What is a logical address, what is a physical address, and what is the relationship between them

in memory. In order to store or retrieve information correctly, each byte unit is given a unique memory address, called a physical address, also known as an actual address or an absolute address.

Physical address (physical

address）

Refers to the address signal that appears on the external address bus of the CPU to address the physical memory, and is the final result of the address transformation. If pagination is enabled, linear addresses are transformed into physical addresses using items in the page directory and page table. If pagination is not enabled, the linear address becomes a physical address.

The relationship between the two is as follows:

Only in Intel real mode, the logical address is equal to the physical address (because real mode has no segmentation or paging mechanism, and the CPU does not perform automatic address translation); The logic is also in Intel

In protected mode, the offset address within the limit of the **segment (assuming that the **segment and the data segment are exactly the same). The application worker only has to deal with the logical address, while the segmentation and paging mechanism is completely transparent to you and is only covered by the system programmer. Although the application worker can directly manipulate the memory himself, he can only operate in the memory segment allocated to you by the operating system.

The "gap" between the logical address and the physical address is 0xc0000000 due to the value of the virtual address->-linear address-> physical address mapping. This value is specified by the operating system.

You're asking about the physical and logical addresses in the CPU, right?

Physical address: In memory, information is stored in bytes, and in order to store or retrieve information correctly, each byte unit is given a unique memory address, called a physical address, also known as an actual address or an absolute address. Addresses are numbered starting from 0 and sequentially incremented by 1 each time, so the physical address space of the memory grows linearly.

It is represented as a binary number, is an unsigned integer, and is written in hexadecimal number.

It is the address signal that appears on the address bus outside the CPU to address the physical memory and is the end result of the address transformation. Cell addressing at the memory chip level, corresponding to the address bus to which the processor and CPU are connected.

Logical address: In a computer with address transformation function, the address (operand) given by the internal access instruction is called the logical address, also known as the relative address. It is necessary to calculate or transform the addressing method to obtain the physical address in the internal memory.

The differences are as follows:

Physical address: Information is stored in bytes in memory, and in order to store or obtain information correctly, each byte unit is given a unique memory address, which is called a physical address, also known as an actual address or an absolute address.

Second, the way of expression:

Tracing back to the source, Intel's 8-bit computer has 8080 CPUs, the data bus (DB) is 8 bits, and the address bus (AB) is 16 bits. Then this 16-bit address information is also transmitted through the 8-bit data bus, and it is also stored in the scratchpad in the data channel, as well as in the registers and memory in the CPU, but because AB is exactly an integer multiple of DB, there will be no contradiction!

However, when it was upgraded to 16-bit computers, the design of Intel8086 8088CPU could not exceed 40 pins due to the limitations of IC integration technology and external packaging and pin technology at that time. However, I feel that the original address addressing capability of the 8-bit machine 2 16=64KB is too little, but it is directly increased to an integer multiple of 16, even if ab=32 bits, it cannot be reached.

Therefore, AB can only be temporarily increased by 4 to 20. Then the addressing capacity of 2 20 = 1MB has increased by a factor of 16. However, this creates a contradiction between the 20-bit AB and the 16-bit DB, and the 20-bit address information cannot be transmitted on the DB or stored in the 16-bit CPU registers and memory cells.

As a result, the principle of CPU segment structure was born.