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Announcements

Announcements. Assignment #2 is due today! Solution will be posted later this week. Tutorials and labs. Lecture slides and videos. No class next week! Midterm exam?. Lecture 7. Chapter 7. Input/Output ( Cont. ). Direct Memory Access.

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  1. Announcements • Assignment #2 is due today! • Solution will be posted later this week. • Tutorials and labs. • Lecture slides and videos. • No class next week! • Midterm exam?

  2. Lecture 7 Chapter 7. Input/Output(Cont.)

  3. Direct Memory Access • Interrupt driven and programmed I/O require active CPU intervention. • CPU tests and services a device  transfer rate is limited. • # of instructions are executed for every I/O  CPU is tied up. • DMA is the answer! • Additional module (hardware) on bus. • DMA controller takes over from CPU for I/O.

  4. DMA Operation • CPU tells DMA controller: • Read/Write. • Device address. • Starting address of memory block for data. • Amount of data to be transferred. • CPU carries on with other work. • DMA controller deals with transfer. • DMA controller sends interrupt when finished.

  5. DMA Transfer - Cycle Stealing • DMA controller transfers data from/to MM over the system bus. • DMA controller takes over bus for a bus cycle to transfer one word of data. • Can use the bus only when CPU is not using it, or • Forces CPU to suspend operation temporarily  DMA steals a bus cycle from the CPU cycle stealing. • Not an interrupt • CPU does not switch context. • CPU suspended just before it accesses bus • i.e. before an operand or data fetch or a data write. • Slows down CPU but not as much as CPU doing transfer.

  6. DMA and Interrupt Breakpoints During an Instruction Cycle

  7. DMA Configurations (1) • Single Bus, Detached DMA controller • Each transfer uses bus twice • I/O to DMA then DMA to memory • CPU is suspended twice

  8. DMA Configurations (2) • Single Bus, Integrated DMA controller. • Controller may support >1 device. • Each transfer uses bus once. • DMA to memory. • CPU is suspended once.

  9. DMA Configurations (3) • Separate I/O Bus • Bus supports all DMA enabled devices • Each transfer uses bus once • DMA to memory • CPU is suspended once

  10. I/O Channels • I/O devices getting more sophisticated. • e.g. 3D graphics cards. • I/O module becomes a processor. • CPU instructs I/O controller to do transfer (execute a program). • CPU directs the I/O processor to fetch and execute an I/O program in memory. • I/O controller does entire transfer • Improves speed • Takes load off CPU. • Dedicated processor is faster.

  11. I/O Channel Architecture – Selector channel • Channel controls multiple high-speed devices. • At any given time, it is dedicated to data transfer with only one of these devices. • Each device/set of devices is/are handled by a controller (I/O module).

  12. I/O Channel Architecture – Multiplexor channel • Can handle multiple low-speed devices at the same time. • A byte multiplexor accepts or transmits characters as fast as possible to multiple devices. • Example: 3 devices with different rates and individual streams A1A2A3A4 ...,B1B2B3B4 . . ., and C1C2C3C4 might result in the character stream A1B1C1A2C2A3B2C3A4 …

  13. I/O Computer • I/O module is a computer: processor and memory. • Large set of I/O devices can be controlled with minimal CPU involvement. • e.g., communication with interactive terminals.

  14. Chapter 8. Operating System Support

  15. What’s an OS? Why? • Operating system (OS) is a program that manages the computer’s resources, provides services for programmers, and schedule the execution of programs. • Scheduling and memory management are most relevant to computer organization. • OS: • Controls the execution of programs. • Acts as an interface between the computer user and the computer hardware. • Objectives: • Convenience: Making the computer easier to use. • Efficiency: Allowing better use of computer resources.

  16. OS as a User/Computer Interface • H/W and S/W can be viewed as layers. • End user sees an application. • Application is expressed in a programming language and developed by a programmer. • Utilities • System programs • Called by an application. • Frequently used functions like I/O, file management, program creation. • OS • Masks the details of H/W from the programmer. • Provides a convenient interface.

