CPSC 121: Models of Computation Module 10: A ... - Ugrad.cs.ubc.ca

Module 10: A Working ComputerHistorical notes:Early 19th century:Joseph Marie Charles dit Jacquard used punched papercards to program looms.CPSC 121 – 2012W T2 7Module 10: A Working ComputerHistorical notes (early 19th century continued):Charles Babbage designed (1837) but could not build thefirst programmable (mechanical) computer, based onJacquard's idea.http://www.computerhistory.org/babbage/CPSC 121 – 2012W T2 8

Module 10: A Working ComputerHistorical notes (continued):1941: Konrad Zuse builds the first electromechanicalcomputer.It had binary arithmetic, including floating point.It was programmable.1946: the ENIAC was the first programmableelectronic computer.It used decimal arithmetic.Reprogramming meant rewiring.All its programmers were women.CPSC 121 – 2012W T2 9Module 10: A Working ComputerHistorical notes (mid 20 thcentury, continued)The first stored-program electronic computers weredeveloped from 1945 to 1950.Programs and data were stored on punched cards.CPSC 121 – 2012W T2 10

Module 10: A Working ComputerA quick roadmap through our courses:CPSC 121: learn about gates, and how we can usethem to design a circuit that executes very simpleinstructions.CPSC 213: learn how the constructs available inlanguages such as Racket, C, C++ or Java areimplemented using these simple instructions.CPSC 313: learn how we can design computers thatexecute programs efficiently and meet the needs ofmodern operating systems.CPSC 121 – 2012W T2 11Module 10: A Working ComputerModule Summary:A little bit of historyImplementing a working computer in LogisimAppendicesCPSC 121 – 2012W T2 12

Module 10: A Working ComputerVon-Neumann architectureMemory (contains both programs and data).Control UnitArithmetic & LogicUnitInput/OutputCPUCPSC 121 – 2012W T2 13Module 10: A Working ComputerMemoryContains both instructions and data.Divided into a number of memory locationsThink of positions in a list: (list-ref mylist pos)Or in an array: myarray[pos] or arrayList arrayl.get(pos).01010111...0 1 2 3 4 5 6 7 8 9 10 11 ...CPSC 121 – 2012W T2 14

Module 10: A Working ComputerMemoryEach memory location contains a fixed number ofbits.Most commonly this number is 8.Values that use more than 8 bits are stored in multipleconsecutive memory locations.Characters use 8 bits (ASCII) or 16/32 (Unicode).Integers use 32 or 64 bits.Floating point numbers use 32, 64 or 80 bits.CPSC 121 – 2012W T2 15Module 10: A Working ComputerArithmetic and Logic UnitPerforms arithmetic and logical operations (+, -, *, /,and, or, etc).Control UnitDecides which instructions to execute.Executes these instructions sequentially.Not quite true, but this is how it appears to the user.CPSC 121 – 2012W T2 16

Module 10: A Working ComputerOur working computer:Implements the design presented in the textbook byBryant and O'Hallaron (used for CPSC 213/313).A small subset of the IA32 (Intel 32-bit) architecture.It has12 types of instructions.One program counter register (PC)contains the address of the next instruction.8 general-purpose 32-bits registersstoresstoresaasinglesinglemulti-bitmulti-bitvalue.value.each of them contains one 32 bit value.used for values that we are currently working with.CPSC 121 – 2012W T2 17Module 10: A Working ComputerExample instruction 1: subl %eax, %ebxThe subl instruction subtracts its arguments.The names %eax and %ebx refer to two registers.This instruction takes the value contained in %eax,subtracts it from the value contained in %ebx, andstores the result back in %ebx.Example instruction 2: irmovl $0x1A, %ecxThis instruction stores a constant in a register.In this case, the value 1A (hexadecimal) is stored in%ecx.CPSC 121 – 2012W T2 18

Module 10: A Working ComputerExample instruction 3: rmmovl %ecx, $8(%ebx)The rmmovl instruction stores a value into memory(Register to Memory Move).In this case it takes the value in register %ecx.And stores it in the memory location whose addressis:The constant 8PLUS the current value of register %ebx.CPSC 121 – 2012W T2 19Module 10: A Working ComputerExample instruction 4: jge $1000This is a conditional jump instruction.It checks to see if the result of the last arithmetic orlogic operation was zero or positive (Greater than orEqual to 0).If so, the next instruction is the instruction stored inmemory address 1000 (hexadecimal).If not, the next instruction is the instruction thatfollows the jge instruction.CPSC 121 – 2012W T2 20

Module 10: A Working ComputerHow does the computer know which instructiondoes what?Each instruction is a sequence of 16 to 48 bits†Some of the bits tell it which instruction it is.Other bits tell it what operands to use.These bits are used as select inputs for severalmultiplexers.Modified slightly from the Y86 presented in the textbook by Bryant and O'HallaronCPSC 121 – 2012W T2 21Module 10: A Working ComputerExample:CPSC 121 – 2012W T2 22

Module 10: A Working ComputerExample 1: subl %eax, %ebxRepresented by6103 (hexadecimal)%ebx%eaxsubtractionarithmetic or logic operation (note: the use of “6” to representthem instead of 0 or F or any other value is completely arbitrary)..CPSC 121 – 2012W T2 23Module 10: A Working ComputerExample 2: rmmovl %ecx, $8(%ebx)Represented by401300000008 (hexadecimal)$8%ebx%ecxignoredregister to memory moveCPSC 121 – 2012W T2 24

