ARM·MMU-EEWORLD

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【MMU】

The main functions of MMU: 1) Setting permissions 2) Address mapping

(Here we mainly talk about address mapping)

[Mapping of virtual address to physical address]

Question 1: Why do we need virtual addresses?

Reason 1: Let's look at a problem first. Program A needs 10MB of memory, and program B needs 110MB of memory. If A and B run at the same time, 20MB of memory is needed. If there is only 10MB of memory left, what if program C needs 20MB of memory? Therefore, the capacity requirement of our physical memory is very large.

Reason 2: If some programs exit at this time, there is 20M memory, but this memory is not continuous, but randomly allocated, but the total is 20M, which can allow the C program to run, but the C program has no fixed running address;

Reason 3:

Here, the memory allocation of physical address is used. Imagine that if program B fails, accesses the address of program A, and modifies the memory of program A, then program A will also fail. In other words, the crash of one program will cause the crash of multiple programs, which will make the system very unstable.

Combining the above two reasons, we can see the necessity of the invention of virtual addresses.

【Virtual address improvements】

（1）

Segmented memory mapping

Although this solves problems 2 and 3, the physical address and virtual address are in a one-to-one correspondence, the physical address has not been expanded, and the capacity is still very small.

(2) Paging method (page size in ARM is 1MB)

In the segmentation method, the entire program is always loaded into the memory each time the program is run; in the paging method, memory is allocated to the page to which the program runs.

Let's use an example to illustrate:

Assume that when a file A is executed in memory, the operating system will first create a 4GB process virtual address space for the program (not a real space, but a data structure, namely the page directory and page table)

The core idea of the paging method is that when the executable file executes to page x, a memory page y is allocated for page x, and then this memory page is added to the mapping table of the process virtual address space. This mapping table is equivalent to a function y=f(x).

Keywords：ARM Reference address：ARM·MMU

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