Orthogonal instruction set

In computer engineering, an orthogonal instruction set is an instruction set architecture where all instruction types can use all addressing modes. It is "orthogonal" in the sense that the instruction type and the addressing mode vary independently. An orthogonal instruction set does not impose a limitation that requires a certain instruction to use a specific register.

In many CISC computers, an instruction could access either registers or memory, usually in several different ways. This made the CISC machines easier to program, because rather than being required to remember thousands of individual instruction opcodes, an orthogonal instruction set allowed a programmer to instead remember just thirty to a hundred operation codes ("ADD", "SUBTRACT", "MULTIPLY", "DIVIDE", etc.) and a set of three to ten addressing modes ("FROM REGISTER 0", "FROM REGISTER 1", "FROM MEMORY", etc.). The DEC PDP-11 and Motorola 68000 computer architectures are examples of nearly orthogonal instruction sets, while the ARM11 and VAX are examples of CPUs with fully orthogonal instruction sets.

With the exception of its floating point instructions, the PDP-11 was very strongly orthogonal. Every integer instruction could operate on either 1-byte or 2-byte integers and could access data stored in registers, stored as part of the instruction, stored in memory, or stored in memory and pointed to by addresses in registers. Even the PC and the stack pointer could be affected by the ordinary instructions using all of the ordinary data modes. In fact, "immediate" mode (hardcoded numbers within an instruction, such as ADD #4, R1 (R1 = R1 + 4) was implemented as the mode "register indirect, autoincrement" and specifying the program counter (R7) as the register to use reference for indirection and to autoincrement.

Since the PDP-11 was an octal-oriented (3-bit sub-byte) machine (addressing modes 0–7, registers R0–R7), there were (electronically) 8 addressing modes. Through the use of the Stack Pointer (R6) and Program Counter (R7) as referenceable registers, there were 10 conceptual addressing modes available.

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