38th BCS Written Exam: 2018প্রশ্ন ও সমাধান

Building blocks of a digital computer (von Neumann model):

graph LR
  IN[Input Unit] --> CPU
  subgraph CPU[Central Processing Unit]
    CU[Control Unit] --> ALU[Arithmetic Logic Unit]
    ALU --> REG[Registers]
  end
  CPU --> MEM[(Primary Memory RAM/ROM)]
  CPU --> OUT[Output Unit]
  CPU --> SEC[(Secondary Storage)]

• Input Unit: converts external data (keyboard, mouse, scanner) into machine-readable form and feeds it to the CPU.
• Control Unit (CU): fetches, decodes and directs execution of instructions; coordinates all other units but does not process data itself.
• ALU (Arithmetic Logic Unit): performs arithmetic (add, subtract, multiply, divide) and logical (AND, OR, comparison) operations.
• Registers: tiny, extremely fast storage inside the CPU holding operands/results during execution.
• Memory Unit: primary memory — RAM (volatile working memory) and ROM (non-volatile boot firmware) — holds the currently running program/data.
• Secondary Storage: non-volatile bulk storage (HDD/SSD) for permanent data.
• Output Unit: presents processed results to the user (monitor, printer).

Exam takeaway: CPU = Control Unit + ALU + Registers; together with Memory and I/O units they form the classical von Neumann architecture, where data and instructions share the same memory.