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Official PYQ · 2013Q28ALU, Data-path and Control UnitMCQ1 mark

GATE CS 2013 Q28 — ALU, Data-path and Control Unit

Explain with AI

Consider the following sequence of micro-operations.

MBR ← PC  
MAR ← X  PC ← Y  
Memory ← MBR

Which one of the following is a possible operation performed by this sequence?

Choose one option
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Answer: D

To identify the operation performed by the given sequence of micro-operations, let us analyze each step:

  1. MBR ← PC:
    The current value of the Program Counter (PC), which holds the return address, is saved into the Memory Buffer Register (MBR).
  2. MAR ← X and PC ← Y:
    • The Memory Address Register (MAR) is loaded with an address X (the location where the return address will be saved, such as a stack pointer or a designated memory location).
    • The Program Counter (PC) is updated with address Y, which is the starting address of the target routine (e.g., the Interrupt Service Routine).
  3. Memory ← MBR:
    The saved return address (currently in the MBR) is written to the memory location specified by the MAR (address X).

This sequence of saving the current program state (PC) to memory and branching to a new service routine address (Y) is the standard procedure for the Initiation of an interrupt service (or a subroutine call).

Let's briefly evaluate the other options:

  • Instruction fetch: Typically loads the instruction from the address in the PC into the Instruction Register (IR) and increments the PC. It does not save the PC to memory.
  • Operand fetch: Fetches data from memory into a register; it does not involve saving the PC.
  • Conditional branch: Updates the PC based on a condition, but does not save the return address to memory.

Therefore, the correct option is D.

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