Understanding how a computer works inside is the foundation of computer science.
This guide explains all concepts from Chapter 1 in a simple way so that every student can understand—from digital circuits to high-level programming languages.
1. Digital Computer: Program and Instructions
A digital computer is an electronic device that processes data using binary digits (0 and 1). It performs operations according to a program, which is a sequence of instructions.
- Program: A set of instructions written to perform a specific task.
- Instruction: A single command that tells the computer what operation to perform.
Machine Language
- The lowest-level language understood directly by the computer.
- Written in binary (0s and 1s).
- Machine dependent and difficult for humans to read and write.
2. Structured Computer Organization
Structured computer organization explains how a computer system is organized into levels, where each level hides the complexity of the lower level.
Key Idea
- Each level provides an abstract view of the computer.
- Higher levels are easier for humans to understand.
- Lower levels deal with hardware details.
3. Languages, Levels, and Virtual Machines
Language
A language is a formal system of communication used to give instructions to a computer.
- Examples: Machine language, Assembly language, High-level languages (C, Java).
Levels
A computer system is organized into multiple levels, each with its own language.
Translation
- Converts a program from one language to another.
- Example: Compiler translates high-level language to machine language.
Interpretation
- Executes instructions line by line without converting the entire program first.
- Example: Interpreter for Python or Java bytecode.
Virtual Machine
- A software-based abstraction of a computer.
- Makes one level appear like a real machine to the level above.
- Example: Java Virtual Machine (JVM).
4. Multiple Levels & Multilevel Viewpoint of a Machine
A multilevel machine consists of several layers, each built on top of the lower one.
Advantages
- Simplicity
- Portability
- Ease of programming
- Hardware independence
5. The Computer Level Hierarchy (Contemporary)
Level 0: Digital Logic Level
- Lowest level of the computer.
- Consists of digital circuits, gates, and wires.
- Uses two-value logic: 0 (OFF) and 1 (ON).
- Physical behavior (voltage/current) is abstracted into logic values.
Level 1: Control Level / Microarchitecture Level
- Executes machine instructions.
- Moves data through the system.
- Implemented using control units.
Hardwired Control
- Control logic built using hardware (gates, flip-flops, decoders).
- Fast operation.
- Difficult to modify.
Microprogrammed Control
- Control signals stored in control memory as microprograms.
- Easier to modify.
- Slightly slower than hardwired control.
Microprogram: A low-level program that controls the execution of machine instructions.
Level 2: Machine Level (Instruction Set Architecture – ISA)
- Also called the ISA level.
- Defines the instructions visible to the programmer.
Instruction Set Architecture (ISA)
- Interface between hardware and software.
- Includes:
- Instruction set (opcodes)
- Registers
- Data types
- Addressing modes
- Memory architecture
- I/O mechanisms
Example:
- ADD instruction stored as binary code.
- Control unit decodes it and performs required microoperations.
Level 3: System Software Level
- Controlled by the Operating System (OS).
- Manages hardware resources.
Functions
- Process management
- Memory protection
- I/O management
Examples
- Operating systems
- Compilers
- Interpreters
- Linkers
- Libraries
Level 4: Assembly Language Level
- Uses mnemonic instructions.
- Easier than machine language.
- Assembler translates assembly code into machine code.
- Lowest human-readable programming level.
Level 5: High-Level Language Level
- Languages like C, C++, Java, Python.
- Easy to read, write, and maintain.
- Machine independent.
- Hides hardware details from users.
Level 6: User Level
- Highest level.
- Interaction through application software.
- Examples: Word processor, Paint, Browser.
- Implementation details are completely hidden.
6. Actual Multilevel Computer
- A real computer system uses all these levels together.
- Each level is implemented on top of the lower one.
7. Evolution of Multilevel Machines
Early Computers
- Only two levels:
- Digital logic level
- ISA level
- Hardware was complex.
Maurice Wilkes (1951)
- Introduced the idea of microprogramming.
- Proposed a three-level computer.
- Simplified hardware design.
Growth of Multilevel Machines
- 1950s–1960s: Few multilevel machines.
- 1970s: Microprogrammed machines became dominant.
Evolution of Operating Systems
- Early computers used punched cards.
- Batch processing systems introduced.
- Later, time-sharing systems allowed multiple users.
- OS automated job control and resource management.
8. Milestones in Computer Architecture
Von Neumann Machine
- Stored-program concept.
- Data and instructions stored in the same memory.
First Bus-Based System
- Common communication pathway.
- Reduced wiring complexity.
IBM 360 Product Line
- Family of compatible computers.
- Same instruction set across models.
9. Moore’s Law
- Proposed by Gordon Moore (1965).
- Number of transistors on a chip doubles every 18 months.
- Led to exponential growth in computing power.
10. The Computer Zoo / Computer Spectrum
Types of Computers
Embedded Computers
- Found in everyday devices.
- Example: TVs, washing machines, toys.
Personal Computers (PCs)
- General-purpose computers.
Game Computers
- Designed mainly for gaming.
Servers
- High-performance machines.
- Handle network services and databases.
NOW / COW
- Network or Cluster of Workstations.
- Multiple computers working together.
Mainframes
- High I/O capacity.
- Used by large organizations.
Supercomputers
- Extremely powerful.
- Highly parallel.
- Used for scientific simulations.
11. Typical Computer Technologies
Intel CPU Family (Pentium II)
- Intel founded in 1968.
- First single-chip CPU: 4004 (4-bit).
- IBM PC used 8088.
- Backward-compatible CPUs followed.
UltraSPARC II
- Developed by Sun Microsystems.
- Based on RISC architecture.
- 32-bit SPARC, later 64-bit UltraSPARC.
- Used in high-end servers.
picoJava II
- Hardware implementation of Java Virtual Machine.
- Improves Java performance.
- Useful in embedded systems.
- Example: Sun microJava CPUs.
Exam Tips for Unit 1
- Draw computer level hierarchy diagram.
- Learn definitions and differences.
- Use tables for generations and computer types.
- Write answers point-wise and simple.