Segment paging and segment page combination

Pagination 、 Before the advent of segmented Technology

Before the technology of segmentation appeared , To run a program, you need to allocate enough continuous memory from memory , Then load the whole program

The size of a program is 10M, then , There needs to be continuous 10M The program can only be loaded into the memory space . If you can't find a continuous 10M Memory , You can't load this program into memory , The program cannot be run

A problem

① Address space is not isolated
② The address when the program is running is uncertain
③ Low memory usage

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The emergence of segmentation technology and virtual address space

piecewise ： Is to divide a program into code segments , Data segment , Stack segment or something , Each segment manages different data

Virtual address space
Simply put, it's an addressable space . If this space is virtual , We call it virtual address space ; If this space is real , We call it physical address space . The virtual address space can be arbitrarily large , Because it's virtual . The physical address space is real , So it's Limited

Segmentation technology maps virtual address space to physical address space , And the program you write operates on a virtual address

Segmentation technology solves the problem of isolation , So that the program does not need to pay attention to the physical address , As long as the virtual address hasn't changed , that , The program will not operate on the two problems of improper address

The emergence of paging technology solves the problem of low efficiency

General principle of paging technology

The basic method of paging is to divide the address space into a fixed size page ; The size of each page is determined by the hardware , Or it's up to the operating system ( If the hardware supports multiple page sizes ).

Page size selection

The page size is fixed , It's up to your machine and hardware , Different machines may have different page sizes
The page size is generally 2 The power of , Usually 512B~8KB

Small page size ： Memory fragmentation is small , High memory utilization , But the number of pages , Make the page table too long , Take up a lot of memory , High management overhead

Large page size ： Page table end , Low memory utilization and large memory fragmentation , Low management overhead

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Pagination 、 Differences in segmentation

Paragraphs and pages combine advantages and disadvantages

advantage

(1) It provides a lot of virtual storage space .

(2) Can effectively use main memory , It provides convenience for organizing the running of multiple programs .

shortcoming ：

(1) Increased hardware costs 、 The complexity of the system and the opening and elimination of management .

(2) There is a risk of system jitter .

(3) There are internal fragments .

(4) There are also various forms that take up main memory space .