Assembly LanguageGoals of Today’s LectureThree Levels of LanguagesHigh-Level LanguageSlide 5Machine LanguageWhy Learn Assembly Language?Why Learn Intel IA32 Assembly?Computer ArchitectureA Typical ComputerVon Neumann ArchitectureControl Unit: Instruction PointerControl Unit: Instruction DecoderRegistersKeeping it Simple: All 32-bit WordsC Code vs. Assembly CodeExample: Kinds of InstructionsExample: Variables in RegistersExample: Immediate and Register AddressingSlide 20Slide 21Slide 22Example: Changing Program FlowExample: Jump and LabelsSlide 25Example: Making More Efficient??Complete ExampleReading IA32 Assembly LanguageDesigning an Instruction SetHow Many Instructions to Have?CISC vs. RISCNext Time: More on Assembly Language1Assembly LanguageProfessor Jennifer RexfordCOS 2172Goals of Today’s Lecture•Assembly languageBetween high-level language and machine codeWhy learn assembly-language programming?•Computer architectureCentral Processing Unit (CPU)Fetch-decode-execute cycle•C code vs. assembly codeSmall IA32 example to illustrate the relationship to CIllustrates storing, manipulating, comparing, and jumping3Three Levels of Languages4High-Level Language•Make programming easier by describing operations in a natural language•Increase the portability of the code•One line may involve many low-level operations•Examples: C, C++, Java, Pascal, …count = 0;while (n > 1) { count++; if (n & 1) n = n*3 + 1; else n = n/2;}5Assembly Language•Tied to the specifics of the underlying machine•Commands and names to make the code readable and writeable by humans•Hand-coded assembly code may be more efficient•E.g., IA32 from Intelmovl %edx, %eaxandl $1, %eaxje .elsejmp .endif.else:.endif:sarl $1, %edxmovl %edx, %eaxaddl %eax, %edxaddl %eax, %edxaddl $1, %edxaddl $1, %ecx.loop:cmpl $1, %edxjle .endloopjmp .loop.endloop:movl $0, %ecx6Machine Language•Also tied to the underlying machine•What the computer sees and deals with•Every command is a sequence of one or more numbers•All stored in memory on the computer, and read and executed•Unreadable by humans0000 0000 0000 0000 0000 0000 0000 00000000 0000 0000 0000 0000 0000 0000 00009222 9120 1121 A120 1121 A121 7211 00000000 0001 0002 0003 0004 0005 0006 00070008 0009 000A 000B 000C 000D 000E 000F0000 0000 0000 FE10 FACE CAFE ACED CEDE1234 5678 9ABC DEF0 0000 0000 F00D 00000000 0000 EEEE 1111 EEEE 1111 0000 0000B1B2 F1F5 0000 0000 0000 0000 0000 00007Why Learn Assembly Language?•Write faster code (even in high-level language)By understanding which high-level constructs are better… in terms of how efficient they are at the machine level•Understand how things work underneathLearn the basic organization of the underlying machineLearn how the computer actually runs a programDesign better computers in the future•Some software is still written in assembly languageCode that really needs to run quicklyCode for embedded systems, network processors, etc.8Why Learn Intel IA32 Assembly?•Program natively on our computing platformRather than using an emulator to mimic another machine•Learn instruction set for the most popular platformMost likely to work with Intel platforms in the future•But, this comes at some cost in complexityIA32 has a large and varied set of instructionsArguably more than are really useful in practice•Fortunately, you won’t need to use everything9Computer Architecture10A Typical Computer CPUChipsetMemoryI/O busCPU. . .NetworkROM11Von Neumann Architecture•Central Processing UnitControl unit–Fetch, decode, and execute Arithmetic and logic unit–Execution of low-level operationsGeneral-purpose registers–High-speed temporary storageData bus–Provide access to memory•MemoryStore instructionsStore dataRandom AccessMemory (RAM)ControlUnitALUCPURegistersData bus12Control Unit: Instruction Pointer•Stores the location of the next instructionAddress to use when reading from memory•Changing the instruction pointer (EIP)Increment by one to go to the next instructionOr, load a new value to “jump” to a new locationEIP13Control Unit: Instruction Decoder•Determines what operations need to take placeTranslate the machine-language instruction •Control what operations are done on what dataE.g., control what data are fed to the ALUE.g., enable the ALU to do multiplication or additionE.g., read from a particular address in memoryALU14Registers•Small amount of storage on the CPUCan be accessed more quickly than main memory•Instructions move in and out of registersLoading registers from main memoryStoring registers to main memory•Instructions manipulate the register contentsRegisters essentially act as temporary variablesFor efficient manipulation of the data•Registers are the top of the memory hierarchyAhead of cache, main memory, disk, tape, …15Keeping it Simple: All 32-bit Words•Simplifying assumption: all data in four-byte unitsMemory is 32 bits wideRegisters are 32 bits wide•In practice, can manipulate different sizes of dataEAXEBX16C Code vs. Assembly Code17Example: Kinds of Instructions•Storing values (in registers)count = 0n•Arithmetic and logic operationsIncrement: count++Multiply: n * 3Divide: n/2Logical AND: n & 1•Checking results of comparisons while (n > 1)if (n & 1)•JumpingTo the end of the while loop (if “n > 1”)Back to the beginning of the loopTo the else clause (if “n & 1” is 0)count = 0;while (n > 1) { count++; if (n & 1) n = n*3 + 1; else n = n/2;}18Example: Variables in RegistersRegistersn %edxcount %ecxReferring to a register: percent sign (“%”)count = 0;while (n > 1) { count++; if (n & 1) n = n*3 + 1; else n = n/2;}19Example: Immediate and Register Addressingcount=0;while (n>1) { count++; if (n&1) n = n*3+1; else n = n/2;}movl $0, %ecxaddl $1, %ecxReferring to a constant: dollar sign (“$”)Read directly from the instructionwritten to a register20Example: Immediate and Register Addressingcount=0;while (n>1) { count++; if (n&1) n = n*3+1; else n = n/2;}movl %edx, %eaxandl $1, %eaxComputing intermediate value in register EAX21movl %edx, %eaxaddl %eax, %edxaddl %eax, %edxaddl $1, %edxExample: Immediate and Register Addressingcount=0;while (n>1) { count++; if (n&1) n = n*3+1; else n = n/2;}Adding n twice is cheaper than multiplication!22sarl $1,
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