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    <p>There are many reasons to do this:</p> <ul> <li><p>If you have a compiled binary, each function has a fixed address in memory and the assembly instructions to call functions have that address hardcoded. If virtual memory didn't exist, two programs couldn't be loaded into memory and run at the same time, because they'd potentially need to have different functions at the same physical address.</p></li> <li><p>If two or more programs are running at the same time (or are being context-switched between) and use direct addresses, a memory error in one program (for example, reading a bad pointer) could destroy memory being used by the other process, taking down multiple programs due to a single crash.</p></li> <li><p>On a similar note, there's a security issue where a process could read sensitive data in another program by guessing what physical address it would be located at and just reading it directly.</p></li> <li><p>If you try to combat the two above issues by paging out all the memory for one process when switching to a second process, you incur a <em>massive</em> performance hit because you might have to page out all of memory.</p></li> <li><p>Depending on the hardware, some memory addresses might be reserved for physical devices (for example, video RAM, external devices, etc.) If programs are compiled without knowing that those addresses are significant, they might physically break plugged-in devices by reading and writing to their memory. Worse, if that memory is read-only or write-only, the program might write bits to an address expecting them to stay there and then read back different values.</p></li> </ul> <p>Hope this helps!</p>
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    1. POWhy do we need virtual memory?
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    1. COThat helps a lot. But what about fragmentation? So certain things (an array for example) needs to be allocated contiguously. If an array is allocated contiguously in virtual memory, does that mean it's also contiguous in physical memory? Or does that level of fragmentation not occur
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    2. CO@user2079802- Not necessarily. If the array spans several pages of virtual memory, the OS can allocate those pages to completely different regions of physical memory. It's up to the OS to make sure that this doesn't lead to performance issues.
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    3. COOne other thing. When you get the address of some variable, for instance using & in C, that is its virtual address, right?
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