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lecture5 - CSCI-365 Computer Organization Lecture 5 Note...

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CSCI-365 Computer Organization Lecture Note : Some slides and/or pictures in the following are adapted from: Computer Organization and Design, Patterson & Hennessy, ©2005 Some slides and/or pictures in the following are adapted from: slides ©2008 UCB 5
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Inequalities in MIPS How do we use this? Compile by hand: if (g < h) goto Less; #g:$s0 , h:$s1 Answer: compiled MIPS code… slt $t0,$s0,$s1 # $t0 = 1 if g<h bne $t0,$0,Less # goto Less # if $t0!=0 # (if (g<h)) Less: Can we implement in one or more instructions using just slt and the branches? (done in class) What about > ? (done in class) What about ? (done in class)
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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, … , $s7 The stack is also used; more later Function Call Bookkeeping
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... sum(a,b);... /* a,b:$s0,$s1 */ } int sum(int x, int y) { return x+y; } address 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 $ra # new instruction Function Call Bookkeeping
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Function Call Bookkeeping ... sum(a,b);... /* a,b:$s0,$s1 */ } int sum(int x, int y) { return x+y; } Question: Why use jr here? Why not simply 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 2000 sum: add $v0,$a0,$a1 2004 jr $ra # new instruction
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Function Call Bookkeeping 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 are very common. Also, you don’t have to know where the code is loaded into memory with jal
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