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Chapter 7: Peripheral Devices

7.1 Peripheral Device Overview

• The function of peripheral devices is to provide connections between the computer and other machines, as well as between the computer and users. In fact, every part of a computer system except for the CPU and main memory can be treated as a peripheral device.

7.1.1 General Functions of Peripheral Devices

• The function of peripheral devices is to provide connections between the computer and other machines, as well as between the computer and users.
  • In terms of specifications: Continuously adopting new technologies, developing toward lower cost, smaller size, higher speed, larger capacity, and lower power consumption.
  • In terms of structure: Evolved from basic serial I/O modes to multiple parallel peripheral operations connected via channels.
  • In terms of variety: Evolved from simple input/output devices to multiple input/output devices, random-access large-capacity external storage, various terminal devices, etc.
  • In terms of performance: Information exchange speed has greatly improved, and input/output formats include not only digital forms but also intuitive images and audio.

Basic Components of Peripheral Devices (Magnetic Disk) (Key Topic)

• (1) Storage Medium: It has the physical characteristics of preserving information. For example, a magnetic disk is a storage medium that uses magnetized elements recorded on the disk surface to represent information.
• (2) Drive Unit: It is used to move the storage medium. For example, in a magnetic disk device, the drive unit is used to spin the disk and perform positioning.
• (3) Control Circuitry: It sends data to or receives data from the storage medium. For example, when reading from a magnetic disk, the control circuitry converts the information represented as magnetized elements on the disk surface into electrical signals needed by the computer and sends these signals to the computer host via cables.

7.1.2 Classification of Peripheral Devices

• What peripheral devices a computer system is equipped with depends on actual needs.
• Peripheral devices can be classified into input devices, output devices, external storage devices, data communication devices, and process control devices.
• Each peripheral device operates under the control of its own device controller, which connects to the host through an I/O interface and is controlled by the host.

7.2 Magnetic Disk Storage Devices (Key Topic)

• 1. Components of a Magnetic Disk:
  • During writing, data sent in parallel from the computer is loaded into a parallel-to-serial converter register, converted to serial data, then sent bit by bit through the write current driver for power amplification and applied to the write head coil to produce current, thereby forming bit-level magnetized storage elements on the disk platter’s magnetic layer.
  • During reading, when the recording medium moves relative to the head, the spatial magnetic field formed by bit-level magnetized storage elements induces an electromotive force in the read head coil. This readout signal is amplified and detected to recover the originally stored data. Since data is read out serially bit by bit, it is sent to a serial-to-parallel converter register for conversion to parallel data, then sent to the computer in parallel.
• 2. Technical Specifications of Magnetic Disk Storage
  • Storage Density: Storage density includes track density, bit density, and surface density. - Track density: Number of tracks per unit length along the disk radius, measured in tracks/inch. - Bit density: Number of binary code bits that can be recorded per unit length of a track, measured in bits/inch. - Surface density: The product of bit density and track density, measured in bits/square inch. - Storage Capacity: The total number of bytes that a magnetic disk storage device can store. - Access Time: The time from issuing a read/write command until the head moves from a starting position to a new recording position and begins reading from or writing to the disk surface, plus the time for data transfer. - Depends on three factors: - First, the time required to position the head on the desired track, called seek time. - Second, the time after seeking until the required information on the track arrives under the head, called latency. Both of these vary randomly, so average values are typically used. Average seek time is the mean of maximum and minimum seek times. Average latency is related to disk rotation speed and is represented as half the time for one disk revolution. - Third is data transfer time. - Total average access time for a magnetic disk: - Ta = Ts + 1/(2r) + b/(rN) - Ts: average seek time - 1/(2r): average latency - b/(rN): data transfer time - r: disk rotation rate in revolutions/second - b: number of bytes transferred - N: bytes per track
  • Data Transfer Rate: The number of bytes a magnetic disk storage device transfers to the host per unit time
    • The transfer rate is related to the storage device and host interface logic. From the host interface perspective, there should be sufficiently fast speed for receiving/sending information to/from the device. From the storage device perspective, assuming disk rotation speed of n revolutions/second with N bytes per track, the data transfer rate is:
    • Dr = nN (bytes/second) or Dr=D*v (bytes/second), where D is bit density and v is linear velocity of disk rotation

[Example 1] A disk pack has 6 disk platters, each with two recording surfaces, with the topmost and bottommost surfaces unused. The storage area has an inner radius of 22cm, outer radius of 33cm, track density of 40 tracks/cm, inner layer bit density of 400 bits/cm, and rotation speed of 6000 RPM. Calculate: (1) How many cylinders are there? (2) What is the total storage capacity of the disk pack? (3) What is the data transfer rate?

Solution: (1) Effective storage area = 16.5 - 11 = 5.5 (cm) Since track density = 40 tracks/cm, 40 x 55 = 220 tracks, i.e., 220 cylinders. (2) Inner track circumference = 2piR = 2 x 3.14 x 11 = 69.08 (cm) Information per track = 400 bits/cm x 69.08cm = 27632 bits = 3454B Information per surface = 3454B x 220 = 759880B Total disk pack capacity = 759880B x 10 = 7598800B (3) Disk data transfer rate Dr = rN N = bytes per track = 3454B r = disk rotation speed = 6000 rev/60s = 100 rev/s Dr = rN = 100 x 3454B = 345400B/s

7.3 Display Devices

Related Concepts (Key Topic)

• Resolution: The number of pixels a display can show. The denser the pixels, the higher the resolution, and the clearer the image.
  • It depends on the granularity of the CRT phosphor, screen size, and CRT electron beam focusing capability.
• Gray Level: In monochrome displays, it represents the brightness difference of displayed pixels; in color displays, it represents different colors. More gray levels result in clearer and more realistic images.
  • It depends on the number of bits in the refresh memory corresponding to each pixel and the CRT’s own performance.
• Refresh: The glow caused by the electron beam hitting the phosphor lasts only tens of milliseconds. To maintain a stable visible image, the electron beam must repeatedly scan the entire screen, a process called refresh.
  • Higher refresh frequency produces less flickering. Based on human visual physiology, a refresh frequency above 30 times/second is needed to avoid perceived flickering.
  • Refresh Memory (video memory, display memory): The memory that provides signals for refresh. Its capacity depends on resolution and gray levels. - Capacity M = rC, where r is resolution and C is color depth - For example, an image with resolution 1024x1024 and 256-level color depth requires 1024 x 1024 x 8bit = 1MB. Its access cycle must meet refresh frequency requirements.