lecture8

# lecture8 - Lecture 8: Procedure Nested Procedure...

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Unformatted text preview: Lecture 8: Procedure + Nested Procedure Lab 1 ­3 Assigned •  •  •  •  Connect 4 Lab 1: input/output Lab2: winner detec?on Lab3: smarter program •  Various due dates: 4/29, 5/13, 5/20 Midterm •  5/3, 75 minutes •  Cover everything from number representa?on to assembly •  Will send out sample from past years •  Closed book, closed notes, no calculators REVIEW: PROCEDURE Instruc?on Support for Func?ons ... sum(a,b);... /* a,b:\$s0,\$s1 */ } C! int sum(int x, int y) { return x+y; } address 1000 add \$a0,\$s0,\$zero # x = a # y = b M  1004 add \$a1,\$s1,\$zero #jump to sum I  1012 jal sum 1016 ... P  2000 sum: add \$v0,\$a0,\$a1 S! 2004 jr \$ra # new instruction Procedure calls Main Program … … … Sum(a,b) … … … … Function loc: … … … … … … … Instruc?on Support for Func?ons • Single instruc?on to jump and save return address: jump and link (jal) 1008 jal sum # \$ra=1012?,go to sum • Why have a jal? Make the common case fast: func?on calls are very common. Also, you don’t have to know where the code is loaded into memory with jal. Instruc?on Support for Func?ons • Syntax for jr (jump register): jr register • Instead of providing a label to jump to, the jr instruc?on provides a register which contains an address to jump to • Only useful if we know exact address to jump • Very useful for func?on calls: – jal stores return address in register (\$ra) – jr \$ra jumps back to that address Rules for Procedures • Called with a jal instruc?on, returns with a jr \$ra • Accepts up to 4 arguments in \$a0, \$a1, \$a2 and \$a3 • Return value is always in \$v0 (and if necessary in \$v1) Vola?le Register Conven?ons •  \$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 vola?le argument registers. Caller needs to save if they’ll need them a^er the call. •  \$t0 ­\$t9: Can change. That’s why they’re called temporary: any procedure may change them at any ?me. Caller needs to save if they’ll need them a^erwards. int sumSquare(int x, int y) { return mult(x,x)+ y; } NESTED PROCEDURES Nested Procedures int sumSquare(int x, int y) { return mult(x,x)+ y; } • Something called sumSquare, now sumSquare is calling mult. caller callee & caller \$ra jal loc … … … … another callee loc: \$ra … jal loc2 … … jr \$ra loc2: … … … … … … … jr \$ra … Problem on Nested Procedure caller callee & caller … \$ra jal loc … … … … another callee loc: \$ra X … jal loc2 … … jr \$ra loc2: … … … … … … … jr \$ra … Nested Procedures 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 overwri6en by the call to mult. •  Need to save sumSquare return address before call to mult. Nested Procedures •  In general, may need to save some other info in addi?on to \$ra. •  When a C program is run, there are 3 important memory areas allocated: – Sta?c: Variables declared once per program, cease to exist only a^er execu?on completes. E.g., C globals – Heap: Variables declared dynamically – Stack: Space to be used by procedure during execu?on; this is where we can save register values C Memory Alloca?on Address!∞! Heap! \$sp ! stack! pointer! ! Stack! Space for saved   procedure information! Explicitly created space,   e.g., malloc(); C pointers! Variables declared! Static! once per program! Code! 0! Program! Using the Stack •  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; } Using the Stack • Hand ­compile int sumSquare(int x, int y) { return mult(x,x)+ y; } sumSquare: addi \$sp,\$sp,-8 “push”! sw \$ra, 4(\$sp) sw \$a1, 0(\$sp) # space on stack # save ret addr # save y add \$a1,\$a0,\$zero # mult(x,x) jal mult # call mult lw \$a1, 0(\$sp) add \$v0,\$v0,\$a1 lw \$ra, 4(\$sp) addi \$sp,\$sp,8 “pop”! jr \$ra mult: ... # # # # restore y mult()+y get ret addr restore stack Nested Procedure caller … \$ra callee & caller jal loc … … … … loc: sub \$sp, 4 sw \$ra, (\$sp) … jal loc2 \$ra … … lw \$ra, (\$sp) add \$sp, 4 jr \$ra another callee loc2: … … … … … … … jr \$ra … Basic Structure of a Func?on Prologue! entry_label: addi \$sp,\$sp, -framesize sw \$ra, framesize-4(\$sp) #save \$ra .. save other regs if need be Body ... Epilogue! ra" (call other functions…)! memory" ..restore other regs if need be lw \$ra, framesize-4(\$sp) addi \$sp,\$sp, framesize jr \$ra #restore \$ra EXERCISE  ­ ­ FIBONACCI NUMBERS F(n) = F(n – 1) + F(n – 2) F(0) = F(1) = 1 ...
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