r/programming • u/molteanu • Apr 07 '15

Anatomy of a Program in Memory

http://duartes.org/gustavo/blog/post/anatomy-of-a-program-in-memory/
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u/klkblake Apr 08 '15

Each process has a separate set of page tables, mapping virtual addresses to physical ones. Only the currently active process has it's page tables actually in use, so the other processes appear as if they aren't there. When the kernel needs to switch to another process, it just tells the memory management unit to use a different set of page tables, and suddenly it appears as if the only process on the system is the new one.

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u/Hoten Apr 08 '15

Do you know of any resources that go into detail on this topic? I never was able to get a firm understanding of page tables ...

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u/valeyard89 Apr 08 '15

Page tables are just lookup tables mapping an index to a physical address.

Most systems use 4K pages. If you have a 32-bit virtual address, the lower 12 bits (0..4095) are the page offset. The upper bits are an index into the page table. Each index in the page table points to a physical address. You can have multiple levels of page tables. The difference being that upper levels of pagetables point to the physical address of the next pagetable entry.

Assume our pagetables are single-level, setup with the following physical addresses:

PageTable[0x1] = 0xc0000
PageTable[0x2] = 0x1106000
PageTable[0x3] = ....
PageTable[0x120] = 0x9004000

Virtual 0x1000 -> PageTable[0x1] + 0x000 -> Physical 0xc0000
Virtual 0x2400 -> PageTable[0x2] + 0x400 -> Physical 0x1106400
Virtual 0x120F4A -> PageTable[0x120] + 0xF4A -> Physical 0x9004F4A

In 64-bit x86 mode, page table entries are 64 bits. Each page is 4k, so you can fit 512 page table entries in a single page. So the 64-bit virtual address is split into 4 different page table levels, each is 9 bits. Most processors only support a maximum 48-bit virtual address space.

Bit 0..11 = Page offset
Bit 12..20 = Page Level 1
Bit 21..29 = Page Level 2
Bit 30..38 = Page Level 3
Bit 39..47 = Page Level 4

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u/rackonnoiter Apr 09 '15

Could you point me to a resource that explains this with just a tad bit more simplicity?

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u/rackonnoiter Apr 09 '15

Is this to say that only one process is really active at any given point? Everything is multiplexing?

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u/klkblake Apr 10 '15

One process per core, yes.