Does an OS lock the entire amount of ram inside your computer

Question

I was wondering if for example. Windows completely lock all the available ram so that some really bored person with too much time on their hands cannot start deleting memory from another process (somehow).

The question originated from what was happening when using the delete function in C++ (was C++ telling the OS that the OS can now release the memory for overwriting or was C++ telling the hardware to unlock the memory)... and then spawned on to me thinking of specific hardware created to interface with the RAM at hardware level and start deleting memory chunks for the fun of it. ie a hacker perhaps.

My thoughts were: The Windows memory management program is told memory is free to be written to again, right? But does that also mean that the memory address is still set to locked at a hardware level so that the memory can only be taken control of by windows rather than another OS. Or is it like the wild west down at hardware level... If Windows isn't locking memory, anything else can use the part that is now free.

I guess the real question is, is there a hardware level lock on memory addresses that operating systems can trigger... so that the memory has locked itself down and cannot be re-assigned then?

Answer 1

I was wondering if Windows completely lock all the available ram

Windows, like any other operating system, uses all the available RAM.

so that some really bored person with too much time on their hands cannot start deleting memory from another process (somehow).

Non sequitur. It does it because that's what it's supposed to do. It's an operating system, and it is supposed to control all the hardware resources.

The question originated from what was happening when you mark memory for deletion in C++.

I don't know what 'mark memory for deletion in C++' means, but if you refer to the delete operator, or the free() function, they in general do not release memory to the operating system.

My thoughts were: The Windows memory management program is told memory is free to be written to again, right?

Wrong, see above.

But does that also mean that the memory address is still set to locked at a hardware level so that the memory can only be taken control of by windows rather than another OS.

What other OS? Unless you're in a virtual environment, there is no other OS, and even if you are, the virtual environment hands control over all the designated RAM to the guest operating system.

Or is it like the wild west down at hardware level... If Windows isn't locking memory, anything else can use the part that is now free.

Anything else such as?

I guess the real question is, is there a hardware level lock on memory addresses that operating systems can trigger?

In general yes, there are hardware definitions of what privilege level is required to access each memory segment. For example, the operating system's own memory is immune to appliucation processes, and application processes are immune to each other: but this is all highly hardware-dependent.

Your question doesn't really make much sense.

Answer 2

The concept you're looking for is mapping , not *locking.

The memory is just there . The OS does nothing special about that.

What it does is map chunks of it into individual processes. Each process can see only the memory that is mapped into its address space. Trying to access any other address just leads to an access violation (or segmentation fault on Unixes). There's just nothing at those addresses. Not "locked memory", just nothing .

And when the OS decides to (or when the process requests it), a page of memory can be unmapped from a given process again.

It's not locking though. The memory isn't "owned" by the process it is mapped to. And the same memory can be mapped into the address spaces of multiple processes at the same time. That's one way to exchange data between processes.

So the OS doesn't "lock" or control ownership of memory. It just controls whether a given chunk of memory is visible to any particular process.

Answer 3

It is not as simple as that, also Windows is not open-source, so exactly what it does may not be published. However all addresses in user space code are virtual and MMU protected - address X in one process does not refer to the same physical memory as address X in another, and one process cannot access that of another. An attempt to access memory outside of the address space of a process will cause an MMU exception.

I believe that when Windows starts a process, it has an initial heap allocation, from which dynamic memory allocation occurs. Deleting a dynamically allocated block simply returns it to the process's existing heap (not to the OS). If the current heap has insufficient memory, additional memory is requested from the OS to expand it.

Memory can be shared between processes in a controlled manner - in Windows this is done via a memory-mapped file, and uses the same virtual-memory mechanisms as the swap-file uses to emulate more memory that is physically available.

I think rather than asking a question on SO for this you'd do better to first do a little basic research, start at About Memory Management on MSDN for example.

With respect to external hardware accessing the memory it is possible to implement shared memory between processors (it is not that uncommon; for example see here for example), but it is not a matter of "wild-west" the mechanisms for doing so are implemented via the OS.

Even on conventional PC architectures, many devices access memory directly via DMA as a method of performing I/O without CPU overhead. Again this is controlled by the OS and not at all "wild west", but an errant device driver could bring down your system - which is why Microsoft have a testing and approvals process for drivers.

Answer 4

No, there isn't. RAM is managed by software, RAM can't lock itself.

Answer 5

You asked the wrong question. There is no such thing as a hardware lock on memory. The trick is virtual memory management which makes only controlled amounts of memory available to a process. The OS controls all available memory, that's its job, and processes only ever see the memory that the OS has given to them.