说明：  MIPS R4400 用户手册，指令集介绍
(The R4000 processor provides complete application software compatibility with the MIPS R2000, R3000, and R6000 processors. Although the MIPS processor architecture has evolved in response to a compromise between software and hardware resources in the computer system, the R4000 processor implements the MIPS ISA for user-mode programs. This guarantees that user programs conforming to the ISA execute on any MIPS hardware implementation.)