Module 10: A Working ComputerHow is an instruction executed?This CPU divides the execution into 6 stages:Fetch: read instruction and decide on new PC valueCPSC 121 – 2012W T2 25Module 10: A Working ComputerSix stages of execution (continued)Decode: read values from registersExecute: use the ALU to perform computationsSome of them are obvious from the instruction (e.g. subl)Other instructions use the ALU as well (e.g. rmmovl)Memory: read data from or write data to memoryWrite-back: store value(s) into register(s).PC update: store the new PC value.Not all stages do something for every instruction.CPSC 121 – 2012W T2 26

Module 10: A Working ComputerSample program:irmovl $3,%eaxirmovl $35, %ebxirmovl $facade, %ecxsubl %eax, %ebxrmmovl %ecx, $8(%ebx)haltCPSC 121 – 2012W T2 27Module 10: A Working ComputerExample 1: subl %eax, %ebxFetch: current instruction ← 6103next PC value ← current PC value + 2Decode: valA ← value of %eaxvalB ← value of %ebxExecute: valE ← valB - valAMemory: nothing needs to be done.Write-back: %ebx ← valEPC update: PC ← next PC valueCPSC 121 – 2012W T2 28

Module 10: A Working ComputerExample 2: rmmovl %ecx, $8(%ebx)Fetch: current instruction ← 401300000008next PC value ← current PC value + 6Decode: valA ← value of %ecxvalB ← value of %ebxExecute: valE ← valB + 00000008Memory: M[valE] ← valBWrite-back: nothing needs to be donePC update: PC ← next PC valueCPSC 121 – 2012W T2 29Module 10: A Working ComputerModule Summary:A little bit of historyImplementing a working computer in LogisimAppendicesCPSC 121 – 2012W T2 30

Module 10: A Working ComputerRegisters (32 bits each):0123%eax%ecx%edx%ebx%esp%ebp%esi%ediInstructions that only need one register use 8 or Ffor the second register.%esp is used as stack pointer.Memory contains 2 32 bytes; all memory accessesload/store 32 bit words.CPSC 121 – 2012W T2 314567Module 10: A Working ComputerInstruction types:register/memory transfers:rmmovl rA, D(rB) M[D + R[rB]] ← R[rA]Example: rmmovl %edx, 20(%esi)mrmovl D(rB), rAR[rA] ← M[D + R[rB]]CPSC 121 – 2012W T2 32

Module 10: A Working ComputerInstruction types:Other data transfer instructionsrrmovl rA, rBirmovl V, rBArithmetic instructionsaddl rA, rBsubl rA, rBandl rA, rBxorl rA, rBR[rB] ← R[rA]R[rB] ← VR[rB] ← R[rB] + R[rA]R[rB] ← R[rB] − R[rA]R[rB] ← R[rB] ∧ R[rA]R[rB] ← R[rB] R[rA]CPSC 121 – 2012W T2 33Module 10: A Working ComputerInstruction types:Unconditional jumpsjmp DestConditional jumpsPC ← Destjle Dest PC ← Dest if last result ≤ 0jl Dest PC ← Dest if last result < 0je Dest PC ← Dest if last result = 0jne Dest PC ← Dest if last result ≠ 0jge Dest PC ← Dest if last result ≥ 0jg Dest PC ← Dest if last result > 0CPSC 121 – 2012W T2 34

Module 10: A Working ComputerInstruction types:Conditional movescmovle rA, rB R[rB] ← R[rA] if last result ≤ 0cmovl rA, rB R[rB] ← R[rA] if last result < 0cmove rA, rB R[rB] ← R[rA] if last result = 0cmovne rA, rB R[rB] ← R[rA] if last result ≠ 0cmovge rA, rB R[rB] ← R[rA] if last result ≥ 0cmovg rA, rB R[rB] ← R[rA] if last result > 0CPSC 121 – 2012W T2 35Module 10: A Working ComputerInstruction types:Procedure calls and return supportcall Dest R[%esp]←R[%esp]-4; M[R[%esp]]←PC; PC←Dest;ret PC←M[R[%esp]]; R[%esp]←R[%esp]+4pushl rA R[%esp]←R[%esp]-4; M[R[%esp]]←R[rA]popl rA R[rA]←M[R[%esp]]; R[%esp]←R[%esp]+4OthershaltnopCPSC 121 – 2012W T2 36

Module 10: A Working ComputerInstructions formatnophaltcmovXX rA, rBirmovl V, rBrmmovl rA, D(rB)mrmovl D(rB), rAOPI rA, rBjXX Destcall Destretpushl rACPSC 121 – 2012W T2 37popl rA0 1 2 3 4 50 0 0 01 0 0 02 fn rA rB3 0 F rB V4 0 rA rB D5 0 rA rB D6 fn rA rB7 fn 0 0 Dest8 0 0 0 Dest9 0 0 0A 0 rA FB 0 rAFModule 10: A Working ComputerInstructions format:Arithmetic instructions:addl → fn = 0 subl → fn = 1andl → fn = 2 xorl → fn = 3Conditional jumps and moves:jump → fn = 0 jle → fn = 1jl → fn = 2 je → fn = 3jne → fn = 4 jge → fn = 5je → fn = 6CPSC 121 – 2012W T2 38