Introduction to Computer System and Computer Organization
What this domain tests
WBCHSE questions here check that you can name the functional units of a computer system, place them inside the von Neumann model, and explain how a stored program is actually executed. Expect one-mark definitions, short notes on memory and devices, and a small numerical part on capacity or address lines.
Core rules
- The stored-program model keeps instructions and data in the same main memory; the CPU repeats fetch, decode, execute.
- The CPU contains the ALU (arithmetic and logic work) and the control unit (fetching, decoding, control signals), plus registers: PC, MAR, MDR, IR, accumulator.
- Memory hierarchy by speed and cost: registers, cache, main memory, secondary storage. Capacity order is the reverse.
- RAM is volatile and holds the running program; ROM is non-volatile and stores firmware such as the bootstrap loader.
- Secondary storage (HDD, SSD, optical disc, pen drive) is non-volatile and cheap per byte but far slower.
- Clock speed alone does not decide performance; cache size, word length and bus width matter too.
Worked example
A machine has 4 GB of RAM with one byte per address. 4 GB = 4 x 2^30 bytes = 2^32 bytes, so there are 2^32 addressable byte locations; the address bus must therefore carry 32 lines. If the data bus is 16 bits wide, each transfer moves 2 bytes, so moving one 32-bit instruction word takes two bus cycles.
Traps that cost marks
- Confusing control unit and ALU: the CU does not add numbers, it directs operations.
- Writing that RAM keeps data after power-off, or that ROM is freely writable.
- Claiming cache is larger than main memory; cache is small and fast.
- Mixing address lines (number of locations) with data lines (bits per location).
- Using 1000 instead of 1024 for KB and MB when a question does not say otherwise.