Digital and Hybrid Computers Overview

Digital Computers

• Digital computers use digital technology to represent information numerically.
• They represent data as discrete electrical pulses, either 'on' or 'off'.
• This results in accurate output.
• Most modern computers are based on digital technology.

Hybrid Computers

• Hybrid computers combine the benefits of analog and digital computers.
• They offer the speed of analog computers and the accuracy of digital computers.
• They accept analog data but process it digitally.
• They require analog-to-digital and digital-to-analog converters for input and output.

Computer Classification by Data Processing

• Computers are categorized based on data processing capabilities into four types.
• Microcomputers, Minicomputers, Mainframe computers, and Supercomputers.

Microcomputers

• These computers use a microprocessor chip as the central processing unit.
• They emerged in the 1970s.
• The first microcomputers were built with 8-bit processors.
• Known as personal computers, designed for individual use (PCs, workstations or notebooks).
• They are relatively small, affordable and widely used.
• Examples include IBM PC, IBM PC/XT, IBM PC/AT.

Minicomputers

• They emerged in the 1960s.
• They are smaller mainframes that perform limited tasks.
• Less expensive than mainframes.
• Less powerful in terms of processing capabilities compared to mainframes.
• Capable of supporting 10 to 100 users simultaneously.
• Early models used 8 or 12-bit processors.
• Later progressed to 16 and 32-bit processors.
• More powerful versions were developed known as supermini computers.
• Examples: IBM AS400.

Mainframe Computers

• They are very powerful computers designed to support thousands of users simultaneously.
• They include powerful data processing systems.
• They can run multiple operating systems.
• They can process up to 100 million instructions per second.
• They are large, expensive, with large storage capacities and high processing speeds.

Optical Character Recognition (OCR)

• Used for converting images of characters to ASCII code.
• It first creates a bitmap image of the document.
• OCR software then translates the image into ASCII code, allowing computers to interpret letters, symbols or numbers.
• The document must be typed using OCR fonts.
• Common standards are OCR-A (American standard) and OCR-B (European standard).

OCR Advantages

• Speedy data entry.
• Accepts a wide range of fonts.

OCR Disadvantages

• Expensive.
• Scanning works correctly only if characters are standard size.
• Dusty paper cannot be scanned properly.

Magnetic Ink Character Recognition (MICR)

• MICR systems use special magnetizable ink to print characters that can be read by magnetic devices.
• Common E13B font is used, containing numbers 0-9 and 4 symbols.
• Character detection occurs in two steps.
• First, the MICR Reader-Sorter reads the data on cheques and sorts them for further processing.
• Magnetized characters are read by the head.

MICR Advantages

• Speedy data entry.
• Accurate output.
• Folded or roughly handled cheques can be scanned accurately.

MICR Disadvantages

• MICR software is required.
• Limited fonts used, primarily in banking industries.

Barcode Reader Devices

• Data is encoded using small lines known as barcodes.
• Barcodes represent alphanumeric data by combinations of vertical lines varying in width and spacing.
• Bar Code Readers are devices that recognize barcode data.
• They scan barcode images, convert them to alphanumeric values, and feed them to computers.
• They often use laser-beam technology.
• The laser scans across the barcode pattern, sensed by a light-sensitive decoder.
• The reflected light patterns are transformed into electrical pulses and then into alphanumeric values.
• Various barcodes exist for different uses.
• The most common is UPC (Universal Product Code), consisting of 10 digits, the first 5 identifying the manufacturer and the remaining 5 identifying a specific product.

Number Complement

• Example of subtraction using a binary system.
• Step 1: Find the complement of the number being subtracted.
• Step 2: Add the complement to the original number.
• Step 3: If a carry occurs, add 1 to the result.
• If no carry occurs, complement the result (in binary) and add a negative sign.

Multiplication in Binary

• Multiplication in binary follows the same rules as decimal multiplication.
• Remember the following table:
  • 0 x 0 = 0
  • 0 x 1 = 0
  • 1 x 0 = 0
  • 1 x 1 = 1

Binary Division

• The following table is used in binary division:
  • 0 / 1 = 0
  • 1 / 1 = 1
• Steps:
  • Begin from the left of the dividend.
  • Perform subtraction where the divisor is subtracted from the dividend.
  • If subtraction is possible, place a 1 in the quotient and subtract the divisor from the matching digits in the dividend; otherwise, place a 0 in the quotient.
  • Bring down the next digit to the right of the remainder.
  • Repeat step 2 until there are no more digits in the dividend.

Units of Information (Codes)

• Most computers use coded versions of binary to represent characters and symbols.
• This standardization enables data transfer between systems.
• Codes include BCD, ASCII, EBCDIC, and Unicode.

BCD (Binary Coded Decimal)

• One of the earliest computer codes.
• Each decimal digit is converted to its binary equivalent (4 bits).
• Convenient for conversion but limited in representing characters (only 16).
• Extended to a 6-bit code for representing more characters.

ASCII (American Standard Code for Information Interchange)

• Each character (letters, digits, symbols) is assigned a unique bit pattern.
• The standard ASCII character set uses 7 bits, representing 128 characters.
• It utilizes one extra parity bit for error detection.
• Extended versions use an extra bit, allowing for 256 characters.
• Popularized by IBM.
• Commonly used for data exchange between systems.

EBCDIC (Extended Binary Coded Decimal Interchange Code)

• Uses 8 bits, representing 256 characters.
• Also utilizes one extra parity bit for error detection.
• Used in IBM mainframes and similar machines.
• Circuits exist to convert characters between ASCII and EBCDIC.

Unicode

• Developed to address the need for representing characters from multiple languages (Chinese, Korean, Japanese).
• ASCII and EBCDIC were insufficient.
• Unicode uses 16 bits, allowing for 65536 characters, plus an extra parity bit.
• It tackles the problem of multilingual data representation.
• Issues: waste of storage space, transmission time, lack of current OS support.
• No consistent sort order.

Parity Check

• Techniques to verify the correctness of transmitted characters.
• A parity bit is added for each character.
• Even Parity System: The total number of '1' bits (including the parity bit) must be even.
• Odd Parity System: The total number of '1' bits must be odd.
• Not foolproof: it can fail when an even number of bits are incorrect.

Languages, Operating System, and Software Packages

• A computer only does what a programmer instructs it to do.
• Programmers write sequences of instructions called programs.
• Each instruction tells the computer to perform a specific task on data.
• Computers understand machine language, while humans use natural languages.
• Language acts as a bridge between humans and computers.