Instruction Types
Common choices for the single instruction are:
- Subtract and branch if less than or equal to zero
- Subtract and branch if negative
- Reverse subtract and skip if borrow
- Move (used as part of a transport triggered architecture)
Only one of these instructions is used in a given implementation. Hence, there is no need for an opcode to identify which instruction to execute; the choice of instruction is inherent in the design of the machine, and an OISC is typically named after the instruction it uses (e.g., an SBN OISC, the SUBLEQ language, etc.). Each of the above instructions can be used to construct a Turing-complete OISC.
This article presents only subtraction-based instructions among those that are not transport triggered. However it is possible to build Turing complete machine using an instruction based on other arithmetic operations, e.g., addition. For example, one variation known as DLN (Decrement and jump if not zero) has only two operands and uses decrement as the base operation. For more information see Subleq derivative languages.
Read more about this topic: One Instruction Set Computer
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