as DDR.

External memory generally includes: floppy disks, hard disks, optical discs, U disks, etc. They can hold large amounts of data for a long time.

Primary memory is also known as internal memory (referred to as memory), and secondary memory is also known as external memory (referred to as external memory). External memory is usually magnetic media or optical discs, such as hard disks, floppy disks, tapes, CDs, etc., which can store information for a long time, and does not rely on electricity to save information, but it is driven by mechanical parts, and the speed is much slower than that of CPU. Memory refers to the storage component on the motherboard, which is the part that the CPU communicates directly with and uses to store data, and stores the data and programs that are currently being used in the file (that is, in execution), and its physical essence is one or more groups of rubber book circuits with data input and output and data storage functions, and the memory is only used to temporarily store programs and data, once the power is turned off or the power is off, the programs and data in it will be lost.

Generally speaking, the memory is DDR memory, which is used to exchange data with the CPU, and the data is lost when the power is off. The external memory may be a regretful late storage card, similar to a USB flash drive, even if the power is off, the data is still there. The external memory of the middle front grip needs to be exchanged with the CPU through the memory to sell the data.

It is equivalent to memory, which is a bridge between external memory and CPU.

The main difference between internal and external memory:

1. The speed of reading data is different: the speed of reading data from memory is fast, and the speed of reading data from external memory is slow.

2. The access rights are different, the CPU can only directly access the memory, and the external storage things can only be processed by the memory CPU first.

3. Different definitions: memory refers to the storage components on the motherboard, which are the parts that the CPU directly communicates with and uses to store data; External memory, also known as auxiliary memory, refers to storage other than computer memory and CPU cache.

4. Different compositions: memory is composed of memory chips, circuit boards, gold fingers and other parts. The external memory is composed of a magnetic rolling platter, a read-write control circuit and a driving mechanism.

5. Different volatility: memory is volatile. This means that when the system loses power, data is lost. In contrast, external storage is non-volatile, so it can hold data even when there is no power supply.

6. Different functions: The function of memory is to temporarily store the computing data in the processor, as well as the data exchanged with external storage such as hard disks. The function of external memory is to store programs and data that need to be saved on the computer but are not used temporarily.

7. The relationship with the CPU is different: in the computer, the CPU is responsible for computing and processing, and the memory is responsible for data exchange. The two are cooperative, and memory is equivalent to the data access channel of the CPU.

In order for external memory to transfer data to the CPU or IO device, it must be done through memory.

How dynamic memory (DRAM) works. Dynamic memory has only one input data line per chip, while there are only eight address pins. In order to form a 64k address, an address-forming circuit must be specially designed between the system address bus and the chip address leads.

This allows the system address bus signal to be time-shared to the pins of the 8 addresses, with the help of a line latch inside the chip.

The column latch and decoding circuit select the memory cells in the chip, and the latch signal is also generated by the external address circuit.

When data is to be read out of the DRAM chip, the CPU first adds the row address to A0-A7 and then sends a RAS latch signal, which locks the address inside the chip by the falling edge of the signal. Then, the column address is added to the A0-A7 of the chip, and then the CAS latch signal is sent, and the column address is locked inside the chip on the descending edge of the signal. Then keep we=1, then the data is output and maintained during the CAS validity period.

When data needs to be written to the chip, the row and column addresses lock RAS and Cas inside the chip, and then, WE is valid, plus the data to be written, the data is written to the selected storage unit.

Since it is impossible for a capacitor to maintain a constant charge for a long time, it is necessary to periodically reread each memory cell of the dynamic memory circuit to keep the charge stable, a process called dynamic memory refresh. The DRAM refresh in a PC XT machine is implemented using DMA. First, the counter of the programmable timer 8253 is applied.

1. Every 1 12 s a DMA request is generated, which is added to the DMA controller.

0 on the channel. When the request of channel 0 of the DMA controller is responded, the DMA controller sends a signal to the refresh address to perform a read operation on the dynamic memory, refreshing one row for each read.