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Old 12-20-2010, 01:10 AM   #1
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Virtual Memory concept in Embedded Sysytem


This is Harinath.

We have designed a board with Cirrus Logic(arm) processor, A Flash memory and some other peripherals have been connected to that.

While building kernel we have selected MMU support.
We have written few custom drivers for keypad,LED,LCD.

But i would like to know how virtual memory mechanism can be helpful here even though there is no any hard disk has been connected.

If it helpful,where will be the virtual memory reside.

Thanks & Regards,
Old 12-21-2010, 10:08 AM   #2
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Hi -

As I'm sure you know, "virtual memory" != "swap". Just because you don't have a swap space, or just because all active processes can fit in available RAM, does *not* mean you're not using "virtual memory".

So far, so good?

Q: So why bother with "virtual memory" if you're not using swap? What's the point?

A: There are literally HUNDREDS of compelling advantages. Including:

1) Increased program security (a buggy program can't easily "step on" a well-behaved neighbor)

2) That's just the way it works (Linux, Windows and MacOS are all "virtual memory" systems
wherein "kernel code" executes in a different, "privileges" address space from "user code").

It's worth noting that BOTH kernel space and user space exist in VIRTUAL memory

This link might help:


VM Advantages
+ Flexible Allocation
+ Isolation
+ Illusion of very large memory
+ Performance

“Any problem in CS can be solved with another layer of indirection” - David Wheeler
Old 12-21-2010, 03:20 PM   #3
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Originally Posted by harinathreddy.c View Post
But i would like to know how virtual memory mechanism can be helpful here even though there is no any hard disk has been connected.
Well, you don't have to have a hard disk for virtual memory to be useful, but you do have to have some kind of memory hierarchy (even if only two levels). If you don't, then you might as well just have a a flat, directly addressed, memory space, and for that you don't need an mmu.

On the other hand, if you do have a memory hierarchy using virtual memory can make life easier in minimising the amount of 'special case' stuff you have to do, if you don't have the memory to load everything in at once.


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