inst.eecs.berkeley.edu/~cs61c UCB CS61C : Machine Structures Lecture 12 – Introduction to MIPS Procedures II & Logical Ops 2008-02-17 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 Lecturer SOE Dan Garcia www.technologyreview.com/computing/24482 Hello to Rob Hunt from Bristol, UK!CS61C L12 Introduction to MIPS : Procedures II & Logical Ops (2) Garcia, Spring 2010 © UCB 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 (3) Garcia, Spring 2010 © UCB 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. Register Conventions (1/4)CS61C L12 Introduction to MIPS : Procedures II & Logical Ops (4) Garcia, Spring 2010 © UCB $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! Register Conventions (2/4) – savedCS61C L12 Introduction to MIPS : Procedures II & Logical Ops (5) Garcia, Spring 2010 © UCB $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. Register Conventions (2/4) – volatileCS61C L12 Introduction to MIPS : Procedures II & Logical Ops (6) Garcia, Spring 2010 © UCB 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. Register Conventions (4/4)CS61C L12 Introduction to MIPS : Procedures II & Logical Ops (7) Garcia, Spring 2010 © UCB 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? Parents leaving for weekend analogy (1/5)CS61C L12 Introduction to MIPS : Procedures II & Logical Ops (8) Garcia, Spring 2010 © UCB 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 Parents leaving for weekend analogy (2/5)CS61C L12 Introduction to MIPS : Procedures II & Logical Ops (9) Garcia, Spring 2010 © UCB 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. Parents leaving for weekend analogy (3/5)CS61C L12 Introduction to MIPS : Procedures II & Logical Ops (10) Garcia, Spring 2010 © UCB 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?!” Parents leaving for weekend analogy (4/5)CS61C L12 Introduction to MIPS : Procedures II & Logical Ops (11) Garcia, Spring 2010 © UCB 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 Parents leaving for weekend analogy (5/5)CS61C L12 Introduction to MIPS : Procedures II & Logical Ops (13) Garcia, Spring 2010 © UCB 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)
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