The x86 has been extended in many ways throughout its history, remaining mostly backwards compatible while adding execution modes and large extensions to the instruction set. A modern x86 processor can operate in one of four major modes: 16-bit real mode, 16-bit protected mode, 32-bit protected mode, and 64-bit long mode. The primary difference between real and protected mode is in the handling of segments: in real mode the segments directly address memory as 16-byte pages, whereas in protected mode the segments are instead indexes into a descriptor table that contains the physical base and size of the segment. 32-bit protected mode allows paging and virtual memory as well as a 32-bit rather than a 16-bit offset.

The 16-bit and 32-bit operating modes both allow for use of both 16-bit and 32-bit registers via instruction prefixes that set the operation and address size to either 16-bit or 32-bit, with the active operating mode setting the default operation size and the “other” size being flagged with a prefix. These operation and address sizes also affect the size of immediate operands: for example, an instruction with a 32-bit operation size with an immediate operand will have a 32-bit value in the encoded instruction, excepting optimizations such as sign-extended 8-bit values.

Unlike the 16-bit and 32-bit modes, 64-bit long mode is more of a break from the “legacy” modes. Long mode obsoletes several instructions. It is also the only mode in which 64-bit registers are available; 64-bit registers cannot be accessed from either 16-bit or 32-bit mode. Also, unlike the other modes, most encoded values in long mode are limited to 32 bits in size. A small subset of the MOV instructions allow 64 bit encoded values, but values greater than 32 bits in other instructions must come from a register. Partly due to this limitation, but also due to the wide use of relocatable shared libraries, long mode also adds a new addressing mode: RIP-relative.