MIPS III instructions

The NEC VR4300 uses an instruction set that is nearly identical to that of the MIPS III Instruction Set Architecture (ISA). While the instruction set is explained in detail in the VR4300 datasheet, this article serves as a reformatted reference for those same instructions, including any quirks or additional reminders that are useful when programming something for the N64.

CPU Instruction Set
Every instruction has a 32-bit instruction word that the CPU uses to identify the instruction and any arguments used in it. The upper 6 bits is the instruction's opcode, however some instructions use additional constant bits to further define the operation. For example, the SPECIAL opcode is used for numerous instructions including ADD, SUB, AND, BREAK, and more.

Typically an assembler will be used when programming in order to format the proper instruction word, when generating a ROM file. Most developers can safely ignore the instruction word column entirely.

FPU Instruction Set
The floating point unit (FPU) is contained within the VR3400 microprocessor and handles floating point operations.