Simple summary of virtual memory

原創

2020-02-20 18:44

Search some materials on the internet and books. Review the knowledge and make a simple summary.

1 Virtual memory

Virtual memory is a memory management technology for modern multitasking operating system. Processes do not get the physical memory directly. Each process runs on its own virtual memory which is automatically translated into real memory by operating system and hardware.

The application program thinks it has a large range of continuous address space. In reality, the parts it is currently using are scattered around the RAM, and the inactive parts are saved in hard disk. No matter how much the physical memory is in a computer, the amount of virtual memory space in a 32 bit system is always 4GB .

The virtual address space starts at zero. It is composed of many fixed size pages. It includes Text, Data, BSS, Heap, and Stack at least. The heap and stack can shrink or grow.

A simple example

char courty = “China”; [global var. “China” is Text and read-only data]

int prvNum; [Not initialized, BSS]

char * fun(void){

char * p; [local var; Stack]

prvNum = 33;

p = malloc(prvNum); [Heap;dynamic memory]

return p;

}

2. page

Page is a fixed-length block of main memory. It is continuous in both physical memory addressing space and

virtual memory addressing space. A page is usually the smallest unit of data for the following:

1. Memory allocation performed by the operating system for a program

2. transfer between main memory and any other auxiliary store.

Two-level page table structure

There are many sub tables that cover all the virtual address space. A master table convers all sub tables. An address refers to the master table. The virtual address is divided into three parts. See the next picture. Using multi-level page table structure accelerates the searching speed. I call the idea as classification.