What is the actual address of a certain storage unit that the CPU wants to access?

The actual address of the storage unit is called "physical address". Information is stored in units of bytes in the memory. In order to store or obtain information correctly, each byte unit is given a unique memory address, which is called a physical address, also called an actual address or an absolute address. The physical address exists in electronic form on the address bus, allowing the data bus to access the memory address of a specific storage unit in the main memory. In real address mode, the physical address is obtained by multiplying the segment address by 16 and adding the offset address. The calculation method is "physical address = segment address * 16d offset address".

The operating environment of this tutorial: Windows 7 system, Dell G3 computer.

The actual address of a certain storage unit that the CPU wants to access is called the "physical address".

In the memory, information is stored in bytes. In order to store or obtain information correctly, each byte unit is given a unique memory address, called a physical address (Physical Address), also called an actual address. or absolute address.

Description

Addresses are numbered starting from 0 and are sequentially increased by 1 each time, so the physical address space of the memory grows linearly. It is represented by a binary number, an unsigned integer, and the writing format is a hexadecimal number. It is the address signal that appears on the CPU external address bus to address physical memory, and is the final result of the address transformation. Used for unit addressing at the memory chip level, corresponding to the address bus connecting the processor and CPU.

In computer science, physical address (English: physical address), also called real address (real address) and binary address (binary address), exists in electronic form on the address bus, so that The data bus provides access to the memory address of a specific location in main memory. In computers with virtual memory, the term physical address is often used to distinguish virtual addresses. Especially in computers that use a memory management unit (MMU) to translate memory addresses, the virtual and physical addresses refer to the addresses before and after the MMU translation, respectively. In computer networks, physical addresses are sometimes synonymous with MAC addresses. This address is actually for the data link layer, not the physical layer as its name implies.

Calculation method

In real address mode, the physical address is obtained by multiplying the segment address by 16 and adding the offset address. The 16-bit segment address multiplied by 16 is equivalent to shifting 4 binary bits to the left, which becomes a 20-bit segment base address. Finally, the segment base address is added to the segment offset address to obtain the physical address.

The 20-bit physical address is calculated as follows:

Physical address = segment address * 16d offset address.

Unaligned addressing

Depending on the computer system, unaligned access to memory may damage the performance of the computer. For example, a computer like the Intel 8086 with a 16-bit data bus will access even-numbered addresses more efficiently. In that case, obtaining a 16-bit value requires only one memory read and one data transfer on the data bus. Obviously, if the 16-bit value is stored at an odd address, the processor actually has to read the memory twice, that is, once to read the part stored at the low address and another time to read the part stored at the high address; both times Discard half of the data read.

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