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→Instruction encoding: make separate section conditional instructions, mention SuperH bf |
→Number of operands: mention that all high-end RISC architectures are forced to have 4-operand FMA |
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*more operands—some CISC machines permit a variety of addressing modes that allow more than 3 operands (registers or memory accesses), such as the [[VAX]] "POLY" polynomial evaluation instruction.
Due to the large number of bits needed to encode the three registers of a 3-operand instruction, RISC architectures that have 16-bit instructions are invariably 2-operand designs, such as the Atmel AVR, [[TI MSP430]], and some versions of [[ARM Thumb]]. RISC architectures that have 32-bit instructions are usually 3-operand designs, such as the [[ARM architecture family|ARM]], [[AVR32]], [[MIPS architecture|MIPS]], [[Power ISA]], and [[SPARC]] architectures. However even 3-operand RISC architectures will, at considerable cost, have [[Multiply%E2%80%93accumulate_operation#Fused_multiply%E2%80%93add|Fused multiply-and-add]] 4-operand instructions out of necessity, due to the increased accuracy provided. Modern examples include [[Power ISA]] and [[RISC-V]].
Each instruction specifies some number of operands (registers, memory locations, or immediate values) ''explicitly''. Some instructions give one or both operands implicitly, such as by being stored on top of the [[stack (data structure)|stack]] or in an implicit register. If some of the operands are given implicitly, fewer operands need be specified in the instruction. When a "destination operand" explicitly specifies the destination, an additional operand must be supplied. Consequently, the number of operands encoded in an instruction may differ from the mathematically necessary number of arguments for a logical or arithmetic operation (the [[arity]]). Operands are either encoded in the "opcode" representation of the instruction, or else are given as values or addresses following the opcode.
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