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Module 2: x86 Architecture, CPU Control, I/O, Memory System Design

20162m

Explain the use of subroutine with the help of suitable example.

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Subroutine Uses (2 Marks)

A subroutine is a self-contained sequence of instructions called from different points in program to avoid code duplication.

Uses:

  1. Modularity & Reuse: Common task (e.g. I/O, math) written once and called many times — saves memory, improves readability.
  2. Return Address Management: On CALL addr, CPU saves return address (next PC) automatically — either in link register or on top of stack: SP ← SP-1, M[SP] ← PC, PC ← addr.
  3. RET instruction: On RET, address popped back: PC ← M[SP], SP ← SP+1 and execution resumes after call.
  4. Nesting & Recursion: Stack LIFO allows nested calls; parameters passed via registers/stack, and CALL/RET use stack pointer.

Thus subroutine supports structured programming with hardware stack mechanism.

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