计算机组成与设计-7.ppt

CS61C L221 Performance ? UC Regents Review Simplifying MIPS: Define instructions to be same size as data word (one word) so that they can use the same memory (compiler can use lw and sw). Computer actually stores programs as a series of these 32-bit numbers. MIPS Machine Language Instruction: 32 bits representing a single instruction I-Format Problems (0/3) Problem 0: Unsigned # sign-extended? addiu, sltiu, sign-extends immediates to 32 bits. Thus, # is a “signed” integer. Rationale addiu so that can add w/out overflow See KR pp. 230, 305 sltiu suffers so that we can have ez HW Does this mean we’ll get wrong answers? Nope, it means assembler has to handle any unsigned immediate 215 ≤ n 216 (I.e., with a 1 in the 15th bit and 0s in the upper 2 bytes) as it does for numbers that are too large. ? I-Format Problem (1/3) Problem: Chances are that addi, lw, sw and slti will use immediates small enough to fit in the immediate field. …but what if it’s too big? We need a way to deal with a 32-bit immediate in any I-format instruction. I-Format Problem (2/3) Solution to Problem: Handle it in software + new instruction Don’t change the current instructions: instead, add a new instruction to help out New instruction: lui register, immediate stands for Load Upper Immediate takes 16-bit immediate and puts these bits in the upper half (high order half) of the specified register sets lower half to 0s I-Format Problems (3/3) Solution to Problem (continued): So how does lui help us? Example: addi $t0,$t0, 0xABABCDCD becomes: lui $at, 0xABAB ori $at, $at, 0xCDCD add $t0,$t0,$at Now each I-format instruction has only a 16-bit immediate. Wouldn’t it be nice if the assembler would this for us automatically? (later) Branches: PC-Relative Addressing (1/5) Use I-Format Branches: PC-Relative Addressing (2/5) How do we typically use branches? Answer: if-else, while, for Loops are generally small: usually up to 50 instructions Function calls and unconditional jump