CS61C Machine Structures Lecture 5 Decisions in C Assembly Language September 13 2000 David Patterson http www inst eecs berkeley edu cs61c cs61c L5 De 1 Review 1 2 In MIPS Assembly Language Registers replace C variables One Instruction simple operation per line Simpler is Better Smaller is Faster Memory is byte addressable but lw and sw access one word at a time A pointer used by lw and sw is just a memory address so we can add to it or subtract from it using offset cs61c L5 De 2 Review 2 2 New Instructions add addi sub lw sw New Registers C Variables s0 s7 Temporary Variables t0 t9 Zero zero cs61c L5 De 3 Overview C Assembly Decisions if if else C Assembly Loops while do while for Inequalities C Switch Statement cs61c L5 De 4 So Far All instructions have allowed us to manipulate data So we ve built a calculator In order to build a computer we need ability to make decisions Heads up pull out some papers and pens you ll do some in class exercises today cs61c L5 De 5 C Decisions if Statements 2 kinds of if statements in C if condition clause if condition clause1 else clause2 Rearrange 2nd if into following if condition goto L1 clause2 go to L2 L1 clause1 L2 Not as elegant as if else but same meaning cs61c L5 De 6 MIPS Decision Instructions Decision instruction in MIPS beq register1 register2 L1 beq is Branch if registers are equal Same meaning as using C if register1 register2 goto L1 Complementary MIPS decision instruction bne register1 register2 L1 bne is Branch if registers are not equal Same meaning as using C if register1 register2 goto L1 Called conditional branches cs61c L5 De 7 MIPS Goto Instruction In addition to conditional branches MIPS has an unconditional branch j label Called a Jump Instruction jump or branch directly to the given label without needing to satisfy any condition Same meaning as using C goto label Technically it s the same as beq 0 0 label since it always satisfies the condition cs61c L5 De 8 Compiling C if into MIPS 1 2 Compile by hand if i j f g h else f g h true i j f g h false i j i j f g h Exit Use this mapping f s0 g s1 h s2 i s3 j s4 cs61c L5 De 9 Compiling C if into MIPS 2 2 true i j f g h Final compiled MIPS code fill in the blank cs61c L5 De false i j i j f g h Exit 10 Loops in C Assembly 1 3 Simple loop in C do g g A i while i h i i j Rewrite this as Loop g g A i i i j if i h goto Loop Use this mapping g s1 h s2 i s3 j s4 base of A s5 cs61c L5 De 12 Loops in C Assembly 2 3 Final compiled MIPS code fill in the blank cs61c L5 De 13 Administrivia Kurt Meinz and Steve Tu heroically volunteer to add to their worloads save Tu Th 5 6 section cs61c L5 De 15 What s This Stuff Good For Breathing Observation Bubble BOB pipes air from a tank under the handlebars into an acrylic dome replacing a diver s face mask and breathing apparatus Wireless technology lets riders talk to other BOBsters darting through the water nearby as well as to armchair divers above in a boat or back on shore Saving energy from not having to kick divers can stay submerged almost an hour with the BOB Like most modern scuba gear the BOB features a computer that tells riders when to come up and calculates decompression times for a safe return to the surface One Digital Day 1998 www intel com onedigitalday cs61c L5 De What do applications apps like these mean for reliability requirements of our technology 16 Loops in C Assembly 3 3 There are three types of loops in C while do while for Each can be rewritten as either of the other two so the method used in the previous example can be applied to while and for loops as well Key Concept Though there are multiple ways of writing a loop in MIPS conditional branch is key to decision making cs61c L5 De 17 Inequalities in MIPS 1 4 Until now we ve only tested equalities and in C General programs need to test and as well Create a MIPS Inequality Instruction Set on Less Than Syntax slt reg1 reg2 reg3 Meaning if reg2 reg3 reg1 1 else reg1 0 In computereeze set means set to 1 reset means set to 0 cs61c L5 De 18 Inequalities in MIPS 2 4 How do we use this Compile by hand if g h goto Less Use this mapping g s0 h s1 cs61c L5 De 19 Inequalities in MIPS 3 4 Final compiled MIPS code fill in the blank cs61c L5 De 20 Inequalities in MIPS 4 4 Now we can implement but how do we implement and We could add 3 more instructions but MIPS goal Simpler is Better Can we implement in one or more instructions using just slt and the branches What about What about cs61c L5 De 22 Immediates in Inequalities There is also an immediate version of slt to test against constants slti Helpful in for loops C if g 1 goto Loop M I P S cs61c L5 De 23 What about unsigned numbers there are unsigned inequality instructions sltu sltiu which set result to 1 or 0 depending on unsigned comparisons s0 FFFF FFFAhex s1 0000 FFFAhex What is value of t0 t1 slt t0 s0 s1 sltu t1 s0 s1 cs61c L5 De 25 Example The C Switch Statement 1 3 Choose among four alternatives depending on whether k has the value 0 1 2 or 3 Compile this C code switch k case 0 f i j break k 0 case 1 f g h break k 1 case 2 f g h break k 2 case 3 f i j break k 3 cs61c L5 De 26 Example The C Switch Statement 2 3 This is complicated so simplify Rewrite it as a chain of if else statements which we already know how to compile if k 0 f i j else if k 1 f g h else if k 2 f g h else if k 3 f i j Use this mapping f s0 g s1 h s2 i s3 j s4 k s5 cs61c L5 De 27 Example The C Switch Statement 3 3 Final compiled MIPS code fill in the blank cs61c L5 De 28 Things to Remember 1 2 A Decision allows us to decide which pieces of code to execute at run time rather than at compile time C Decisions are made using conditional statements within an if while do while or for MIPS Decision making instructions are the conditional branches beq and bne In order to help the conditional branches make decisions concerning inequalities we introduce a single instruction Set …
View Full Document
Unlocking...