Reputation: 75

C accessing memory location

I mean the physical memory, the RAM.

In C you can access any memory address, so how does the operating system then prevent your program from changing memory address which is not in your program's memory space?

Does it set specific memory adresses as begin and end for each program, if so how does it know how much is needed.

Upvotes: 2

Answers (8)

Brian White

Reputation: 8756

Most computers (and all PCs since the 386) have something called the Memory Management Unit (or MMU). It's job is to translate local addresses used by a program into the physical addresses needed to fetch real bytes from real memory. It's the operating system's job to program the MMU.

As a result of this, programs can be loaded into any region of memory and appear, from that program's point of view while executing, to be be any any other address. It's common to find that the code for all programs appear (locally) to be at the same address and their data always appears (locally) to be at the same address even though physically they will be in different locations. With each memory access, the MMU transparently translates from the local address space to the physical one.

If a program trys to access a memory address that has not been mapped into its local address space, the hardware generates an exception and typically gets flagged as a "segmentation violation", followed by the forcible termination of the program. This protects from accessing the memory of other processes.

But that doesn't have to be the case! On systems with "virtual memory" and current resource demands on RAM that exceed the amount of physical memory, some pages (just blocks of memory of a common size, often on the order of 4-8kB) can be written out to disk and given as RAM to a program trying to allocate and use new memory. Later on, when that page is needed by whatever program owns it, the memory access causes an exception and the OS swaps out some other memory page and re-loads the needed one from disk. The program that "page-faulted" gets delayed while this happens but otherwise notices nothing.

There are lots of other tricks the MMU/OS can do as well, like sharing memory between processes, making a disk file appear to be direct-memory-accessible, setting some pages as "NX" so they can't be treated as executable code, using arbitrary sections of the logical memory space regardless of how much and at what address the physical ram uses, and more.

Upvotes: 0

pratZ

Reputation: 3356

The logical address is generated by the CPU which is mapped to the physical address by the memory mapping unit. Unlike the physical address space the logical address is not restricted by memory size and you just get to work with the logical address space. The address binding is done by MMU. So you never deal with the physical address directly.

Upvotes: 0

Aki Suihkonen

Reputation: 20037

The operating system does "memory management" often coupled with TLB's (Translation Lookaside Buffers) and Virtual Memory, which translate any address to pages, which the operation system can tag readable or executable in the current processes context.

The minimum requirement for a processors MMU or memory management unit is in current context restrict the accessable memory to a range which can be only set in processors registers in supervisor mode (as opposed to user mode).

Upvotes: 0

user405725

Reputation:

This is what is called a memory protection. It may be implemented using different methods. I'd recommend you start with a Wikipedia article on this subject — http://en.wikipedia.org/wiki/Memory_protection

Upvotes: 2

foobarfuzzbizz

Reputation: 58725

This has to do with a thing called 'paging', which is provided by the CPU itself. In old operating systems, you had 'real mode', where you could directly access memory addresses. In contrast, paging gives you 'virtual memory', so that you are not accessing the raw memory itself, but rather, what appears to your program to be the entire memory map.

Upvotes: 0

mah

Reputation: 39837

Your operating system kernel works closely with memory management (MMU) hardware, when the hardware and OS both support this, to make it impossible to access memory you have been disallowed access to.

Generally speaking, this also means the addresses you access are not physical addresses but rather are virtual addresses, and hardware performs the appropriate translation in order to perform the access.

Upvotes: 3

Michael Krelin - hacker

Reputation: 143259

It depends on the architecture, on some it's not even possible to prevent a program from crashing the system, but generally the platform provides some means to protect memory and separate address space of different processes.

Upvotes: 1

Luchian Grigore

Reputation: 258638

Actually, your program is allocated virtual memory, and that's what you work with. The OS gives you a part of the RAM, you can't access other processes' memory (unless it's shared memory, look it up).

Upvotes: 1

C accessing memory location

Answers (8)

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