inst.eecs.berkeley.edu/~cs61c UCB CS61C : Machine Structures Lecture 11 – Introduction to MIPS Procedures I 2010-02-12 “Google is planning to launch an experiment that we hope will make Internet access better and faster for everyone. We plan to test 1 GB/s networks (100x faster) in one or more trial locations across the country, fiber-to-the-home connections. We'll offer service at a competitive price to at least 50,000 and potentially up to 500,000 people.” Lecturer SOE Dan Garcia www.google.com/appserve/fiberrfi CS61C L11 Introduction to MIPS : Procedures I (2) Garcia, Spring 2010 © UCB Review In order to help the conditional branches make decisions concerning inequalities, we introduce a single instruction: “Set on Less Than” called slt, slti, sltu, sltiu One can store and load (signed and unsigned) bytes as well as words Unsigned add/sub don’t cause overflow New MIPS Instructions: sll, srl, lb, sb slt, slti, sltu, sltiu addu, addiu, subu CS61C L11 Introduction to MIPS : Procedures I (3) Garcia, Spring 2010 © UCB C functions main() { int i,j,k,m; ... i = mult(j,k); ... m = mult(i,i); ... } /* really dumb mult function */ int mult (int mcand, int mlier){ int product = 0; while (mlier > 0) { product = product + mcand; mlier = mlier -1; } return product; } What information must compiler/programmer keep track of? What instructions can accomplish this? 2010-02-01 @ Faculty Lunch CS10 : The Beauty and Joy of Computing http://inst.eecs.berkeley.edu/~cs39n/fa10/ Function Call Bookkeeping Registers play a major role in keeping track of information for function calls. Register conventions: Return address $ra Arguments $a0, $a1, $a2, $a3 Return value $v0, $v1 Local variables $s0, $s1, … , $s7 The stack is also used; more later. CS61C L11 Introduction to MIPS : Procedures I (5) Garcia, Spring 2010 © UCB Instruction Support for Functions (1/6) ... sum(a,b);... /* a,b:$s0,$s1 */ } int sum(int x, int y) { return x+y; } address (shown in decimal) 1000 1004 1008 1012 1016 … 2000 2004 C M I P S In MIPS, all instructions are 4 bytes, and stored in memory just like data. So here we show the addresses of where the programs are stored. CS61C L11 Introduction to MIPS : Procedures I (6) Garcia, Spring 2010 © UCB Instruction Support for Functions (2/6) ... sum(a,b);... /* a,b:$s0,$s1 */ } int sum(int x, int y) { return x+y; } address (shown in decimal) 1000 add $a0,$s0,$zero # x = a 1004 add $a1,$s1,$zero # y = b 1008 addi $ra,$zero,1016 #$ra=1016 1012 j sum #jump to sum 1016 … 2000 sum: add $v0,$a0,$a1 2004 jr $ra # new instruction C M I P SCS61C L11 Introduction to MIPS : Procedures I (7) Garcia, Spring 2010 © UCB Instruction Support for Functions (3/6) ... sum(a,b);... /* a,b:$s0,$s1 */ } int sum(int x, int y) { return x+y; } 2000 sum: add $v0,$a0,$a1 2004 jr $ra # new instruction • Question: Why use jr here? Why not use j? • Answer: sum might be called by many places, so we can’t return to a fixed place. The calling proc to sum must be able to say “return here” somehow. C M I P S CS61C L11 Introduction to MIPS : Procedures I (8) Garcia, Spring 2010 © UCB Instruction Support for Functions (4/6) Single instruction to jump and save return address: jump and link (jal) Before: 1008 addi $ra,$zero,1016 #$ra=1016 1012 j sum #goto sum After: 1008 jal sum # $ra=1012,goto sum Why have a jal? Make the common case fast: function calls very common. Don’t have to know where code is in memory with jal! CS61C L11 Introduction to MIPS : Procedures I (9) Garcia, Spring 2010 © UCB Instruction Support for Functions (5/6) Syntax for jal (jump and link) is same as for j (jump): jal label jal should really be called laj for “link and jump”: Step 1 (link): Save address of next instruction into $ra! Why next instruction? Why not current one? Step 2 (jump): Jump to the given label CS61C L11 Introduction to MIPS : Procedures I (10) Garcia, Spring 2010 © UCB Instruction Support for Functions (6/6) Syntax for jr (jump register): jr register Instead of providing a label to jump to, the jr instruction provides a register which contains an address to jump to. Very useful for function calls: jal stores return address in register ($ra) jr $ra jumps back to that address CS61C L11 Introduction to MIPS : Procedures I (11) Garcia, Spring 2010 © UCB Nested Procedures (1/2) int sumSquare(int x, int y) { return mult(x,x)+ y; } Something called sumSquare, now sumSquare is calling mult. So there’s a value in $ra that sumSquare wants to jump back to, but this will be overwritten by the call to mult. Need to save sumSquare return address before call to mult. CS61C L11 Introduction to MIPS : Procedures I (12) Garcia, Spring 2010 © UCB Nested Procedures (2/2) In general, may need to save some other info in addition to $ra. When a C program is run, there are 3 important memory areas allocated: Static: Variables declared once per program, cease to exist only after execution completes. E.g., C globals Heap: Variables declared dynamically via malloc Stack: Space to be used by procedure during execution; this is where we can save register valuesCS61C L11 Introduction to MIPS : Procedures I (13) Garcia, Spring 2010 © UCB C memory Allocation review 0 ∞"Address Code Program Static Variables declared once per program; e.g., globals Heap Explicitly created space, i.e., malloc() Stack Space for saved procedure information $sp stack pointer CS61C L11 Introduction to MIPS : Procedures I (14) Garcia, Spring 2010 © UCB Using the Stack (1/2) So we have a register $sp which always points to the last used space in the stack. To use stack, we decrement this pointer by the amount of space we need and then fill it with info. So, how do we compile this? int sumSquare(int x, int y) { return mult(x,x)+ y; } CS61C L11 Introduction to MIPS : Procedures I (15) Garcia, Spring 2010 © UCB Using the Stack (2/2) Hand-compile sumSquare: addi $sp,$sp,-8 # space on stack sw $ra, 4($sp) # save ret addr sw $a1, 0($sp) # save y add $a1,$a0,$zero # mult(x,x) jal mult # call mult lw $a1, 0($sp) # restore y add $v0,$v0,$a1 # mult()+y
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