Orthogonal instruction set

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Definition:
"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."

The story of orthogonal instruction set:
In the past, because the memory is very precious, so the processor designer tends to make instructions as powerful as possible. So they hope one instruction can perform full function like get data from memory and then perform the calculation. They make all instructions can have all the addressing modes. By this way, one instruction can fetch data and do the calculation at the same time. 

But this will inscrease the complexity of the processor design. Also, accessing memory directly is very time-consuming, so some companies choose to use RISC processor (like ARM) which has a LOAD-STORE arhitecture to reduce the complexity of the design.

Examples: the DEC PDP-11 and Motorola 68000 are nearly orthogonal instruction sets, and VAX is an example of CPU with fully orthogonal instruction set.


References:

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