ModRM

ModRM is a byte used in x86 instruction encoding to specify operand locations and addressing modes. It is part of many instructions that operate on memory or on registers, and is often followed by displacement bytes and, when present, a scale-index-base (SIB) byte. The ModRM byte encodes three fields: mod, reg/opcode, and r/m.

The reg/opcode field can designate a register operand. In some instructions it also serves as an opcode extension, selecting an operation or sub-operation when the main opcode requires additional bits.

The mod field selects the addressing mode and displacement size. In general, mod=11 means both operands are registers. For memory operands, mod=00, 01, or 10 indicate no displacement, 8-bit displacement, or 32-bit displacement respectively (with exceptions in certain r/m cases). In 32- and 64-bit modes, r/m values determine the memory operand or a special case such as absolute or RIP-relative addressing. In 64-bit mode, mod=00 and r/m=101 encodes a RIP-relative disp32 address rather than a simple absolute address, while mod values with other r/m values produce standard memory operands or require a SIB byte when r/m=100.

The r/m field selects the memory address or a register operand. When r/m=100, a SIB byte follows to specify a scalable index and base, with fields scale (2 bits), index (3 bits), and base (3 bits). The effective address is computed from base + index*scale + displacement, with several special cases for absent index or base values and for displacement size dictated by mod.

ModRM is a core element of many instructions across 16, 32, and 64-bit modes, functioning as both a register selector and a memory addressing descriptor, often in conjunction with SIB encoding.