R/M (r/m) (3 bits) Register/Memory operand Opcode the opcode is a 6 bit quantity that determines what instruction family the code is MOD (Mod) (2 bits) Register mode.
1 = Register is Destination, 0 = Register is source. Prefixes Optional prefixes which change the operation of the instruction D (1 bit) Direction. This is the general instruction form for the 8086 sequentially in main memory:ĭisplacement or data (occasional: 1, 2 or 4 bytes) The length is defined separately for each instruction, depending on the available modes of operation of the instruction, the number of required operands and more.Ĩ086 instruction format (16 bit) X86 instructions can be anywhere between 1 and 15 bytes long. This can make the processes of assembly, disassembly and instruction decoding more complicated, because the instruction length needs to be calculated for each instruction.
Amongst other things, this means that the instructions for the x86 architecture are of varying lengths. The x86 architecture is a complex instruction set computer (CISC) architecture.
This page will talk about some of the conversions from assembly language to machine language. This means that essentially we can convert assembly instructions into machine instructions with a look-up table. X86 assembly instructions have a one-to-one relationship with the underlying machine instructions.
