Galvin operating system

Question

CPU generates logical address and that logical address is converted to physical address.

What does this means exactly ??

Assume run time binding is used.

When process P1 is loaded in memory lets say at 10k

Lets say OS scheduler schedules P1,so CPU should start executing that P1 ,So cpu's PC should be loaded with starting address to fetch first instruction so what should be loaded in PC.

And cpu generates address means we put that address in MAR to get some data/instruction from memory right ?

Please help me in this 

Answer 1

Any process doesn't have a direct view of the physical memory as it would lead to unnecessary complications (eg: accessing other process' private data, accessing kernel memory etc). Basically, the OS doesn't trust the user with the actual physical memory and would like to manage everything on its own. 

So, it's the job of the OS to virtualise the view of memory. It is one of the three primary tasks of an OS (the others being concurrency control and persistence.)

This has loads of advantages. Mainly, it makes the process feel that the entire memory is available to itself and this memory might be even more than the actual physical memory available. It also provides protection against process' accessing each other's parts and so on.

This is done mostly with the help of a mix of paging and segmentation in modern operating systems. The CPU generates a logical address, which is then converted into the physical address by the operating system. Every process has its own logical address space, in which it stores its data(code, heap, stack etc). This ensure protection, as well as the illusion of having a larger memory( as the logical address space can be much larger than the actual physical memory.)

When you say P1 is loaded at 10k, you are talking about the virtual address 10k. In fact, whatever address is visible to you use as a programmer, is a virtual address. When you do malloc() in C, it returns a virtual address. 

Refer to sections on memory management from any standard OS textbook, you'd understand it.
 

Comments

Logical address space is just to visualise how process will be loaded in memory ..addresses will be differing only right?

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I don't understand your question.

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Virtual address space is only imaginary concept right ? Just for user view how process will be stored in main memory.

For actual execution cpu generates logical address means in PC program counter also logical address will be loaded ??

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I am not sure if it can be called an "imaginary concept". All the operations do you use logical addresses - the program counter stores the virtual address as only virtual addresses are generated by the process. Later, the virtual address is converted into a physical address to be stored/accessed in the memory.

Also, it depends. In the x86 architecture, the addressing depends if paging is activated or not. If paging is deactivated it uses real addresses, if paging is enabled it uses virtual addresses (the translation is then done by the MMU).

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when CPU generates logical address means CPU assume that memory address is starting from 0, (but actually that particular is stored at random position)so first MMU will check whether required address value is less than value present in a limit register if yes then the relocatable address is added to it which gives a physical address