GATE CS 2020 Set 1 — Question 14
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Question
Consider the following data path diagram.
Consider an instruction: . The following steps are used to execute it over the given data path. Assume that PC is incremented appropriately. The subscripts r and w indicate read and write operations, respectively.
1.
2.
3.
4.
5.
Which one of the following is the correct order of execution of the above steps?Correct answer
(C) 3, 5, 2, 1, 4
Solution
The instruction execution cycle typically consists of Fetch, Decode, and Execute phases.
1.Fetch Phase: The instruction must be fetched from memory.
- Step 3: transfers the Program Counter to the Memory Address Register and initiates a memory read.
- Step 5: transfers the data from the Memory Data Register to the Instruction Register.
- So, the sequence starts with 3, 5.
- First, one operand must be moved to a temporary register (input to ALU).
- Step 2: moves the value of R1 to TEMP1.
- Next, the second operand is read, added to the first, and the result is stored.
- Step 1: reads R2, reads TEMP1 (holding R1), performs addition, and writes the result to TEMP2.
- Finally, the result is written to the destination register.
- Step 4: moves the result from TEMP2 to R0.
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