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is usually encoded within the instruction code(e.g. IBM System/360 and successors, most RISC). But when there are many addressing modes, a specific field is often set aside in the instruction to specify the addressing mode. The DEC VAX allowed multiple memory operands for almost all instructions, and so reserved the first few bits of each operand specifier to indicate the addressing mode for that particular operand.Keeping the addressing mode specifier bits separate from the opcode operation bits produces an orthogonal instruction set.

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