The encoding of instructions, which is a key aspect of an Instruction Set Architecture, defines how instuctions and arguments are encoded as binary values in the machine code of a system.
Some architectures encode instructions as multi-byte sequences, where one or more bytes specify the operation to be performed and the addressing mode(s) to be used, and additional bytes specify the operands (such as the register numbers, immediate values, addresses, or offsets to be used). Other architectures encode instructions as fixed-length bitfields, where various (and varying) subfields within the bitfield specify the operation, addressing mode(s), and operands.
The 6502 architecture uses variable-length byte sequences, from 1 to 3 bytes (including the opcode) depending on the addressing mode.
The x86_64 architecture uses variable-length byte sequences, from 1 to approximately 17 bytes.
The AArch64 architecture uses 32-bit long instructions. Since this instruction length is insufficient to adequately contain a 64-bit operand (such as an address), some operands are encoded using run-length encoding (RLE) or shifted-bitfield techniques. For certain operand values, it may be necessary to build the required value in a register with a few instructions.