X86 memory segmentation

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x86 memory segmentation refers to the implementation of memory segmentation on the x86 architecture. Memory is divided into portions that may be addressed by a single index register without changing a 16-bit segment selector. In real mode or V86 mode, a segment is always 64 kilobytes in size (using 16-bit offsets). In protected mode, a segment can have variable length.


Real mode

In 16-bit real mode, enabling applications to make use of multiple memory segments (in order to access more memory than available in any one 64K-segment) was quite complex, but was viewed as a necessary evil for all but the smallest tools (which could do with less memory). The root of the problem was that no appropriate address-arithmetic instructions suitable for flat addressing of the entire memory range were available. Flat addressing is possible by applying multiple instructions, which however leads to slower programs.

In real mode, the 16-bit segment selector is interpreted as the most significant 16 bits of a linear 20-bit address of which the remaining four least significant bits are all zeros. The segment selector is always added with a 16-bit offset to yield a linear address. For instance, the linear address 06EF0h has a segment selector of 06EFh; the segmented address 06EFh:1234h has a segment selector of 06EFh to which we add the offset, yielding the linear address 06EF0h + 1234h = 08124h (cf. hexadecimal).

A single linear address can be mapped to 4096 distinct segment:offset pairs. For example, the linear address 08124h can have the segmented addresses 06EFh:1234h, 0812h:0004h, 0000h:8124h, etc. This could be confusing to programmers accustomed to unique addressing schemes. (Note that the leading zeros of the linear address, segmented addresses, and the segment and offset fields, which are usually neglected, were shown here for clarity).

The effective 20-bit address space of real mode limited the addressable memory to 220 bytes, or 1,048,576 bytes.

Protected mode

In protected mode, segmentation is used as a virtual memory mechanism, providing memory isolation and contiguous addressing of non-contiguous physical memory.

On the 386 and later, programs issue logical (46-bit) addresses which go through the segmentation unit to be checked and translated into linear 32-bit addresses, before being sent to the paging unit (if enabled) which ultimately translates them into physical addresses (which are also 32-bit on the 386, but can be larger on more modern processors which support Physical Address Extension).

Detailed Segmentation Unit Workflow

A logical address consists of a 16-bit segment selector (supplying 13+1 address bits) and a 32-bit offset (16-bit on the 286). The segment selector must be located in one of the segment registers. That selector consists of a 2-bit Requested Privilege Level (RPL) where the lowest number is the highest privilege level, a 1-bit Table Indicator (TI), and a 13-bit index.

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