inst eecs berkeley edu cs61c UCB CS61C Machine Structures Lecture 12 Introduction to MIPS Procedures II Logical Ops Lecturer SOE Dan Garcia 2008 02 17 Hello to Rob Hunt from Bristol UK IBM has created graphene transistors that operate at 100 GHz 10x silicon Graphene is a flat monolayer of carbon atoms packed into a 2D honeycomb lattice Wikipedia www technologyreview com computing 24482 Review Functions called with jal return with jr ra The stack is your friend Use it to save anything you need Just leave it the way you found it Instructions we know so far Arithmetic add addi sub addu addiu subu Memory lw sw lb sb Decision beq bne slt slti sltu sltiu Unconditional Branches Jumps j jal jr Registers we know so far All of them There are CONVENTIONS when calling procedures CS61C L12 Introduction to MIPS Procedures II Logical Ops 2 Garcia Spring 2010 UCB Register Conventions 1 4 CalleR the calling function CalleE the function being called When callee returns from executing the caller needs to know which registers may have changed and which are guaranteed to be unchanged Register Conventions A set of generally accepted rules as to which registers will be unchanged after a procedure call jal and which may be changed CS61C L12 Introduction to MIPS Procedures II Logical Ops 3 Garcia Spring 2010 UCB Register Conventions 2 4 saved 0 No Change Always 0 s0 s7 Restore if you change Very important that s why they re called saved registers If the callee changes these in any way it must restore the original values before returning sp Restore if you change The stack pointer must point to the same place before and after the jal call or else the caller won t be able to restore values from the stack HINT All saved registers start with S CS61C L12 Introduction to MIPS Procedures II Logical Ops 4 Garcia Spring 2010 UCB Register Conventions 2 4 volatile ra Can Change The jal call itself will change this register Caller needs to save on stack if nested call v0 v1 Can Change These will contain the new returned values a0 a3 Can change These are volatile argument registers Caller needs to save if they are needed after the call t0 t9 Can change That s why they re called temporary any procedure may change them at any time Caller needs to save if they ll need them afterwards CS61C L12 Introduction to MIPS Procedures II Logical Ops 5 Garcia Spring 2010 UCB Register Conventions 4 4 What do these conventions mean If function R calls function E then function R must save any temporary registers that it may be using onto the stack before making a jal call Function E must save any S saved registers it intends to use before garbling up their values Remember caller callee need to save only temporary saved registers they are using not all registers CS61C L12 Introduction to MIPS Procedures II Logical Ops 6 Garcia Spring 2010 UCB Parents leaving for weekend analogy 1 5 Parents main leaving for weekend They caller give keys to the house to kid callee with the rules calling conventions You can trash the temporary room s like the den and basement registers if you want we don t care about it BUT you d better leave the rooms registers that we want to save for the guests untouched these rooms better look the same when we return Who hasn t heard this in their life CS61C L12 Introduction to MIPS Procedures II Logical Ops 7 Garcia Spring 2010 UCB Parents leaving for weekend analogy 2 5 Kid now owns rooms registers Kid wants to use the saved rooms for a wild wild party computation What does kid callee do Kid takes what was in these rooms and puts them in the garage memory Kid throws the party trashes everything except garage who ever goes in there Kid restores the rooms the parents wanted saved after the party by replacing the items from the garage memory back into those saved rooms CS61C L12 Introduction to MIPS Procedures II Logical Ops 8 Garcia Spring 2010 UCB Parents leaving for weekend analogy 3 5 Same scenario except before parents return and kid replaces saved rooms Kid callee has left valuable stuff data all over Kid s friend another callee wants the house for a party when the kid is away Kid knows that friend might trash the place destroying valuable stuff Kid remembers rule parents taught and now becomes the heavy caller instructing friend callee on good rules conventions of house CS61C L12 Introduction to MIPS Procedures II Logical Ops 9 Garcia Spring 2010 UCB Parents leaving for weekend analogy 4 5 If kid had data in temporary rooms which were going to be trashed there are three options Move items directly to garage memory Move items to saved rooms whose contents have already been moved to the garage memory Optimize lifestyle code so that the amount you ve got to shlep stuff back and forth from garage memory is minimized Mantra Minimize register footprint Otherwise Dude where s my data CS61C L12 Introduction to MIPS Procedures II Logical Ops 10 Garcia Spring 2010 UCB Parents leaving for weekend analogy 5 5 Friend now owns rooms registers Friend wants to use the saved rooms for a wild wild party computation What does friend callee do Friend takes what was in these rooms and puts them in the garage memory Friend throws the party trashes everything except garage Friend restores the rooms the kid wanted saved after the party by replacing the items from the garage memory back into those saved rooms CS61C L12 Introduction to MIPS Procedures II Logical Ops 11 Garcia Spring 2010 UCB Bitwise Operations So far we ve done arithmetic add sub addi mem access lw and sw branches and jumps All of these instructions view contents of register as a single quantity e g signed or unsigned int New Perspective View register as 32 raw bits rather than as a single 32 bit number Since registers are composed of 32 bits wish to access individual bits or groups of bits rather than the whole Introduce two new classes of instructions Logical Shift Ops CS61C L12 Introduction to MIPS Procedures II Logical Ops 13 Garcia Spring 2010 UCB Logical Operators 1 3 Two basic logical operators AND outputs 1 only if all inputs are 1 OR outputs 1 if at least one input is 1 Truth Table standard table listing all possible combinations of inputs and resultant output A B A AND B A OR B 0 0 1 1 0 1 0 1 0 0 0 1 0 1 1 1 CS61C L12 Introduction to MIPS Procedures II Logical Ops 14 Garcia Spring 2010 UCB Logical Operators 2 3 Logical Instruction Syntax 1 2 3 4 where 1 operation name 2 register that will receive value 3 first operand register 4 second operand register or
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