Virtual address space

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• 2021/11/27

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When the processor reads or writes to a memory location , It uses a virtual address . During a read or write operation , The processor converts the virtual address to the physical address . Accessing memory through a virtual address has the following advantages ：

• Programs can use a series of contiguous virtual addresses to access discontinuous large memory buffers in physical memory .

• A program can use a series of virtual addresses to access memory buffers larger than the available physical memory . When the supply of physical memory changes , The memory manager will put physical memory pages （ Usually the size is 4 KB） Save to disk file . Data or code pages move between physical memory and disk as needed .

• Virtual addresses used by different processes are isolated from each other . Code in one process cannot change the physical memory being used by another process or operating system .

The range of virtual addresses available to a process is called the “ Virtual address space ” . Each user mode process has its own private virtual address space . about 32 Bit process , The virtual address space is usually 2 GB, Range from 0x00000000 to 0x7FFFFFFF. about 64 position Windows Upper 64 Bit process , The virtual address space is 128 TB, Range from 0x000'00000000 to 0x7FFF'FFFFFFFF. A series of virtual addresses is sometimes called a series “ Virtual memory ” . For more information , see also Memory and address space limitations .

This figure illustrates some important functions of virtual address space .

The figure shows two 64 Virtual address space of bit process ：Notepad.exe and MyApp.exe. Each process has its own virtual address space , Range from 0x000'0000000 to 0x7FF'FFFFFFFF. Each shadow block represents a page of virtual memory or physical memory （ The size is 4 KB）. Be careful ,Notepad Process use from 0x7F7'93950000 Three consecutive pages of the starting virtual address . However, these three consecutive pages of the virtual address will be mapped to discontinuous pages in physical memory . Attention, please. , Both processes use from 0x7F7'93950000 Start virtual memory page , But these virtual pages map to different pages of physical memory .

User space and system space

Such as Notepad.exe and MyApp.exe The process of runs in user mode . The core operating system components and multiple drivers run in a more privileged kernel mode . More about processor mode , see also User mode and kernel mode . Each user mode process has its own private virtual address space , But all code running in kernel mode is shared as “ System space ” A single virtual address space . The virtual address space of user mode processes is called “ User space ” .

stay 32 position Windows in , The total virtual address space available is 2^32 byte （4 GB）. Usually , Lower 2 GB For user space , Higher 2 GB For system space .

stay 32 position Windows in , You can choose to specify （ When it starts ） exceed 2 GB Available in user space . As a result, less virtual addresses are available in the system space . You can increase the size of user space to 3 GB, under these circumstances , The system space is only 1 GB You can use . To increase the size of user space , Please use  BCDEdit /set increaseuserva.

stay 64 position Windows in , The theoretical size of virtual address space is 2^64 byte （16 Ebyte ）, But in fact, only 16 A small part of the byte range .

Code running in user mode can access user space , But you cannot access the system space . This restriction prevents user mode code from reading or changing protected operating system data structures . Code running in kernel mode can access user space , You can also access system space . namely , Code running in kernel mode can access the system space and the virtual address space of the current user mode process .

Drivers running in kernel mode must be very careful when reading or writing these addresses directly from user space addresses . This scheme explains the reason .

1. The user mode program initiates a request to read some data from the device . The program provides the starting address of the buffer to receive data .

2. The device driver routine running in kernel mode starts the read operation and returns control to its caller .

3. later , The device interrupts any currently running thread to show that the read operation is complete . Interrupt this arbitrary thread （ Belongs to any process ） Kernel mode driver routines running on .

4. here , The driver must not write data to the user mode program in step 1 The starting address provided in . This address is in the virtual address space of the process that initiated the request , This process is likely to be different from the current process .

Paging buffer pool and non paging buffer pool

In user space , All physical memory pages can be called out to disk files as needed . In system space , Some physical pages can be called , Other physical pages cannot . The system space has two areas for dynamically allocating memory ： Paging buffer pool and non paging buffer pool .

The memory allocated in the paging buffer pool can be called out to disk files as needed . The memory allocated in the non paged buffer pool can never be called out to disk files .