Question 100 about virtual storage and cache

The similarity is that they all use the principle of program locality, divide the program into many information blocks, and automatically schedule the information blocks from slow memory to fast memory during operation, and use a certain replacement strategy to improve the hit rate when continuing to run. They use the same address transformation, address mapping method, and substitution algorithm.

The difference is that cache is used to compensate for the speed difference between the main memory and the CPU, while virtual memory is used to make up for the lack of main memory capacity; The cache sends a fixed-length block of information each time, with only a few dozen bytes. The virtual information block can be fixed length (page) or indefinite length (segment), and the length is relatively large; The CPU can directly access the cache, but not the auxiliary memory. The information exchange process of the cache is all implemented by hardware, and the information exchange between the main and auxiliary storage is completed by auxiliary hardware and storage management software.

First of all, they are both memory, which is their main similarity.

Cache is commonly referred to as caching SRAM. SRAM is called static memory, and "static" means that when we write a piece of data to SRAM, the written data remains unchanged unless new data is rewritten or the power is turned off. Since the speed of the CPU is much faster than the speed of the memory and hard disk, it will cause the CPU to wait when accessing data, affecting the speed of the computer.

SRAM accesses faster than other memory and hard disks, so it is used as a cache for computers.

With the cache, you can pre-write the data to it first, and read it directly from it when needed, which shortens the CPU waiting time. The reason why the cache can improve the speed of the system is based on a statistical law, the control system on the motherboard will automatically count which data in the memory will be used frequently, and store the data in the cache, and when the CPU wants to access the data, it will first go to the cache to find it, so as to improve the overall running speed. Generally speaking, the 256K cache can increase the speed of the whole machine by about 10% on average.

The so-called virtual memory is to unify the use of internal and external memory, and the basic idea is to keep the part that is being used in memory. The capacity of the virtual memory is related to the address structure of the CPU, such as the address bit of the Pentium is 32 bits, then his addressing space is 2 to the 32nd power, 4GB, that is, his maximum addressing space is 4GB.

Cache and vmem data are stored on the memory, taking memory as an example, cache is generally some temporary data, once the program is run, or the operating system exits, or power off, the data is gone, vmem is equivalent to the memory as a hard disk, when reading, the data is read from the hard disk to the memory, and the read and write operations are to read and write the memory as a hard disk, which greatly improves the reading and writing efficiency. When the program exits, the data is written back to the hard drive, or when the system is not busy.

Cache, commonly known as cache, storage speed is a storage medium between memory and CPU, so that the internal memory is used to solve the speed mismatch between memory and CPU; Virtual storage generally refers to storage devices, such as disk arrays, which belong to external storage devices. Another is virtual memory, which is a part of the hard disk that is divided into a part of the disk to store frequently used data, solving the speed mismatch between memory and disk.

Similarities: The most recently used part of the program resides in high-speed memory;

Once these parts become incommon, send them back to the low-speed memory;

This swap-in and swap-out is done by the hardware or operating system and is transparent to the user;

Attempt to make the storage system's performance close to that of high-speed memory,**close to that of low-speed memory.

The main difference: the performance penalty of a miss in the virtual storage is much greater than the loss of a miss in the cache system.

Staging data for the processor, waiting for calls.

In fact, there are also in the encyclopedia

The cache has three parts:

cache: stores the instructions and data blocks that are called in from the main memory.

Virtual memory: Also known as virtual memory. All programs running on the computer need to be executed through memory, and if the program is large or numerous, it will cause the memory to be exhausted. To solve this problem, virtual memory technology is used in Windows, that is, a portion of the hard disk space is used as memory usage.

When the memory is depleted, the computer will automatically call the hard disk to act as memory to relieve the strain on memory. If your computer lacks the random access memory (RAM) needed to run programs or operations, Windows uses that to compensate. It combines your computer's RAM with the temporary space on your hard drive.

When RAM runs slowly, it moves data from RAM into a space called a "paging file". Moving data in and out of a paging file frees up RAM so you can get the job done. Generally speaking, the more RAM a computer has, the faster the program will run.

If your computer's speed is slowed down due to lack of RAM available space, you can try to compensate by adding virtual memory. However, computers can read data from RAM at a faster rate than from a hard drive, so expanding the RAM capacity (with memory sticks) is the best option. Reference.