Nios® II Processor Reference Guide

ID 683836
Date 10/22/2020
Public
Document Table of Contents

8.1.2. R-Type

The defining characteristic of the R-type instruction word format is that all arguments and results are specified as registers. R-type instructions contain:
  • A 6-bit opcode field OP
  • Three 5-bit register fields A, B, and C
  • An 11-bit opcode-extension field OPX

In most cases, fields A and B specify the source operands, and field C specifies the destination register.

Some R-Type instructions embed a small immediate value in the five low-order bits of OPX. Unused bits in OPX are always 0.

R-type instructions include arithmetic and logical operations such as add and nor; comparison operations such as cmpeq and cmplt; the custom instruction; and other operations that need only register operands.

Table 88.  R-Type Instruction Format
Bit Fields
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
A B C OPX
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
OPX OP

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