inst eecs berkeley edu cs61c CS61C Machine Structures Lecture 10 Introduction to MIPS Decisions II 2004 09 22 Lecturer PSOE Dan Garcia www cs berkeley edu ddgarcia Bill Gates visits Cal Oct 1 9am he ll speak Zel Only Eng L S CS students fac allowed in free tix Sep 24th 9am at E side of McLaughlin Hall www coe engnews Fall04 EN04F bill html Garcia Fall 2004 UCB CS 61C L10 Introduction to MIPS Decisions II 1 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 offset 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 CS 61C L10 Introduction to MIPS Decisions II 2 Garcia Fall 2004 UCB From 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 CS 61C L10 Introduction to MIPS Decisions II 3 Garcia Fall 2004 UCB Loading Storing bytes 2 2 What do with other 24 bits in the 32 bit register lb sign extends to fill 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 CS 61C L10 Introduction to MIPS Decisions II 4 Garcia Fall 2004 UCB Overflow in Arithmetic 1 2 Reminder Overflow 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 CS 61C L10 Introduction to MIPS Decisions II 5 Garcia Fall 2004 UCB Overflow in Arithmetic 2 2 Some languages detect overflow Ada some don t C MIPS solution is 2 kinds of arithmetic instructions to recognize 2 choices add add add immediate addi and subtract sub cause overflow to be detected add unsigned addu add immediate unsigned addiu and subtract unsigned subu do not cause overflow detection Compiler selects appropriate arithmetic MIPS C compilers produce addu addiu subu CS 61C L10 Introduction to MIPS Decisions II 6 Garcia Fall 2004 UCB Two Logic Instructions 2 lectures ago we saw add addi sub 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 0000 0002hex Before 0000 0000 0000 0000 0000 0000 0000 0010 two 0000 0008hex After 0000 0000 0000 0000 0000 0000 0000 1000two What arithmetic effect does shift left have Shift Right srl is opposite shift CS 61C L10 Introduction to MIPS Decisions II 7 Garcia Fall 2004 UCB Loops in C Assembly 1 3 Simple loop in C A is an array of ints 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 h i j base of A s1 s2 s3 s4 s5 CS 61C L10 Introduction to MIPS Decisions II 8 Garcia Fall 2004 UCB Loops in C Assembly 2 3 Final compiled MIPS code Loop sll add lw add add bne t1 s3 2 t1 4 I t1 t1 s5 t1 addr A t1 0 t1 t1 A i s1 s1 t1 g g A i s3 s3 s4 i i j s3 s2 Loop goto Loop if i h Original code Loop g g A i i i j if i h goto Loop CS 61C L10 Introduction to MIPS Decisions II 9 Garcia Fall 2004 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 while and for loops as well Key Concept Though there are multiple ways of writing a loop in MIPS the key to decision making is conditional branch CS 61C L10 Introduction to MIPS Decisions II 10 Garcia Fall 2004 UCB Inequalities in MIPS 1 3 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 reg1 reg2 reg3 if reg2 reg3 reg1 1 else reg1 0 Same thing In computereeze set means set to 1 reset means set to 0 CS 61C L10 Introduction to MIPS Decisions II 11 Garcia Fall 2004 UCB Inequalities in MIPS 2 3 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 bne t0 0 Less t0 1 if g h goto Less if t0 0 if g h Less Branch if t0 0 g h 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 CS 61C L10 Introduction to MIPS Decisions II 12 Garcia Fall 2004 UCB Inequalities in MIPS 3 3 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 CS 61C L10 Introduction to MIPS Decisions II 13 Garcia Fall 2004 UCB 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 Loop I slti t0 s0 1 P S beq t0 0 Loop if g 1 t0 1 if s0 1 g 1 goto Loop if t0 0 A slt beq pair means if goto CS 61C L10 Introduction to MIPS Decisions II 14 Garcia Fall 2004 UCB What about unsigned numbers Also unsigned inequality instructions sltu sltiu which sets result to 1 or 0 depending on unsigned comparisons What is value of t0 t1 s0 FFFF FFFAhex s1 0000 FFFAhex slt t0 s0 s1 sltu t1 s0 s1 CS 61C L10 Introduction to MIPS Decisions II 15 Garcia Fall 2004 UCB MIPS Signed vs Unsigned diff meanings MIPS Signed v Unsigned is an overloaded term Do Don t sign extend lb lbu Don t overflow addu addiu subu multu divu Do signed unsigned compare slt slti sltu sltiu CS 61C L10 Introduction to MIPS Decisions II 16 Garcia Fall 2004 UCB Administrivia Proj1 due in 9 days start EARLY Out on Wed due Friday extended date The following hw smaller still …
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