Chapter 2: Computer-System Structures

1. Chapter 2: Computer-System Structures • Computer System Architecture • Computer System Operation • I/O Structure, Interrupts • Storage Structure • Storage Hierarchy, Caching • Hardware Protection

2. Computer-System Architecture • A general purpose computer-system consists ofoneor more CPUs and a number of device controllers that are connected through a common bus that provides access to a shared memory. • Each device controller is in charge of a specific type of device (for example, disk drives, audio devices or video displays). system bus memory controller

3. Computer-System Operation • Device controllers and the CPU can execute concurrently (in parallel), competing for memory. • Memory controller synchronize access to the memory. • Each device controller is responsibe from a particular device type. • Each device controller has a local buffer. • Input/Output is from the device to local buffer of device controller. • After transfering device controller informs CPU that it has finished its operation(i.e. read request) by causing an interrupt(a signal that shows occurance of an event). • CPU moves data from main memory tolocal buffersor from local buffers to main memory. Note: Operating Systems have a device driver for each device controller. Device driver provides an interface between rest of the OS and the device.

4. Common Functions of Interrupts • Interrupt, transfers control to the appropriate interrupt service routine (responsible for dealing with the interrupt), through the interrupt vector, which contains the starting addresses of all the service routines for the various devices. • Interrupt architecture must save the address of the interrupted instruction (loaded into the program counter). • Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt, so any incoming interrupts are delayed until operating system is done with the current one; then, interrupts are enabled. • A trapor exceptionis a software-generated interrupt caused either by an error in the program or a request from a user program. • An operating system is managed by interrupts(interrupt driven).

5. Common Functions of Interrupts(cont.) Interrupt driven (hardware and software) Hardware interrupt by one of the devices Software interrupt (exception or trap): Software error (e.g., division by zero) Invalid memory access Request for operating system service (using system calls) Other process problems include infinite loop, processes modifying each other or the operating system

6. Interrupt Handling • When an interrupt (or trap) occurs, the hardware transfers control to the operating system. • The operating system preserves the state of the CPU by storing registers and the program counter. • Then OS determines which type of interrupt has occured. • Separate segments of code (in the OS) determine what action should be taken for each type of interrupt. • An interrupt service routine is provided to deal with the interrupt and control is passed to the corresponding interrupt service routine. Note: Operating system waits for some events to occur. The occurrence of an event is usually signaled by an interrupt from either the hardware or the software. When the CPU is interrupted, it stops what it is doing and immediately transfers execution to a fixed location(contains the starting address of interrupt service routine). Then interrupt service routine executes; on completion, the CPU is resuming the interrupted computation.

7. Interrupt Time Line For a Single Process Doing Output Completion of an I/O triggers an interrupt Duration of the time spent in interrupt service routine

8. Storage Definitions and Notation Review The basic unit of computer storage is the bit. A bit can contain one of two values, 0 and 1. All other storage in a computer is based on collections of bits. Given enough bits, it is amazing how many things a computer can represent: numbers, letters, images, movies, sounds, documents, and programs, to name a few. A byte is 8 bits, and on most computers it is the smallest convenient chunk of storage. For example, most computers don’t have an instruction to move a bit but do have one to move a byte. A less common term is word, which is a given computer architecture’s native unit of data. A word is made up of one or more bytes. For example, a computer that has 64-bit registers and 64-bit memory addressing typically has 64-bit (8-byte) words. A computer executes many operations in its native word size rather than a byte at a time. Computer storage, along with most computer throughput, is generally measured and manipulated in bytes and collections of bytes. A kilobyte, or KB, is 1,024 bytes a megabyte, or MB, is 1,0242 bytes a gigabyte, or GB, is 1,0243 bytes a terabyte, or TB, is 1,0244 bytes a petabyte, or PB, is 1,0245 bytes Computer manufacturers often round off these numbers and say that a megabyte is 1 million bytes and a gigabyte is 1 billion bytes. Networking measurements are an exception to this general rule; they are given in bits (because networks move data a bit at a time).

9. Metric Units Operating System Concepts

10. Storage Structure Main memory (primary storage, RAM)– only large storage media that the CPU can access directly Randomaccess Typically volatile Magnetic disks(secondary storage)– extension of main memory that provides large nonvolatilestorage capacity Hard 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 Solid-state disks – faster than hard disks, nonvolatile Various technologies Becoming more popular

11. Storage Structure(cont.) Notes: The programs must be in the main memory to be executed. CPU automatically loads instruction from main memory for execution. Main memory is an array of words or bytes ranging in size. Each word has it own size. Secondary storage provides storage for programs and data Most programs(web browsers, compilers, word processors) are stored on the disk until they are loaded into memory. Instruction-execution cycle: fetch the instruction from memory, decode instruction, execute instruction, store instruction back in memory.

12. Storage Hierarchy Storage systems organized in hierarchy according to Speed Cost Volatility Caching – copying information into faster storage system; main memory can be viewed as a cache for secondary storage

13. Storage-Device Hierarchy Expensive, fast Cheap, slow Increasing access time

14. Caching • Important principle, performed at many levels in a computer (in hardware, operating system, software) • Use of high-speed memory to hold recently-accessed data. • Information in use copied from slower to faster storage temporarily • Faster storage (cache) checked first to determine if information is there • If it is, information used directly from the cache (fast) • If not, data copied to cache and used there • Requires a cache management policy, since caches have limited memory size. • Caching introduces another level in storage hierarchy. This requires data that is simultaneously stored in more than one level to be consistent. • Ex: Value of a variable can be different in register and in the various storage systems.

15. Direct Memory Access (DMA) Structure • Used for high-speed I/O devices, able to transmit information at close to memory speeds. • Device controller transfers blocks of data to or from its local buffer directly to main memory without CPU intervention. • Only one interrupt is generated per blockby DMA controller, rather than an interrupt per byte.

16. How a Modern Computer Works A von Neumann architecture

17. Memory Protection • We must protect the OS from access by user programs and protect user programs from one another. • We must provide memory protection at least for the interrupt vector and the interrupt service routines of the operating system. • 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.

18. Use of A Base and Limit Registers Kernel mode Supervisor mode System mode Privilaged mode Monitor mode User mode

19. Hardware Address Protection

20. Operating-System Operations Dual-mode operation allows OS to protect itself and other system components User mode and kernel mode Mode bit provided by hardware Provides ability to distinguish when system is running user code or kernel code Some instructions designated as privileged, only executable in kernel mode(I/O instructions, instruction to modify the memory-management registers or the timers, halt instructions). System call changes mode to kernel, return from call resets it to user mode