ICS 143 - Principles of Operating SystemsComputer System StructuresComputer System ArchitectureComputer System OrganizationInterruptsInterrupt HandlingI/O StructureDirect Memory Access (DMA)Storage StructureStorage HierarchyStorage Device HierarchyHardware ProtectionDual-mode operationDual-mode operation(cont.)I/O ProtectionMemory ProtectionHardware Address ProtectionProtection Hardware (cont.)CPU ProtectionGeneral System ArchitectureOperating System StructuresProcess Management (Chapters 4-7)Memory Management (Chapters 8-9)Secondary Storage and I/O Management (Chapter 10)File System Management (Chapters 11-12)Protection and Security (Chapter 14)Networking (Distributed Systems)Command Interpreter SystemOperating System ServicesSystem CallsSystem ProgramsSystem Design and ImplementationSystem GenerationOS Structure - Simple ApproachUNIX System StructureLayered OS StructureVirtual MachinesSummary of OS StructuresICS 143 - Principles of Operating SystemsLecture 2 - Operating System StructuresProf. Nalini [email protected] System Structures Computer System Operation I/O Structure Storage Structure Storage Hierarchy Hardware Protection General System ArchitectureComputer System ArchitectureComputer System Organization I/O devices and the CPU execute concurrently. Each device controller is in charge of a particular device type Each device controller has a local buffer. I/O is from the device to local buffer of controller CPU moves data from/to main memory to/from the local buffers Device controller interrupts CPU on completion of I/OInterrupts Interrupt transfers control to the interrupt service routine Interrupt Service Routine: Segments of code that determine action to be taken for each type of interrupt. Interrupt vector contains the address of service routines. OS preserves the state of the CPU stores registers and the program counter (address of interrupted instruction). Trap software generated interrupt caused either by an error or a user request.Interrupt Handling Types of interrupt Polling Vectored interrupt system Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt.I/O Structure Synchronous I/O wait instruction idles CPU until next interrupt no simultaneous I/O processing, at most one outstanding I/O request at a time. Asynchronous I/O After I/O is initiated, control returns to user program without waiting for I/O completion. System call Device Status table - holds type, address and state for each device OS indexes into I/O device table to determine device status and modify table entry to include interrupt.Direct Memory Access (DMA) Used for high speed I/O devices able to transmit information at close to memory speeds. Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention. Only one interrupt is generated per block, rather than one per byte (or word).MemoryMemoryCPUCPUI/O devicesI/O devicesI/O instructionsStorage Structure Main memory - only large storage media that the CPU can access directly. Secondary storage - extension of main memory that has large nonvolatile storage capacity. Magnetic disks - rigid metal or glass platters covered with magnetic recording material. Disk surface is logically divided into tracks, subdivided into sectors. Disk controller determines logical interaction between device and computer.Storage Hierarchy Storage systems are organized in a hierarchy based on Speed Cost Volatility Caching - process of copying information into faster storage system; main memory can be viewed as fast cache for secondary storage.Storage Device HierarchyHardware Protection Dual Mode Operation I/O Protection Memory Protection CPU ProtectionDual-mode operation Sharing system resources requires operating system to ensure that an incorrect program cannot cause other programs to execute incorrectly. Provide hardware support to differentiate between at least two modes of operation:1. User mode -- execution done on behalf of a user.2. Monitor mode (supervisor/kernel/system mode) --execution done on behalf of operating system.Dual-mode operation(cont.) Mode bit added to computer hardware to indicate the current mode: monitor(0) or user(1). When an interrupt or fault occurs, hardware switches to monitor mode. Privileged instructions only in monitor mode.UserMonitorSet usermodeInterrupt/faultI/O Protection All I/O instructions are privileged instructions. Must ensure that a user program could never gain control of the computer in monitor mode, for e.g. a user program that as part of its execution, stores a new address in the interrupt vector.Memory Protection Must provide memory protection at least for the interrupt vector and the interrupt service routines. To provide memory protection, add two registers that determine the range of legal addresses a program may address. Base Register - holds smallest legal physical memory address. Limit register - contains the size of the range. Memory outside the defined range is protected.0300040120900Base registermonitorJob1Job 2Job 4Job 302560003000040420940Limit register8800001024000Hardware Address ProtectionProtection Hardware (cont.) When executing in monitor mode, the OS has unrestricted access to both monitor and users’ memory. The load instructions for the base and limit registers are privileged instructions.CPU Protection Timer - interrupts computer after specified period to ensure that OS maintains control. Timer is decremented every clock tick. When timer reaches a value of 0, an interrupt occurs. Timer is commonly used to implement time sharing. Timer is also used to compute the current time. Load timer is a privileged instruction.General System Architecture Given the I/O instructions are privileged, how do users perform I/O? Via system calls - the method used by a process to request action by the operating system.Operating System Structures Operating System Components Process Management, Memory Management, Secondary Storage Management, I/O System Management, File Management, Protection System, Networking, Command-Interpreter. Operating System Services, System calls, System Programs Virtual Machine Structure and Organization A Structural Approach to Operating Systems