What is the virtual address protection mode, and what are the physical addresses of the virtual addr

The physical address in the computer can be virtual.

If the paging flag in the CPU register is set, the CPU (to be precise, the memory management unit) will automatically convert the virtual address into a physical address based on the information in the page directory and page table when executing the machine instruction for memory operation.

Physical Address: An address placed on the addressing bus. On the addressing bus, if it is a read, the circuit will put the data in the physical memory of the corresponding address into the data bus according to the value of each bit of this address.

In the case of writes, the circuit places the contents of the data bus in the physical memory of the corresponding address based on the value of each bit of the address. Physical memory is addressed in bytes (8 bits).

Virtual Address: When the CPU starts protected mode, the program runs in the virtual address space. Note that not all "programs" run in a virtual address.

The CPU runs in real mode when it is booted, and the bootloader and kernel do not use the virtual address before initializing the page table, but directly use the physical address.

Not only Linux uses a virtual memory address, but Windows also uses a virtual memory address, and the virtual memory address is used to access the memory in the protected mode of the CPU, which is determined by the memory management mode in the CPU protection mode, and the physical memory will be directly accessed only in the real mode.

Real mode: the application directly accesses all IO addresses (including memory addresses, of course) Protection mode: the operating system copies a copy of the relevant information for access, and the space accessed by the application does not affect other programs (but when you really need access, you can apply for permission to access the real address).

Virtual 8086 is a 16-bit access to 80386 and above 32-bit processors, and the processor and operating system emulate an 8086 to provide the application and copy a copy of the relevant information.

Real mode: it is pure DOS, with a guide disk; Character-only interface environment. Protected mode: It is a graphical interface environment such as Windows.

In 32-bit Windows, the total amount of virtual address space available is 2 32 bytes (4 GB). Typically the lower 2 GB is for user space and the top 2 GB is for system space.

In 64-bit Windows, the theoretical size of the virtual address space is 2 64 bytes (16 exabytes), but in practice only a fraction of the 16 exabytes range is used. Ranges from 0x000'00000000 to 0x7ff'8 TB of ffffffff is used in user space, ranging from 0xffff0800'00000000 to 0xffffffff'The 248 TB portion of FFFFFFFF is used for system space.

Under Windows it is a virtual address (logical address). It is also different on different machines: in the 8086 real mode, the segmentation mechanism is turned on, and the pointer is in 20-bit form; In protected mode, paging and segmentation is enabled, the pointer points to the 32-bit logical address of the process, and the CPU is responsible for converting it into a physical address.

Real-time embedded systems are not counted, and the pointers in the current microcomputer operating system all point to virtual addresses. The management of virtual addresses is done by the processor hardware and operating system, and the program does not need to be considered, only the linear address space. It is important to note that the virtual addresses of different processes are not comparable.

Addresses in one process don't make sense in another.

When the assembler or compiler converts the source program into a target program, the address range occupied by a target program is called the address space, and the number of these addresses is determined relative to the starting address, and the starting address is generally zero, which is called the logical address or relative address. Storage space Storage space refers to the series of physical units occupied by the target program when it is loaded into the main memory.