inst eecs berkeley edu cs61c UCB CS61C Machine Structures Lecture 10 Introduction to MIPS Decisions II Lecturer SOE Dan Garcia Obama sweeps 8th state in a row it s getting tight 2008 02 13 In what may be regarded as the most anticipated game in a long time EA will be releasing Maxis Spore on PCs Macs Nintendo DS and mobile phones in the fall Players will be able to create and evolve life establish tribes build civilizations sculpt entire worlds and explore others universes www spore com Review 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 o set A Decision allows us to decide what to execute at run time rather than compile time C Decisions are made using conditional statements within if while do while for MIPS Decision making instructions are the conditional branches beq and bne New Instructions lw sw beq bne j CS61C L10 Introduction to MIPS Decisions II 2 Garcia Spring 2008 UCB Last time Loading Storing bytes 1 2 In addition to word data transfers lw sw MIPS has byte data transfers load byte lb store byte sb same format as lw sw E g lb s0 3 s1 contents of memory location with address sum of 3 contents of register s1 is copied to the low byte position of register s0 CS61C L10 Introduction to MIPS Decisions II 3 Garcia Spring 2008 UCB Loading Storing bytes 2 2 What do with other 24 bits in the 32 bit register lb sign extends to ll upper 24 bits xxxx xxxx xxxx xxxx xxxx xxxx xzzz zzzz byte is copied to sign extend loaded This bit Normally don t want to sign extend chars MIPS instruction that doesn t sign extend when loading bytes load byte unsigned lbu CS61C L10 Introduction to MIPS Decisions II 4 Garcia Spring 2008 UCB Over ow in Arithmetic 1 2 Reminder Over ow occurs when there is a mistake in arithmetic due to the limited precision in computers Example 4 bit unsigned numbers 15 1111 3 0011 18 10010 But we don t have room for 5 bit solution so the solution would be 0010 which is 2 and wrong CS61C L10 Introduction to MIPS Decisions II 5 Garcia Spring 2008 UCB Over ow in Arithmetic 2 2 Some languages detect over ow Ada some don t C MIPS solution is 2 kinds of arithmetic instructs These cause over ow to be detected add add add immediate addi subtract sub These do not cause over ow detection add unsigned addu add immediate unsigned addiu subtract unsigned subu Compiler selects appropriate arithmetic MIPS C compilers produce addu addiu subu CS61C L10 Introduction to MIPS Decisions II 6 Garcia Spring 2008 UCB Two Logic Instructions Here are 2 more new instructions Shift Left sll s1 s2 2 s1 s2 2 Store in s1 the value from s2 shifted 2 bits to the left inserting 0 s on right in C Before 0000 0002hex 0000 0000 0000 0000 0000 0000 0000 0010two After 0000 0008hex 0000 0000 0000 0000 0000 0000 0000 1000two What arithmetic e ect does shift left have Shift Right srl is opposite shift CS61C L10 Introduction to MIPS Decisions II 7 Garcia Spring 2008 UCB Loops in C Assembly 1 3 Simple loop in C A is an array of ints do g g A i i i j while i h Rewrite this as Loop g g A i i i j if i h goto Loop Use this mapping g h i j base of A s1 s2 s3 s4 s5 CS61C L10 Introduction to MIPS Decisions II 8 Garcia Spring 2008 UCB Loops in C Assembly 2 3 Final compiled MIPS code Loop sll t1 s3 2 addu t1 t1 s5 lw t1 0 t1 addu s1 s1 t1 addu s3 s3 s4 bne s3 s2 Loop t1 4 I t1 addr A 4i t1 A i g g A i i i j goto Loop if i h Original code Loop g g A i i i j if i h goto Loop CS61C L10 Introduction to MIPS Decisions II 9 Garcia Spring 2008 UCB 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 these loops as well Key Concept Though there are multiple ways of writing a loop in MIPS the key to decision making is conditional branch CS61C L10 Introduction to MIPS Decisions II 10 Garcia Spring 2008 UCB Administrivia Project 1 due Friday ok Saturday but tell your brain it s Friday How useful was Faux Exam 1 Other administrivia CS61C L10 Introduction to MIPS Decisions II 11 Garcia Spring 2008 UCB Inequalities in MIPS 1 4 Until now we ve only tested equalities and in C General programs need to test and as well Introduce MIPS Inequality Instruction Set on Less Than Syntax slt reg1 reg2 reg3 Meaning reg1 reg2 reg3 if reg2 reg3 reg1 1 Same thing else reg1 0 set means change to 1 reset means change to 0 CS61C L10 Introduction to MIPS Decisions II 12 Garcia Spring 2008 UCB Inequalities in MIPS 2 4 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 Register 0 always contains the value 0 so bne and beq often use it for comparison after an slt instruction A slt bne pair means if goto CS61C L10 Introduction to MIPS Decisions II 13 Garcia Spring 2008 UCB Inequalities in MIPS 3 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 branches What about What about CS61C L10 Introduction to MIPS Decisions II 14 Garcia Spring 2008 UCB Inequalities in MIPS 4 4 a s0 b s1 slt t0 s0 s1 t0 1 if a b beq t0 0 skip skip if a b stuff do if a b skip Two independent variations possible Use slt t0 s1 s0 instead of slt t0 s0 s1 Use bne instead of beq CS61C L10 Introduction to MIPS Decisions II 15 Garcia Spring 2008 UCB Immediates in Inequalities There is also an immediate version of slt to test against constants slti Helpful in for loops C M I P S if g 1 goto Loop Loop slti t0 s0 1 beq t0 1 if s0 1 g 1 t0 0 Loop goto Loop if t0 0 if g 1 An slt beq pair means if goto CS61C L10 Introduction to MIPS Decisions II 16 Garcia Spring 2008 UCB What about unsigned numbers Also unsigned inequality instructions sltu sltiu which sets result to 1 or 0 …
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