  17. OS Services • Program creation • Utility programs help programmer to create a program. • e.g., editors and debuggers. • Program execution • Loads data and instructions into MM. • Initializes I/O devices and files. • Prepares other resources. • Access to I/O devices • Takes care of details of I/O devices. • Controlled access to files • Understands I/O device and file format. • Protection is multi-user. • System access • Protects system and resources from unauthorized users. • Error detection and response • Memory error, device failure, overflow, access forbidden memory. • Accounting: Usage statistics, performance, and billing.

  18. O/S as a Resource Manager • OS is a computer program that manages computer’s resources. • It directs the CPU in the use of other system resources and in the timing of execution of other programs. • OS relinquishes control for CPU to do some useful work, and resumes control to prepare CPU to do the next piece. • Kernel: core of OS loaded in MM. • Controls access to I/O devices & files. • CPU management.

  19. Types of Operating Systems • Batch vs. interactive • Batch OS (rare) • User’s program is batched with other users’ and submitted by a computer operator. • After completion, results are printed out for the user. • Interactive OS • User interacts with the computer thru a keyboard/display. • User may communicate with the computer during the execution of a job. • Uniprogramming vs. multiprogramming • Uniprogramming: one program at a time. • Multiprogramming • CPU works on > 1 program at a time  keep it busy. • Programs loaded into MM, CPU switches among them.

  20. Early Systems • Late 1940s to mid 1950s. • No operating system  programs interact directly with hardware. • Console = display lights + toggle switches (input device) + printer. • Programs loaded via an input device. • Program halted by an error • Error is indicated by lights. • Programmer examine register and MM to find error cause. • Program completes  output on printer. • Two main problems: • Scheduling: sign-up sheet: reserve CPU time in multiples of 30-min slots. May finish early or error occurs. • Setup time: mounting/dismounting tapes, card decks. If an error occurs, start setup sequence over again.

  21. Simple Batch OS • CPUs expensive  maximize its utilization. • User has no direct access to CPU. • Users submit jobs to operator. • Operator batches jobs sequentially and places the batch on an input device. • Simple batch OS = Monitor. • Resident Monitor program: part of Monitor always in memory. • Monitor controls sequence of events to process batch. • When one job finishes, control returns to Monitor which reads next job from I/P device and loads it into memory. • Monitor handles scheduling.

  22. Job Control Language (JCL) • Special type of programming language that contains instructions to Monitor. • Usually denoted by $. • Example: user submitting a Fortran program. $JOB $FTN ... .. Some Fortran instructions $LOAD $RUN ... .. Some data $END • $FTN  Monitor loads compiler  translates program and saves on tape. • $LOAD  Monitor loads program into memory and transfers control to it.

  23. Desirable Hardware Features • Memory protection • User program must not alter the memory containing Monitor. • If happens, CPU H/W detects it and transfers control to the Monitor. Monitor stops job, print error message, and loads next job. • Timer • To prevent a job monopolizing the system. • If expires  interrupt and control goes to Monitor. • Privileged instructions • Only executed by Monitor. • If CPU encounters one by user program  interrupt. • e.g. I/O  prevents program from reading JCL from next job. Program requests Monitor to do I/O. • Interrupts • Allows Monitor to relinquish and regain control.

  24. Multiprogrammed Batch OS • I/O devices very slow compared to CPU. • Uniprogramming • One program at a time. • CPU executes  encounters I/O instruction  waits till it finishes. • When one program is waiting for I/O, another can use the CPU.

  25. Multiprogramming with Two Programs • Assume memory can hold OS (resident Monitor) and two programs. • When one job needs to wait for I/O, the processor can switch to the other job.

  26. Multiprogramming with Three Programs • Additional feature for multiprogrammed batch system: H/W that supports interrupts and DMA. • CPU can issue I/O command for a job and proceed with the execution of another job while the I/O is carried out by the device controller. • When the I/O operation is complete, CPU is interrupted and control is passed to ISR in OS. The OS will then pass control to another job.

  27. Time-Sharing OS • Allow users to interact directly with the computer. • i.e. Interactive • Like multiprogramming OS, handles multiple interactive jobs at a time. • CPU’s time is shared among multiple users. • Multiple users access the system simultaneously through terminals. • n users  each one will see 1/n of computer speed. • Response time still acceptable from a human.

  28. Reading Material • Stallings, chapter 7, pages 236-239, 242-244 • Stallings, chapter 8, pages 259-271

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