1. Basic Computer Organisation
The basic computer follows the Von Neumann architecture, which consists of:
- CPU (Control Unit + ALU + Registers)
- Memory Unit (RAM and ROM)
- I/O Unit (Input and Output devices)
- System Bus (connects all components)
2. CPU Architecture
The CPU has three main units:
- Arithmetic Logic Unit (ALU): Performs arithmetic (add, subtract, multiply, divide) and logical (AND, OR, NOT, comparison) operations.
- Control Unit (CU): Fetches instructions from memory, decodes them, and controls the execution sequence.
- Register Set: Small, ultra-fast storage inside the CPU.
3. Registers
| Register | Full Form | Purpose |
|---|---|---|
| PC | Program Counter | Holds address of next instruction |
| IR | Instruction Register | Holds current instruction being executed |
| ACC | Accumulator | Stores intermediate results |
| MAR | Memory Address Register | Holds memory address to access |
| MDR | Memory Data Register | Holds data read from or written to memory |
4. Instruction Cycle
The CPU follows a cycle to execute instructions:
- Fetch: CU fetches instruction from memory (address in PC) and stores in IR. PC increments.
- Decode: CU decodes the instruction (determines what operation to perform).
- Execute: ALU performs the operation (or data is moved). Results may be stored in ACC or memory.
5. Memory Hierarchy
Memory in a computer is organized in a hierarchy based on speed, size, and cost:
- Registers (fastest, smallest, most expensive)
- Cache Memory (L1, L2, L3)
- RAM (Primary Memory)
- SSD/HDD (Secondary Storage) (slowest, largest, cheapest)
6. I/O Organisation
Input/Output devices communicate with the CPU through:
- Programmed I/O: CPU continuously checks device status.
- Interrupt-driven I/O: Device alerts CPU when ready.
- DMA (Direct Memory Access): Device transfers data directly to/from memory without CPU involvement.
7. Revision Questions and Answers
Very Short Answer Questions
1. What is the Von Neumann architecture?
A computer architecture where data and instructions are stored in the same memory.
2. Name the three main units of the CPU.
ALU, Control Unit, and Registers.
3. What does PC register store?
Address of the next instruction to be executed.
4. What is the full form of DMA?
Direct Memory Access.
5. Arrange in increasing order of speed: RAM, Cache, Register.
RAM → Cache → Register.
Short Answer Questions
1. Explain the instruction cycle.
The instruction cycle has three phases: Fetch (load instruction from memory), Decode (interpret the instruction), Execute (perform the operation). This repeats for each instruction.
2. Differentiate between MAR and MDR.
MAR holds the memory address to read/write. MDR holds the actual data being transferred to/from memory.
Long Answer Questions
1. Explain the memory hierarchy of a computer system with a diagram.
From top (fastest, smallest, most expensive) to bottom (slowest, largest, cheapest):
1. CPU Registers
2. Cache Memory (L1 > L2 > L3)
3. Primary Memory (RAM)
4. Secondary Storage (SSD > HDD)
The CPU accesses faster memory first. If not found, it goes to the next level (locality of reference principle).