Networks & I/O Devices

Interconnecting wires I/O Overview Central Processor Main Memory Interconnecting wires Input Controller Comms Controller Output Controller May 28, 2019

Device Controllers Hardware View - A device controller is a piece of electronic hardware that is responsible for handling the electrical signals and data formats used by a particular external I/O device and converts them into signals and data formats that are compatible with the computer’s system bus May 28, 2019

Device Controllers Software view – a device controllers function is to allow software in the computer to control the attached I/O device and transfer data to or from it. Hardware view - each device controller has a number of registers in it that can be read or written to under the control of a program. May 28, 2019

Device Controllers Some of the registers are control registers and are used by the program to select how the device will work i.e. screen resolution Other data registers are used to transfer data between the device and the program i.e. keys hit on keyboard displayed onto the screen May 28, 2019

Communication The system communicates to the user through a variety of hardware devices The GUI is the interface between the system and the user If an output device is not working, error messages are shown often with codes referring to specific types of errors May 28, 2019

Data Storage Two types of data storage Primary: includes RAM, processor registers and processor cache

Data Storage Secondary: hard drive, sequential access storage devices and media, direct access storage devices and media

Data Storage

Data Storage Types of Memory Access RAM – items are independently addressed, access time is constant DIRECT ACCESS – items are independently addressed in regions, access times are variable though not significantly SEQUENTIAL ACCESS – items are organized in sequence (linearly), access time is significantly variable

Data Storage Sequential media include magnetic tapes Direct media include magnetic hard disks optical disks (DVD, CD) USB – flash memory

Data Storage In modern computers, hard disk drives are usually used as secondary storage. The time taken to access a given byte of information stored on a hard disk is typically a few thousandths of a second, or milliseconds. By contrast, the time taken to access a given byte of information stored in random access memory is measured in billionths of a second, or nanoseconds.

Data Storage This illustrates the significant access-time difference which distinguishes solid-state memory from rotating magnetic storage devices: hard disks are typically about a million times slower than memory

Data Storage Rotating optical storage devices, such as CD and DVD drives, have even longer access times

Data Storage With disk drives, once the disk read/write head reaches the proper placement and the data of interest rotates under it, subsequent data on the track is very fast to access. As a result, in order to hide the initial seek time and rotational latency, data is transferred to and from disks in large contiguous blocks.

Reading a Hard Disk

Data Storage Hard disks are rigid platters, composed of a substrate and a magnetic medium. The substrate - the platter's base material - must be non-magnetic and capable of being machined to a smooth finish. It is made either of an aluminium alloy or a mixture of glass and ceramic.

Data Storage To allow data storage, both sides of each platter are coated with a magnetic medium - formerly magnetic oxide, but now, almost exclusively, a layer of metal called a thin-film medium. This stores data in magnetic patterns, with each platter capable of storing a billion or so bits per square inch (bpsi) of platter surface.

Data Storage

Data Storage A Sector is 512 bytes in size - or big enough to hold about 256 characters. Windows chunks these out into Clusters, each of which holds about 64 Sectors. Every time you create a file, Windows sets aside - allocates - at least one Cluster, and then writes your data to it.

Data Storage When a file is saved, there are several attributes saved with it. One is the date the file was created; one is the date the file was last changed, or modified; one is the date the file was last accessed. This information is kept as part of a file listing directory. This directory is viewed by the user as the contents of a folder.