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Concept
Orthogonal instruction set
Summary
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 so there is little overlapping of instruction functionality.
Orthogonality was considered a major goal for processor designers in the 1970s, and the VAX-11 is often used as the benchmark for this concept. However, the introduction of RISC design philosophies in the 1980s significantly reversed the trend against more orthogonality.
Modern CPUs often simulate orthogonality in a preprocessing step before performing the actual tasks in a RISC-like core. This "simulated orthogonality" in general is a broader concept, encompassing the notions of decoupling and completeness in function libraries, like in the mathematical concept: an orthogonal function set is easy to use as a basis into expanded functions, ensuring that parts don’t affect another if we change one part.
At their core, all general purpose computers work in the same underlying fashion; data stored in a main memory is read by the central processing unit (CPU) into a fast temporary memory (e.g. CPU registers), acted on, and then written back to main memory. Memory consists of a collection of data values, encoded as numbers and referred to by their addresses, also a numerical value. This means the same operations applied to the data can be applied to the addresses themselves. While being worked on, data can be temporarily held in processor registers, scratchpad values that can be accessed very quickly. Registers are used, for example, when adding up strings of numbers into a total.
In early computers, the instruction set architecture (ISA) often used a single register, in which case it was known as the accumulator. Instructions included an address for the operand.
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