So far Binary numbers Logic gates Digital circuits process data using gates – Half and full adder Data storage – Electronic memory – Magnetic memory – Optical memory – Solid state memory

This week Software Operating systems Shells

4 Learning outcomes Describe the functions of the Operating System and its components. Explain where the operating system is usually stored and how it gets started. Explain how the operating system manages memory – Partitioning, swapping, paging, demand paging Explain how the operating system manages I/O devices – Programmed I/O, Interrupt-driven I/O, DMA

5 Additional Reading Essential Reading – Stalling (2003): Chapter 7 and 8

6 What is an operating system? A program that runs on the hardware and supports – Resource Abstraction – Resource Sharing Abstracts and standardises the interface to the user across different type of hardware – Virtual machine hides the messy details witch must be performed. Manages the hardware resources – Each program gets time with the resource – Each program gets space on the resource

7 Main functions of an operating system 1.User/computer interface: – Provides an interface between the user and the computer 2.Resource manager: – manages all computers resources. Process manager Memory manager Device manager File manager, etc.

8 Operation System User command interface Resource management Process Manager Memory manager Device Manager File manager Network manager A model of an operation System

9 Operating system as a resource manager Resource management Process Manager: Next program to be executed? Time to be given to each program ? Memory manager: Best use of the memory to run as many programs as possible I/O Device (e.g.printer) Manager: Which program should use a particular I/O device? Network manager: which computer should execute a particular program?

10 How the operating system get started? Operating System Main memory Bootstrap program Main memory Bootstrap Program Operating System Disk storage ROMROM ROMROM RAMRAM RAMRAM

11 Operating system as a process manager A program is divided into a sequence of instructions Each instructions is divided into a sequence of processes Operating System – Manages the execution of all processes – First come first served – Shortest job first – Round robin

12 Process State As a process executes, it changes state – new: The process is being created. – running: Instructions are being executed. – waiting: The process is waiting for some event to occur. – ready: The process is waiting to be assigned to a process. – terminated: The process has finished execution.

13 Memory partition How processes are arranged in the main memory before been executed? Fixed-sized partitions Variable-sized partitions Swapping Paging

14 Swapping I/O operations are slow If a running process requires an I/O operation. The CPU will move to another process in the main memory. Suppose the main memory is full of processes waiting on I/O. CPU becomes idle To solve this problem Swapping technique is used.

15 os P1 os P1 p2 os P1 p2 p3 os P1 P3 os P1 P4 P3 os P4 p3 os p2p2 P4 p3 a d c b e h g f

16 I/O devices Called peripherals: – Keyboard – Mouse – Speakers – Monitor – scanner – Printer – Disk drive – CD-drive. OS – manages all I/O operations and devices

17 I/O modules System bus CPU Main memory I/O module I/O device I/O device

18 Advantages of I/O modules They are 3 ways in which I/O modules can work – Programmed I/O – Interrupt-driven I/O – Direct memory access.

19 Programmed I/O The CPU controls I/O device directly Via the I/O modules. The CPU sends an I/O command to the I/O module. And waits until the I/O operation is completed before sending another I/O command. The performance is poor as the CPU spends too much time waiting the I/O device.

20 Interrupt-driven I/O The CPU issues a command to the I/O module and then gets on with executing other instructions. The I/O module interrupts the CPU when it is ready to exchange data with the CPU. The CPU then executes the data transfer. Most computer have interrupt lines to detect and record the arrival of an interrupt request.

21 Disadvantages of Interrupt-driven I/O CPU is responsible for managing I/O data transfer. Every transferred word must go through the CPU. Devices with large transfer, e.g. disk drive, the CPU wastes time dealing with data transfer. Solution: Direct-memory-access(DMA).

22 Direct-memory-access - DMA Special-purpose processor. Handles data transfer. CPU issues to the DMA: starting address in main memory to read/write to. Starting address in the I/O device to read/write to. The number of words to be transferred. DMA transfers data without intervention from the CPU. DMA sends interrupt to the CPU when transfer is completed.

23 DMA/CPU - bus system DMA take care data transfer. CPU free to do other jobs. However, they can not use the bus at the same time. DMA can use the bus only when the CPU is not using it. Some times it has to force to CPU to free the bus, cycles stealing.

24 DMA/CPU System bus CPU Main memory I/O module I/O device DMA

25 Summery OS- memory manager Fixed-sized partition: waist of memory. Variable-sized partition: fragmentation. Swapping. Time wasted in swapping the whole process Simple paging: process divided into pages and loaded into main memory(divided into frames). OS- I/O manager Programmed I/O: CPU waste waiting for I/O operation. Interrupt-driven I/O: CPU responsible for data transfer. DMA: takes care of data transfer instead the CPU.