Lecture 3:Lecture 3:I/OI/O and Processesand ProcessesCSE 120: Principles of Operating SystemsAlex C. SnoerenLab 0 & HW 1 Due Tuesday 10/03CSE 120 – Lecture 3: Processes 2Input/Output (I/O)Input/Output (I/O) Input and Output devices operate independently of main CPU◆ Speeds vary; often much slower than CPU instructions◆ Device uses interrupts to signal OS when ready/done» Have you ever heard of an IRQ (Interrupt Request Level)? I/O devices are managed by the OS◆ Enforces sharing, protection◆ Requires privileged instructions to initiate I/O◆ Kernel maintains table to map device interrupts to handlers, a.k.a devicedrivers Communication handled through memory abstraction◆ Device buffers and registers mapped into address space◆ Data copied into address range send data to I/O deviceCSE 120 – Lecture 3: Processes 3I/O ExampleI/O Example1) Ethernet receives packet, writes packet into buffer2) Network Interface Card (NIC) signals an interrupt3) CPU stops current instruction, saves current context4) CPU reads handler address from kernel vector table,indexed by IRQ, jumps to appropriate address(Ethernet driver)5) Ethernet device driver reads packet from buffer,processes packet6) Kernel restores saved context, reissues interruptedinstructionCSE 120 – Lecture 3: Processes 4Interrupt IssuesInterrupt IssuesOperating SystemHardwareApplications Interrupts suspend current process and transfercontrol to the OS◆ Can the OS itself be interrupted?◆ Sometimes interrupts need to be masked until the OS is readyto handle them Interrupts are default model for device interaction◆ What is the alternative?◆ What are the drawbacks of the alternative approach?◆ When might an OS want to avoid interrupts?CSE 120 – Lecture 3: Processes 5AtomicityAtomicity Interrupts are pesky◆ They can occur at any time, including inconvenient ones◆ In particular, a handler may interfere with currently executingcode paths» Can a handler interrupt itself? OS needs some way to ensure certain instructions orsequence of instructions are not interrupted◆ One option is to disable interrupts during critical section◆ Another alternative is to have special atomic instructions» Guaranteed not to be interrupted» E.g., test and conditionally set a bit based on previous value What if the machine has more than one CPU?CSE 120 – Lecture 3: Processes 6Coordinating ExecutionCoordinating Execution This lecture starts a class segment that coversprocesses, threads, and synchronization◆ These topics are perhaps the most important in this class.◆ You can rest assured that they will be covered in the exams. Today’s topics are processes and processmanagement◆ What are the units of execution?◆ How are those units of execution represented in the OS?◆ How is work scheduled in the CPU?◆ What are the possible execution states of a process?◆ How does a process move from one state to another?CSE 120 – Lecture 3: Processes 7The ProcessThe Process The process is the OS abstraction for execution◆ It is the unit of execution◆ It is the unit of scheduling◆ It is the dynamic execution context of a program A process is sometimes called a job or a task or asequential process. A sequential process is a program in execution◆ It defines the sequential, instruction-at-a-time execution of aprogram◆ Programs are static entities with the potential for executionCSE 120 – Lecture 3: Processes 8Process ComponentsProcess Components A process contains all of the state for a program inexecution◆ An address space◆ The code for the executing program◆ The data for the executing program◆ An execution stack encapsulating the state of procedure calls◆ The program counter (PC) indicating the next instruction◆ A set of general-purpose registers with current values◆ A set of operating system resources» Open files, network connections, etc. A process is named using its process ID (PID)CSE 120 – Lecture 3: Processes 9Process Address SpaceProcess Address SpaceStack0x000000000xFFFFFFFFCode(Text Segment)Static Data(Data Segment)Heap(Dynamic Memory Alloc)AddressSpaceSPPCCSE 120 – Lecture 3: Processes 10Process StateProcess State A process has an execution state that indicates what itis currently doing◆ Running: Executing instructions on the CPU» It is the process that has control of the CPU» How many processes can be in the running state simultaneously?◆ Ready: Waiting to be assigned to the CPU» Ready to execute, but another process is executing on the CPU◆ Waiting: Waiting for an event, e.g., I/O completion» It cannot make progress until event is signaled (disk completes) As a process executes, it moves from state to state◆ Unix “ps”: STAT column indicates execution state◆ What state do you think a process is in most of the time?◆ How many processes can a system support?CSE 120 – Lecture 3: Processes 11Process State GraphProcess State GraphNew ReadyRunningWaitingTerminatedCreateProcessProcessExitI/O, PageFault, etc.I/O DoneScheduleProcessUnscheduleProcessCSE 120 – Lecture 3: Processes 12Process Data StructuresProcess Data StructuresHow does the OS represent a process in the kernel? At any time, there are many processes in the system,each in its particular state The OS data structure representing each process iscalled the Process Control Block (PCB) The PCB contains all of the info about a process The PCB also is where the OS keeps all of a process’hardware execution state (PC, SP, regs, etc.) whenthe process is not running◆ This state is everything that is needed to restore the hardwareto the same configuration it was in when the process wasswitched out of the hardwareCSE 120 – Lecture 3: Processes 13PCB Data StructurePCB Data Structure The PCB contains a huge amount of information inone large structure» Process ID (PID)» Execution state» Hardware state: PC, SP, regs» Memory management» Scheduling» Accounting» Pointers for state queues» Etc. It is a heavyweight abstractionCSE 120 – Lecture 3: Processes 14PCBs and Hardware StatePCBs and Hardware State When a process is running, its hardware state (PC,SP, regs, etc.) is in the CPU◆ The hardware registers contain the current values When the OS stops running a process, it saves thecurrent values of the registers into the process’ PCB When the OS is ready to start executing a newprocess, it loads the hardware registers from thevalues
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