Address space

From Academic Kids

In computing, an address space defines a context in which an address makes sense. Two addresses may be numerically the same, but refer to different things, if they belong to different address spaces.

Some example address spaces include:

In general, things in one address space are physically in a different location than things in another address space. For example, "house number 101 South" on one particular southward street is completely different from any house number (not just the 101st house) on a different southward street.

However, sometimes different address spaces overlap (some physical location exists in both address spaces). When overlapping address spaces are not aligned, translation is necessary. For example, virtual-to-physical address translation is necessary to translate addresses in the virtual memory address space to addresses in physical address space -- one physical address, and one or more numerically different virtual address, all refer to the same physical byte of RAM.

Many programmers prefer to use a flat memory model, in which there is no distinction between code space, data space, and virtual memory -- in other words, numerically identical pointers refer to exactly the same byte of RAM in all three address spaces.

Unfortunately, many early computers did not support a flat memory model -- in particular, Harvard architecture machines force program storage to be completely separate from data storage. Many modern DSPs (such as the Motorola 56000) have 3 separate storage areas -- program storage, coefficient storage, and data storage. Some commonly-used instructions fetch from all three areas simultaneously -- fewer storage areas (even if there were the same or more total bytes of storage) would make those instructions run slower.

In the Linux kernel, address spaces include:

  • Kernel memory
  • User memory, accessed through copy_to_user(), copy_from_user and similar functions
  • I/O memory, accessed through readb(), writel(), memcpy_toio(), etc.pl:Przestrzeń adresowa
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