Operating Systems

1. Operating Systems Input/Output Management

2. What is the I/O System • A collection of devices that different sub-systems of a computer use to communicate with each other. • Inputs are the signals received by the device, and outputs are the signals sent from it. • Input Device: keyboard, mouse. • Output Device: monitor, printer.

3. The I/O Bus • The CPU communicates with the I/O system by means of an I/O bus. • The I/O bus is simply a common set of wires that connect all the I/O devices to the CPU. These wires are used to transmit data, housekeeping signals (such as clock pulses), addresses and instructions. • The size or width of a bus is how many bits it carries in parallel. • The speed of a bus is how fast it moves data along the path. This is usually measured in MHz.

4. I/O Bus Architectures • The ISA (Industry Standard Architecture) bus was first introduced in1984. It had a 16-bit width and ran at a speed of 8 MHz. • EISA (Extended ISA) was introduced in 1988 as an extension to the ISA standard. It had a 32-bit width but only ran at 8 MHz to be compatible with ISA devices.

5. I/O Bus Architectures • PCI (Peripheral Component Interconnect) was created in 1993. PCI is available in both a 32 bit version running at 33 MHz and a 64 bit version running at 66 MHz. PCI is currently the standard bus for PCs. • AGP (Accelerated Graphics Port) was created in 1997. The first version of AGP, now called AGP 1.0 or AGP 1x, had a 32-bit width and operated at 66 MHz. Newer versions of AGP increase the speed up to 266 MHz. AGP is used only for video controllers.

6. How I/O Devices Communicate • The O/S sends commands or data to an I/O device by writing to its device registers. • The O/S retrieves status or data from an I/O device by reading from its registers. • Remember, registers are like memory storage spaces.

7. How I/O Devices Communicate • The address which the O/S uses to communicate with an I/O device is called the I/O address or the I/O port. • I/O devices use interrupts (IRQs – Interrupt ReQuests) to signal to the CPU that a task has been completed. Interrupts enable I/O devices to operate independently of, and at the same time with the CPU. • On ISA buses, data can be sent directly from the I/O controller to memory without the involvement of the CPU. • This is referred to as DMA (Direct Memory Access).

8. I/O Addresses • If more than one I/O device attempt to use the same I/O address an I/O conflict occurs. This can cause information to get mixed up and overwritten. • I/O addresses vary in size, from 4 to 32 bytes.

9. I/O Address Assignments • Some I/O address assignments in Windows XP Device Manager.

10. Interrupts • Device interrupts are fed to the processor using an interrupt controller. • The interrupt controller has 8 input lines that take requests from one of 8 different devices. The controller then passes the request on to the processor, telling it which device issued the request. • Modern PCs have 2 interrupt controllers. The 2nd controller is cascaded onto the 1st through input line 2 (IRQ2).

11. Interrupts • Interrupts 0, 1, 2, 8 and 13 are reserved for internal use, the remainder are used by I/O devices. • On an ISA bus when more than one I/O device attempt to use the same interrupt at the same time an interrupt conflict occurs. The CPU is unable to determine which device raised the interrupt. • Devices on a PCI bus can share interrupts.

12. Interrupts

13. Typical Interrupt Usage

14. Typical Interrupt Usage • Interrupt assignments in Windows XP Device Manager.

15. DMA • DMA transfers are managed by a DMA controller. A DMA controller has 4 channels, numbered 0 to 3. • Most PCs have 2 DMA controllers, with the 1st controller cascaded to the 2nd on channel 0. This leaves 7 usable DMA channels. • A DMA conflict arises if two I/O devices try to use the same DMA channel at the same time. • DMA channel 0 is reserved for system use.

16. DMA

17. DMA Channel Assignment • DMA channel assignment in Windows XP Device Manager.

18. Objectives of the I/O System • Efficiency • I/O devices must do useful work at the maximum rate. • Device independence • programs can access any I/O device without specifying device in advance. • Uniform naming • name of I/O device is independent of how the device is manufactured. • Error handling • What to do if something goes wrong! • Retransmitting data

19. Structure of the I/O System • Input-output control system (IOCS) • The part of the O/S that deals with I/O activity. • Performs initial processing and validation on the I/O request from the application and routes it to the appropriate device driver at the next stage. • Device Driver • A software module which manages the communication with, and control of, a specific I/O device. • It converts requests from the application to specific commands to the I/O device. • Device drivers are considered to be part of the O/S. • Frequently written in assembly language

20. Application Program Input-output Control System (IOCS) I/O Device Device Driver Device Controller } System Calls Operating System I/O Bus Structure of the I/O System • Device Controller • Hardware device that is attached to the I/O bus and provides an interface between the computer and the I/O device. • Responsible for sending data to the device in a way the device will understand

21. I/O Buffering • A buffer is an intermediate main memory storage area under the control of the O/S. • A buffer holds data in-transit between a process’ memory area and an I/O device. • More than two buffers can be used to let the I/O activity keep up with the CPU processing. • The buffers are organised into a circular queue with data being transferred into the queue at one end and being moved out of the queue at the other.