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Chapter 7: Peripheral Devices#

7.1 Overview of Peripheral Devices#

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

7.1.1 General Functions of Peripheral Devices#

• The function of peripheral devices is to provide communication between computers and other machines, as well as between computers and users.
  • In terms of indicators: continuously adopting new technologies to develop in the aspects of low cost, small size, high speed, large capacity, and low power consumption.
  • In terms of structure: from the initial serial operation input/output mode to various peripheral devices with channel connections and parallel operation modes.
  • In terms of types: from simple input/output devices to various input/output devices, random access large-capacity external storage, and various terminal devices.
  • In terms of performance: the speed of information exchange has been greatly improved, and the input/output forms include not only digital form, but also intuitive forms such as images and sounds.

Basic Components of Peripheral Devices (Disk)#

• (1) Storage medium, which has physical characteristics for storing information. For example, a disk is an example of a storage medium, which uses magnetized elements recorded on the disk to represent information.
• (2) Drive device, which is used to move the storage medium. For example, in a disk device, the drive device is used to rotate the disk and position it.
• (3) Control circuit, which sends data to or receives data from the storage medium. For example, when reading from a disk, the control circuit converts the information represented by magnetized elements on the disk into electrical signals required by the computer, and sends these signals to the computer host via cables.

7.1.2 Classification of Peripheral Devices#

• The selection of peripheral devices for a computer system is determined based on actual needs.
• Peripheral devices can be divided into several categories: input devices, output devices, external storage devices, data communication devices, and process control devices.
• Each type of peripheral device works under the control of its own device controller, and the device controller is connected to the host via an I/O interface and controlled by the host.

7.2 Disk Storage Devices#

• 1. Composition of a Disk:
  • During writing, the data sent in parallel by the computer is taken into the serial-to-parallel conversion register, converted into serial data, and then driven by the write current driver to amplify the power and add it to the write head coil to generate current, thereby forming bit-by-bit magnetized storage elements on the disk surface.
  • During reading, when the recording medium moves relative to the head, the spatial magnetic field formed by the bit-by-bit magnetized storage elements in the read head coil generates an induced electromotive force. This read information can be restored to the original data after amplification and detection. Since the data is read out bit by bit in serial, it needs to be converted to parallel data and then sent to the computer in parallel.
• 2. Technical Specifications of Disk Storage
  • Storage density: Storage density includes track density, bit density, and surface density.
    • Track density: The number of tracks per unit length in the radial direction of the disk, measured in tracks per inch.
    • Bit density: The number of binary code bits that can be recorded per unit length of the track, measured in bits per inch.
    • Surface density: The product of bit density and track density, measured in bits per square inch.
    • Storage capacity: The total number of bytes that a disk storage device can store, known as the storage capacity of the disk storage device.
    • Access time: Access time refers to the time required for the head to move from a certain starting position to a new recording position and start reading or writing information and transmitting data after issuing a read/write command.
    • It depends on the following three factors:
      • The first is the time required to position the head on the required track, called seek time.
      • The second is the time from completing the seek to the arrival of the information to be accessed on the track under the head, called waiting time. Both of these times are random, so the average value is often used to represent them. The average seek time is the average of the maximum and minimum seek times. The average waiting time is related to the disk rotation speed, and it is represented by half of the time required for the disk to rotate one revolution.
      • The third is the data transfer time.
    • Overall average access time of the disk:
      • Ta = Ts + 1 / (2r) + b / (rN)
      • Ts: average seek time
      • 1 /(2r): average waiting time
      • b / (rN): data transfer time
      • r: disk rotation rate, measured in revolutions per second
      • b: number of bytes transferred
      • N: number of bytes per track
  • Data transfer rate: The number of bytes of data that the disk storage device can transfer to the host in one unit of time, called the data transfer rate.
    • The transfer rate depends on the storage device and the interface logic of the host. From the perspective of the host interface logic, it should have a fast enough transfer speed to receive/send information from/to the device. From the perspective of the storage device, assuming the disk rotation speed is n revolutions per second and each track has a capacity of N bytes, the data transfer rate is:
    • Dr = nN (bytes per second) or Dr = D * v (bytes per second), where D is the bit density and v is the linear speed of disk rotation.

【Example 1】A disk group has 6 disks, each with two recording surfaces, and the top and bottom surfaces are not used. The storage area has an inner diameter of 22cm and an outer diameter of 33cm. The track density is 40 tracks/cm, and the inner layer bit density is 400 bits/cm. The rotation speed is 6000 revolutions per minute. Questions:
(1) How many cylinders are there in total?
(2) What is the total storage capacity of the disk group?
(3) What is the data transfer rate?

Solution: (1) Effective storage area = 16.5 - 11 = 5.5 (cm)
Since the track density is 40 tracks/cm, there are 40 × 55 = 220 tracks, which means 220 cylinders.
(2) The circumference of the inner track is 2πR = 2 × 3.14 × 11 = 69.08 (cm)
Information per track = 400 bits/cm × 69.08 cm = 27632 bits = 3454 bytes
Information per surface = 3454 bytes × 220 = 759880 bytes
Total capacity of the disk group = 759880 bytes × 10 = 7598800 bytes
(3) Disk data transfer rate Dr = rN
N is the capacity per track, N = 3454 bytes
r is the disk rotation speed, r = 6000 revolutions per 60 seconds = 100 revolutions per second
Dr = rN = 100 × 3454 bytes = 345400 bytes/s

7.3 Display Devices#

Concepts (Focus)#

• Resolution: The number of pixels that a display can show. The higher the pixel density, the higher the resolution, and the clearer the image.
  • It depends on the granularity of the phosphor in the cathode ray tube, the size of the fluorescent screen, and the focusing ability of the CRT electron beam.
• Gray level: The brightness difference of pixels displayed on a black and white display, or the different colors displayed on a color display. The more gray levels, the clearer and more realistic the image.
  • It depends on the number of bits in the refresh memory corresponding to each pixel and the performance of the CRT itself.
• Refresh: The light emitted by the electron beam hitting the phosphor can only last for tens of milliseconds. In order to display a stable image that can be seen by the human eye, the electron beam must continuously scan the entire screen, which is called refresh.
  • The higher the refresh rate, the less flickering the display. According to human visual physiology, the refresh rate should be greater than 30 times per second to avoid flickering.
  • Refresh memory (video memory, frame buffer): The memory that provides signals for refreshing. Its capacity depends on the resolution and gray level.
    • Capacity M = rC, where r is the resolution and C is the color depth.
    • For example, for an image with a resolution of 1024×1024 and a color depth of 256 levels, the storage capacity is 1024×1024×8 bits = 1 MB, and its access cycle must meet the requirements of the refresh rate.