Practice Question (Gate 2010)
Duration: 6 min
This video lesson is available to enrolled students.
AI Summary
An AI-generated summary of this video lecture.
This educational video analyzes a GATE 2010 computer architecture problem involving a 5-stage pipelined processor. The core task is to calculate the total clock cycles required for a sequence of four instructions: MUL, DIV, ADD, and SUB. The processor architecture features five stages: Instruction Fetch (IF), Instruction Decode (ID), Operand Fetch (OF), Perform Operation (PO), and Write Back (WO). The IF, ID, OF, and WO stages each consume one clock cycle. However, the PO stage duration varies based on instruction type: 1 cycle for ADD and SUB, 3 cycles for MUL, and 6 cycles for DIV. The problem explicitly states that operand forwarding is utilized to mitigate data hazards, allowing dependent instructions to access results as soon as they are produced in the PO stage rather than waiting for the WO stage. The video demonstrates how to construct a pipeline timing diagram to visualize instruction execution and identify stalls caused by data dependencies, specifically read-after-write hazards where an instruction requires a result from a preceding instruction that has not yet completed its PO stage.
Chapters
0:00 – 2:00 00:00-02:00
The video opens by presenting the GATE 2010 problem statement on screen, displaying a 5-stage pipelined processor configuration. The text explicitly lists the instruction latencies: IF, ID, OF, and WO take 1 cycle each. The PO stage duration is variable: 1 cycle for ADD/SUB, 3 cycles for MUL, and 6 cycles for DIV. The instructor introduces the instruction sequence I0 through I3: MUL R2, R0, R1; DIV R5, R3, R4; ADD R2, R5, R2; SUB R5, R2, R6. The problem asks for the total clock cycles needed to execute this sequence given that operand forwarding is used. An empty grid appears on screen, intended for mapping the pipeline execution cycle by cycle.
2:00 – 5:00 02:00-05:00
The instructor begins constructing the pipeline timing diagram to solve the problem. He maps the first instruction, MUL R2, R0, R1 (I0), into the pipeline stages. I0 enters IF at cycle 1 and proceeds through ID, OF, and PO. Since MUL requires 3 cycles in the PO stage, it occupies cycles 4, 5, and 6. The instructor highlights that the result of I0 becomes available at the end of cycle 6 due to operand forwarding. Next, he places DIV R5, R3, R4 (I1) into the pipeline. I1 starts at cycle 2 and proceeds through IF, ID, OF. It encounters a dependency check but since it does not depend on I0's result immediately in the OF stage, it proceeds to PO. The DIV instruction requires 6 cycles in PO, occupying cycles 7 through 12. The instructor marks the timeline to show that I1's result is available at the end of cycle 12.
5:00 – 5:55 05:00-05:55
The final segment focuses on resolving data hazards for the remaining instructions, ADD and SUB. The instructor analyzes I2: ADD R2, R5, R2. This instruction depends on the result of I0 (R2) and I1 (R5). Since operand forwarding is used, the instructor checks if results are available. R2 from I0 is ready after cycle 6, and R5 from I1 is ready after cycle 12. The ADD instruction enters the OF stage and must wait for R5, causing a stall or delay until cycle 13. The instructor calculates the total cycles by extending the timeline to ensure all instructions complete their WO stage. The final calculation suggests a total of 17 clock cycles, matching option (C) in the multiple-choice question displayed on screen. The video concludes with the completed pipeline diagram showing stalls and varying PO durations.
The video effectively demonstrates the application of pipeline timing diagrams to solve complex computer architecture problems involving variable instruction latencies and data hazards. The key takeaway is the critical role of operand forwarding in reducing stalls, as it allows dependent instructions to proceed without waiting for the Write Back stage. The variable duration of the PO stage is a crucial factor that complicates the timing, requiring careful tracking of when each instruction's result becomes available. The problem-solving approach involves mapping each instruction to the pipeline stages, identifying dependencies between registers (R2 and R5), and calculating stalls based on when the required operands are produced. The final answer of 17 cycles is derived by ensuring all four instructions complete their WO stage, accounting for the extended PO durations of MUL and DIV. This method is essential for students preparing for exams like GATE, where understanding pipeline hazards and forwarding mechanisms is fundamental.