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Linux Virtual Memory Model

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whisper's picture
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Joined: 2006-09-06

"Kernel use of virtual memory begins very early on in the boot process. head.S contains code to create provisional page tables and get the kernel up and running, however that is beyond this overview.

Every physical page of memory up to 896MB is mapped directly into the kernel space. Memory greater than 896MB (High Mem) is not permanently mapped, but is instead temporarily mapped using kmap and kmap_atomic (see HighMemory).
(...)
User Space Virtual Memory:

Every process in linux is able to address 4 gigabytes of linear address space. In a standard kernel config, the first 3 gigabytes (0x00000000 - 0xC0000000) are referred to as 'user space' and represent data, functions and the stack of user processes. The top 1 gigabyte (0xC0000000 - 0xFFFFFFFF) of memory is 'kernel space'. User processes typically do not have access to kernel memory space, and will normally not address this region."

I really don't get It. How can a process address 4 gigs of address space When my pc (or almost any PC) has 512MB
of physical memory? There's a lot of stuff about this I don't grasp. Could anyone enlight me? I'm lost, I really can't understand this virtual memory stuff...

Thank You Smiling

a thing's picture
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Joined: 2005-12-20
doesn't go that high

1. It can't; it just stops when it runs out.
2.

Virtual memory or virtual memory addressing is a memory management technique, used by computer operating systems, more common in multitasking OSes, wherein non-contiguous memory is presented to a software (aka process) as contiguous memory. This contiguous memory is referred to as the virtual address space

whisper's picture
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Joined: 2006-09-06
This is a good explanation :)

Physical and virtual memory

"To understand how memory gets allocated within your program, you first need to understand how memory gets allocated to your program from the operating system. Each process on your computer thinks that it has access to all of your physical memory. Obviously, since you are running multiple programs at the same time, each process can't own all of the memory. What happens is that your processes are using virtual memory.

Just for an example, let's say that your program is accessing memory address 629. The virtual memory system, however, doesn't necessarily have it stored in RAM location 629. In fact, it may not even be in RAM -- it could even have been moved to disk if your physical RAM was full! Because the addresses don't necessarily reflect the physical location where the memory is located, this is called virtual memory. The operating system maintains a table of virtual address-to-physical address translations so that the computer hardware can respond properly to address requests. And, if the address is on disk instead of in RAM, the operating system will temporarily halt your process, unload other memory to disk, load in the requested memory from disk, and restart your process. This way, each process gets its own address space to play in and can access more memory than you have physically installed.

On 32-bit x86 systems, each process can access 4 GB of memory. Now, most people don't have 4 GB of memory on their systems, even if you include swap, must less 4 GB per process. Therefore, when a process loads, it gets an initial allocation of memory up to a certain address, called the system break. Past that is unmapped memory -- memory for which no corresponding physical location has been assigned either in RAM or on disk. Therefore, if a process runs out of memory from its initial allocation, it has to request that the operating system "map in" more memory. (Mapping is a mathematical term for one-to-one correspondence -- memory is "mapped" when its virtual address has a corresponding physical location to store it in.)"

http://www-128.ibm.com/developerworks/linux/library/l-memory/

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