Chapter 2: Computer-System Structures The traditional/general computer-system architecture consists of one or more CPUs and a number of device controllers connected via a common bus that provides access to shared memory The CPU(s) and device controllers execute concurrently competing for memory cycles (access synchronized by the memory controller) CEG 433/633 - Operating Systems I

Computer-System Operation I/O devices and the CPU can execute concurrently Each device controller has a local buffer I/O is from the device to local buffer of controller OS provides mechanism for issuing commands to device controllers Device driver - portion of OS which converts OS requests into controller register loads/reads OS provides main memory “buffer” for I/O CPU issues commands by setting device control registers CPU moves data from/to main memory to/from local buffers w/o DMA, CPU must transfer each word (bus size) w/DMA, CPU is invoked once per transaction (block) Device controller informs CPU that it requires attention by causing an interrupt CEG 433/633 - Operating Systems I

Common Functions of Interrupts When powered-up, a computer needs an initial program to run Bootstrap program - initializes the machine Loads the OS The OS kernel starts the first process, after which it only processes events. Hardware Interrupt - generally from I/O devices Trap (Exception) - software-generated interrupt caused either by an error or a user request An operating system is event (or interrupt) driven An interrupt is a request to transfers control of the CPU from the current process to the OS via the interrupt service routine Some devices do not support the interrupt structure and simply set flags in one of their local registers to request service The OS must periodically poll such devices CEG 433/633 - Operating Systems I

CEG 433/633 - Operating Systems I Interrupt Handling An interrupt is handled by: A hardware check against the interrupt mask to see if the interrupt is enabled Incoming interrupts of lower priority are disabled to prevent a lost interrupt Storing the current state of the process The operating system preserves the state of the CPU by storing registers and the program counter Calling the interrupt handling routine which determines the type of interrupt and determines the OS response Re-enable interrupts and restore normal operation Separate segments of code determine what action should be taken for each type of interrupt The OS response is dependent upon the interrupt vector, which contains the addresses of all the service routines CEG 433/633 - Operating Systems I

Interrupt Time Line For a Single Process Doing Output CEG 433/633 - Operating Systems I

CEG 433/633 - Operating Systems I I/O Structure Synchronous I/O Operation After I/O request, OS returns control to user program only upon I/O completion CPU is idle (wait loop) in OS until the next interrupt. The next interrupt will be from the specified device since at most one I/O request is outstanding (No vectoring) Simple, but not efficient No overlapping of computation or additional I/O with I/O CEG 433/633 - Operating Systems I

CEG 433/633 - Operating Systems I I/O Structure Asynchronous I/O Operation After I/O request, control returns without waiting for I/O to complete Blocking I/O library function: includes a system call to to allow the user to wait for I/O completion CPU idles or control is passed to another process Non-Blocking I/O: The user program continues Requires more complex user code to handle the completion of the I/O event Pro: The OS may have multiple outstanding I/O requests One process may make several concurrent requests Multiple processes may make requests on multi-programmed systems Con: The OS must maintain a device-status table to record current and requested transactions for each I/O device CEG 433/633 - Operating Systems I

CEG 433/633 - Operating Systems I Storage Structure Main memory – only large storage media that the CPU can access directly Secondary storage – extension of main memory that provides large nonvolatile storage capacity (requires I/O operation) Storage systems organized in hierarchy Speed, cost, volatility Memory Hierarchy Speed Vs. Cost . . . registers, cache, main memory, disk, CD, tape . . . Caching – copying information into faster storage system; main memory can be viewed as a last cache for secondary storage Cache management is generally handled below the OS level by dedicated hardware requires an policy for replacement, coherency, etc. CEG 433/633 - Operating Systems I

Moving-Head Disk Mechanism Magnetic disks – rigid metal or glass platters covered with magnetic recording material Disk surface is logically divided into tracks, which are subdivided into sectors The disk controller determines the logical interaction between the device and the computer Transfer rate - drive to memory (MB/s) Access time - seek time (arm movement) + rotational delay (platter movement) (ms) CEG 433/633 - Operating Systems I

CEG 433/633 - Operating Systems I Hardware Protection How do we keep erroneous/malicious processes from interfering with other processes on a timesharing system? Dual-Mode Operation Provide hardware support to differentiate between at least two modes of operations 1. User mode – execution done on behalf of a user 2. Monitor mode (also supervisor mode or system mode) – execution done on behalf of operating system When an interrupt or fault occurs hardware switches to monitor mode monitor user Interrupt/fault set user mode CEG 433/633 - Operating Systems I

CEG 433/633 - Operating Systems I Dual-Mode Operation Mode bit added to computer hardware to indicate the current mode: monitor (0) or user (1) The OS is called (via trap) to handle hardware-generated exceptions (ex: seg. fault) in monitor mode Privileged instructions can be issued only in monitor mode All I/O instructions are privileged instructions (I/O Protection) Must ensure that a user program could never gain control of the computer in monitor mode (i.e., a user program that, as part of its execution, stores a new address in the interrupt vector) Load-timer is a privileged instruction (CPU Protection) Timer – interrupts computer after specified period to ensure operating system maintains control When timer reaches the value 0, an interrupt occurs Timer commonly used to implement time sharing CEG 433/633 - Operating Systems I

CEG 433/633 - Operating Systems I Memory Protection Problem: We must protect the OS and other user’s routines from a deranged or malicious process without compromising the speed of memory access Must provide memory protection at least for the interrupt vector and the interrupt service routines In order to have memory protection, add two registers that determine the range of legal addresses a program may access: base register – holds the smallest legal physical memory address. Limit register – contains the size of the range Memory outside the defined range is protected. CEG 433/633 - Operating Systems I

A Base And A limit Register Define A Logical Address Space CEG 433/633 - Operating Systems I

CEG 433/633 - Operating Systems I Protection Hardware When executing in monitor mode, the operating system has unrestricted access to both monitor and user’s memory. The load instructions for the base and limit registers are privileged instructions. CEG 433/633 - Operating Systems I

General-System Architecture Time-sharing led directly to hardware modifications to allow the OS to maintain control of the system Problem: How do users programs access necessary privileged instructions (such as I/O)? The user must ask the OS to perform the operation on the user’s behalf System call – the method used by a process to request action by the operating system Usually takes the form of a trap to a specific location in the interrupt vector Control passes through the interrupt vector to a service routine in the OS, and the mode bit is set to monitor mode The monitor verifies that the parameters are correct and legal, executes the request, and returns control to the instruction following the system call CEG 433/633 - Operating Systems I

Use of A System Call to Perform I/O System calls are complex Information must be passed via registers or ordered in the stack Library functions exist which act as wrappers for the trap These library functions are the system functions provided in each language’s system library Example: Memory Management The OS only provides sbrk() which increases or decreases the address space of the process by some number of bytes C provides the language specific malloc() and free() routines to manage the heap Consider java CEG 433/633 - Operating Systems I

System Calls An application written to a standard set of supported system calls (such as POSIX) is dependent upon the OS, not the machine! User Interface System Programs Programmer Interface Application Code C function library System Interface System Calls Kernel Hardware Abstraction Layer Hardware CEG 433/633 - Operating Systems I