Peripheral Devices

Peripherals are the electromechanical and electromagnetic devices connected to a computer that provide input, output or auxiliary storage. They are outside the CPU-memory core.

1. Why Peripherals Need an Interface

   Four fundamental mismatches between the CPU and a peripheral:

   1. SPEED       : CPU works in nanoseconds; a keyboard in
                    hundreds of milliseconds. Ratio can exceed 10^6.

   2. DATA FORMAT : peripherals may be serial, byte-wide, or use a
                    different code (ASCII, Unicode, analogue voltages);
                    the CPU works in fixed-width parallel words.

   3. SIGNAL LEVEL: peripherals may use 12 V (RS-232) or analogue
                    signals; the CPU uses 3.3 V / 5 V logic.

   4. OPERATING MODE: each device has its own control protocol; the
                    CPU should not have to know the details.

   -> Therefore every peripheral is connected through an INTERFACE UNIT.

2. Classification by Function

3. Input Devices in Detail

4. Output Devices in Detail

Monitors

   Resolution and memory:

   A 1024 x 768 display with 24-bit colour needs:
      1024 x 768 x 3 bytes = 2,359,296 bytes ≈ 2.25 MB of video RAM per frame.

   At 60 frames per second:
      2.25 MB x 60 = 135 MB/s of bandwidth.

Printers

   Impact printers    : noisy, slow, cheap consumables, multi-part forms
   Non-impact printers: quiet, fast, better quality, costlier consumables

5. Data Rates — the reason for interface design

   The spread between a keyboard and main memory is about 10^9.
   No single transfer technique can serve both efficiently — which is
   exactly why Unit IV defines THREE modes of transfer:
   programmed I/O, interrupt-driven I/O and DMA.

6. ASCII — the character code peripherals speak

   ASCII = 7-bit code -> 128 characters (an 8th bit is often a parity bit)

   0x00 - 0x1F : control characters (NUL, BEL, BS, HT, LF, CR, ESC, ...)
   0x20        : space
   0x30 - 0x39 : digits '0' to '9'
   0x41 - 0x5A : 'A' to 'Z'
   0x61 - 0x7A : 'a' to 'z'
   0x7F        : DEL

   Useful facts:
      'A' = 65 = 0x41,  'a' = 97 = 0x61   ->  difference is exactly 32 (bit 5)
      '0' = 48 = 0x30   ->  digit value = ASCII code - 48

7. Where Peripherals Attach

Every device hangs off an interface unit that resolves the four mismatches listed in section 1. The structure of that interface unit is the subject of the next lesson.

Summary

   Peripherals = input, output, storage and communication devices
   Four mismatches: speed, data format, signal level, operating mode
   -> every device needs an INTERFACE
   Data rates span 10^9 -> three different transfer modes are needed
   ASCII = 7-bit character code used by most character devices