逻辑地址和物理地址之间的区别?

在过去只有物理地址的糟糕年代，代码必须可重定位(或在加载时固定)，因为它可以在任何地方加载.使用虚拟内存，该代码(和数据)可以同时在多个不同进程中的逻辑内存位置50中-但是它的实际物理地址将有所不同.甚至可以共享它，以便在许多进程的地址空间中一次存在一个物理副本.这是共享代码(因此我们使用的物理内存不超过需要的数量)和共享内存的关键，以允许轻松的进程间通信.它的效率当然不如纯物理地址环境，但是CPU制造商试图使它效率尽可能高，因为它已被大量使用.优点 far 大于缺点.I am reading Operating Systems Concept and I am on the 8th chapter! However I could use some clarification, or reassurance that my understanding is correct.Logical Addresses: Logical addresses are generated by the CPU, according to the book. What exactly does this mean? (In an execute-generated address system..) I assume when code is compiled for a program, the program has no idea where the code will be loaded in memory. All the compiler does is set up a general sketch of the program layout and how the image should be laid out, but doesn't assign any real addresses to it. When the program is executed the CPU takes this layout image that the compiler made and hands out some addresses (logical ones) to the ones generated from the code.Physical Addresses: The physical addresses are not generated until after the CPU generates some set of logical addresses (consisting of a base address and an offset). The logical addresses go through the MMU or another device and somewhere along the line the logical addresses are mapped to physical RAM addresses.What then is the actual difference? I can see one benefit. Using logical addresses gives more freedom to the applications. If the physical addresses were hard coded, then the program success would depend heavily on the physical computer machine, available RAM addresses etc.Doesn't the use of logical addresses converted to physical address impose two steps instead of a one to one, and therefore more over head?Where then do the logical addresses reside after generation? They may exist in a register on the CPU while the CPU is servicing a process, but before and after, where do they go? I understand this is implementation dependent. I assume they may be stored in some special register space or buffer on the CPU such as a TLB, correct? If not, then the table may exist in the actual RAM itself, and the CPU only holds a pointer/address to the base address of the table in RAM, correct?It seems holding the addresses in RAM is counter productive to the purpose of logical memory addresses. I can only assume my understanding is incorrect. 解决方案 This answer is by no means exhaustive but it may explain it enough to make things click.In virtual memory systems, there is a disconnect between logical and physical addresses.An application can be given a virtual address space of (let's say) 4G. This is its usable memory and it's free to use it as it sees fit. It's a nice contiguous block of memory (from the point of view of the application).However, it is not the only application running, and the OS has to mediate between them all. Underneath that nice contiguous model, there is a lot of mapping going on to convert logical to physical addresses.With this mapping, the OS and hardware (I'll just call these the lower layers from here on in) is free to put the application pages anywhere it wants (either in physical memory or swapped out to secondary storage).When the application tries to access memory at logical address 50, the lower levels can translate that to a physical address using translation tables. And, if it tries to access logical memory that's been swapped out to disk, a page fault is raised and the lower levels can bring the relevant data back into memory, at whatever physical address it wants.In the bad old days when physical addresses were all you had, code had to be relocatable (or fixed up on load) since it could load anywhere. With virtual memory, that code (and data) can be at logical memory location 50 in a dozen different processes at the same time - it's actual physical address will be different however.It can even be shared so that one physical copy exists in the address space of many processes at once. This is the crux of shared code (so we don't use more physical memory than we need) and shared memory to allow easy inter-process communication).It is, of course, less efficient than a pure physical-address environment but the CPU manufacturers try to make it as insanely efficient as possible, since it's used heavily. The advantages far outweigh the disadvantages. 这篇关于逻辑地址和物理地址之间的区别?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！ 上岸，阿里云！