OS X, gcc, x86, segmentation, paging, seg fault, bus error

In the case of osx, gcc, modern x8开发者_开发技巧6:

How is the x86 segmentation h/w and paging h/w used?

For the most part¹, the segmentation hardware isn't used. Most current OSes set CS, DS, SS, and ES to all point to all memory (base address of 0, limit of 4Gig). Each is set to allow full access to all that memory (CS->execute, DS, ES, SS->read/write).

That means nearly all real access control is done with the paging unit. The basic idea is that pages accessible by a particular process are mapped to that process. Pages that are in virtual memory are mapped, but marked not present, so attempting to read/write them will cause an exception; the OS reads the data from the paging file into RAM, marks the data as present, and re-starts the instruction.

As far as how pages are marked, most executable code will be marked read-only, and will be shared between processes. Most data and stack will be marked read/write and will not be shared. Depending on the exact system, stack space will usually have the NX bit set to prevent it from being executed.

There are a few other bits and pieces that are a bit different. For example, most OSes (including OS/X, if memory serves) set up a stack guard page -- a page at the top of the stack that allows no access. When/if you try to access it, the OS catches an exception, allocates another page of stack space, and re-starts the instruction. This means you can allocate (say) 4 megabytes of address space for the stack, but only allocate actual RAM for roughly the space that's been used (obviously in page-sized increments).

The hardware also supports "large" (4 megabyte) pages. These are used primarily for mapping large chunks of contiguous memory like the part of the memory on the graphics card that's directly visible to the CPU.

That's only a very high-level view, but it's hard to provide more detail without knowing what you care about. Trying to cover all the use of paging by an entire OS could occupy an entire (large) book.

¹ Windows (unlike most other systems) does make a minimal use of segmentation -- it sets up FS as a pointer to a Thread Information Block (TIB), which gives access to some basic information about the current thread. This is useful (and used) particularly by Windows' Structured Exception Handling (and Vectored Exception Handling).