CS61C - Machine Structures Lecture 13 - Input/Output: Polling and InterruptsReview 1/2Review 2/2OutlineAnatomy: 5 components of any ComputerMotivation for Input/OutputI/O Device Examples and SpeedsWhat do we need to make I/O work?Instruction Set Architecture for I/OMemory Mapped I/OProcessor-I/O Speed MismatchProcessor Checks Status before ActingSPIM I/O SimulationSPIM I/OI/O ExampleAdministrivia“What’s This Stuff Good For?”Cost of Polling?% Processor time to poll mouse, floppy% Processor time to hard diskWhat is the alternative to polling?I/O InterruptDefinitions for ClarificationInterrupt Driven Data TransferInstruction Set Support for I/O InterruptSlide 26SPIM I/O Simulation: Interrupt Driven I/OBenefit of Interrupt-Driven I/OQuestions Raised about Interrupts4 Responsibilities leading to OS4 Functions OS must provideThings to RememberCS61C L13 I/O © UC Regents1CS61C - Machine StructuresLecture 13 - Input/Output: Polling and Interrupts October 11, 2000David Pattersonhttp://www-inst.eecs.berkeley.edu/~cs61c/CS61C L13 I/O © UC Regents2Review 1/2°Pointer is high level language version of address°Like goto, with self-imposed discipline can achieve clarity and simplicity•Dereferences in C turn into loads or stores in MIPS code•Powerful, dangerous concept: can cause difficult to fix bugs°C supports pointers, pointer arithmetic°Java structure pointers have many of the same potential problemsCS61C L13 I/O © UC Regents3Review 2/2°MIPS Signed v. Unsigned "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) •Immediate sign extension independent of term (andi, ori zero extend; rest sign extend)°Assembler uses $at to turn MAL into TALCS61C L13 I/O © UC Regents4Outline°I/O Background°Polling°InterruptsCS61C L13 I/O © UC Regents5Anatomy: 5 components of any Computer Processor (active)ComputerControl(“brain”)Datapath(“brawn”)Memory(passive)(where programs, data live whenrunning)DevicesInputOutputKeyboard, MouseDisplay, PrinterDisk (where programs, data live when not running)CS61C L13 I/O © UC Regents6Motivation for Input/Output°I/O is how humans interact with computers°I/O lets computers do amazing things:•Read pressure of synthetic hand and control synthetic arm and hand of fireman•Control propellers, fins, communicate in BOB (Breathable Observable Bubble)•Read bar codes of items in refrigerator°Computer without I/O like a car without wheels; great technology, but won’t get you anywhereCS61C L13 I/O © UC Regents7I/O Device Examples and Speeds°I/O Speed: bytes transferred per second(from mouse to display: million-to-1) °Device Behavior Partner Data Rate (Kbytes/sec)Keyboard Input Human 0.01Mouse Input Human 0.02Line Printer Output Human 1.00Floppy disk Storage Machine 50.00Laser Printer Output Human 100.00Magnetic Disk Storage Machine 10,000.00Network-LAN I or O Machine 10,000.00Graphics Display Output Human 30,000.00CS61C L13 I/O © UC Regents8What do we need to make I/O work?°A way to connect many types of devices to the Proc-Mem°A way to control these devices, respond to them, and transfer data°A way to present them to user programs so they are usefulProcMemPCI BusSCSI Buscmd reg.data reg.Operating SystemWindowsFilesCS61C L13 I/O © UC Regents9Instruction Set Architecture for I/O°Some machines have special input and output instructions°Alternative model (used by MIPS):•Input: ~ reads a sequence of bytes •Output: ~ writes a sequence of bytes°Memory also a sequence of bytes, so use loads for input, stores for output•Called “Memory Mapped Input/Output”•A portion of the address space dedicated to communication paths to Input or Output devices (no memory there)CS61C L13 I/O © UC Regents10Memory Mapped I/O°Certain addresses are not regular memory°Instead, they correspond to registers in I/O devices00xFFFFFFFF0xFFFF0000cmd reg.data reg.addressCS61C L13 I/O © UC Regents11Processor-I/O Speed Mismatch°500 MHz microprocessor can execute 500 million load or store instructions per second, or 2,000,000 KB/s data rate•I/O devices from 0.01 KB/s to 30,000 KB/s°Input: device may not be ready to send data as fast as the processor loads it•Also, might be waiting for human to act°Output: device may not be ready to accept data as fast as processor stores it°What to do?CS61C L13 I/O © UC Regents12Processor Checks Status before Acting°Path to device generally has 2 registers:•1 register says it’s OK to read/write (I/O ready), often called Control Register•1 register that contains data, often called Data Register°Processor reads from Control Register in loop, waiting for device to set Ready bit in Control reg to say its OK (0 1)°Processor then loads from (input) or writes to (output) data register•Load from device/Store into Data Register resets Ready bit (1 0) of Control RegisterCS61C L13 I/O © UC Regents13SPIM I/O Simulation°SPIM simulates 1 I/O device: memory-mapped terminal (keyboard + display)•Read from keyboard (receiver); 2 device regs•Writes to terminal (transmitter); 2 device regsReceived ByteReceiver Data0xffff0004Unused (00...00)(IE)Receiver Control0xffff0000Ready(I.E.)Unused (00...00)TransmittedByteTransmitter Control0xffff0008Transmitter Data0xffff000cReady(I.E.)Unused (00...00)UnusedCS61C L13 I/O © UC Regents14SPIM I/O°Control register rightmost bit (0): Ready•Receiver: Ready==1 means character in Data Register not yet been read; 1 0 when data is read from Data Reg•Transmitter: Ready==1 means transmitter is ready to accept a new character;0 Transmitter still busy writing last char-I.E. bit discussed later°Data register rightmost byte has data•Receiver: last char from keyboard; rest = 0•Transmitter: when write rightmost byte, writes char to displayCS61C L13 I/O © UC Regents15I/O Example°Input: Read from keyboard into $v0lui $t0, 0xffff #ffff0000Waitloop: lw $t1, 0($t0) #controlandi $t1,$t1,0x0001beq $t1,$zero, Waitlooplw $v0, 4($t0) #data°Output: Write to display from $a0lui $t0, 0xffff #ffff0000Waitloop: lw $t1, 8($t0) #controlandi $t1,$t1,0x0001beq $t1,$zero, Waitloopsw $a0, 12($t0) #data°Processor waiting for I/O called “Polling”CS61C L13 I/O © UC Regents16Administrivia°Midterm will be in 2 weeks 5-8 pm•Old midterms will be onlineCS61C L13 I/O © UC Regents17“What’s This Stuff Good For?”Remote Diagnosis:“NeoRest ExII,” a high-tech toilet
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