Logical Address, linear address, physical address, and virtual address

Article Title: Logical Address, linear address, physical address, and virtual address. Linux is a technology channel of the IT lab in China. Includes basic categories such as desktop applications, Linux system management, kernel research, embedded systems, and open source.

I. Concepts

Physical address)

It is used for memory chip-level unit addressing, which corresponds to the address bus connected to the processor and CPU.

-- This concept should be better understood among these concepts, but it is worth mentioning that although physical addresses can be directly understood as the memory itself inserted on the machine, consider the memory as a large array of serial numbers from 0 bytes to the maximum number of NULL bytes, and then call this array a physical address. But in fact, this is just a figure provided by hardware to the software, the memory addressing method is not like this. Therefore, it is more appropriate to say that it is "corresponding to the address bus", but aside from considerations of the physical memory addressing method, it directly maps the physical address to the physical memory one by one, it is also acceptable. Maybe wrong understanding is more conducive to the metaphysical image extraction.

Virtual memory)

This is the image description of the entire memory (do not insert the upper number with the machine. It is relative to the physical memory, and can be directly understood as "not straight", "fake" memory, for example, a 0x08000000 memory address, it is not the address element of 0x08000000-1 in the large array on the physical address;

This is because the modern operating system provides a kind of memory management image, namely virtual memory ). The process uses the address in the virtual memory, and the operating system assists the relevant hardware to "convert" it into a real physical address. This "conversion" is the key to all discussions.

With this image, a program can use a much larger address space than a real physical address. (The east wall, the west wall, and the bank do the same), and even multiple processes can use the same address. It's not surprising because the converted physical addresses are not the same.

-- You can decompile the connected program and check that the connector has allocated an address to the program. For example, to call A function A, the code is call 0x0811111111 instead of call, that is to say, the address of function A has been fixed. Without such a "Conversion", there is no virtual address concept, and this is simply not feasible.

The problem persists.

Logical address)

Intel retains the segmented memory management methods of the ancient times for compatibility purposes. A logical address refers to the address in a machine language instruction that is used to specify an operand or an instruction. In the above example, the address 0x08111111 allocated by the connector for A is the logical address.

-Sorry, it seems that it violates Intel's middle-stage management requirements for logical addresses, "a logical address, it is expressed as [segment identifier: Intra-segment offset] by adding an offset of the relative address in the specified segment to a segment identifier, that is, the 0x08111111 in the preceding example, it should be represented as [A code segment identifier: 0x08111111], so that it is complete"

Linear address or virtual address)

Similar to the logical address, it is also an invalid address. If the logical address is the address before the segment Management Switch on the hardware platform, the linear address corresponds to the address before the hardware page memory conversion.

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The CPU needs to take two steps to convert the addresses in a virtual memory space to physical addresses: first, we need to give a logical address (in fact, it is a segment offset. This must be understood !!!), The CPU needs to use its segmented Memory Management Unit to convert a logical address into a thread address, and then use its webpage Memory Management Unit to convert it to the final physical address.

The two conversions are indeed very troublesome and unnecessary, because linear addresses can be directly extracted to the process. The reason for this redundancy is that Intel is completely compatible.

2. CPU memory management, how to convert logical addresses to linear addresses

A logical address is composed of two parts. The segment identifier is the intra-segment offset. A segment identifier is a 16-bit long field, which is called a segment selector. The first 13 digits are an index number. The last three digits contain some hardware details,

The last two items involve permission checks, which are not included in this post.

Index number, or directly understood as an array subscript -- it always corresponds to an array. What is its index? This is "segment descriptor (segment descriptor)". Haha, the specific address of the segment descriptor describes a segment (I think of it as an understanding of the word "segment, take a knife and cut the virtual memory into several blocks ). In this way, many segment descriptors are grouped into an array called the "segment descriptor table". In this way, the first 13 digits of the segment identifier can be used, find a specific segment descriptor directly in the segment descriptor table. This descriptor describes a segment. The image of the segment is not accurate just now, let's take a look at what is in the descriptor-that is, how it describes it, and understand what the segment actually has. Each segment descriptor consists of 8 bytes, for example:

These things are very complicated. Although we can use a Data Structure to define them, I only care about the same here, that is, the Base field, which describes the linear address at the beginning of a segment.

Intel designed some global segment descriptors to be placed in the global segment descriptor table (GDT). Some local descriptors, such as, put it in the so-called "local segment descriptor table (LDT. So when should we use GDT and LDT? This is represented by the T1 field in the segment selection operator, = 0, indicating GDT, = 1 indicating LDT.

The GDT address and size in the memory are stored in the gdtr control register of the CPU, while the LDT is in the ldtr register.

Many concepts, like tongue twisters. This figure looks more intuitive:

 

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