12 Nov - COA -Pipeline Hazards & Instruction Format-1
Duration: 2 hr 3 min
This video lesson is available to enrolled students.
AI Summary
An AI-generated summary of this video lecture.
This educational video is a comprehensive lecture on computer architecture, focusing on instruction pipelining, data hazards, and instruction formats. The session begins with an introduction to pipeline hazards, specifically defining Read After Write (RAW), Write After Read (WAR), and Write After Write (WAW) dependencies, and explaining how they can cause incorrect program execution. The instructor then transitions to a series of solved problems from past GATE exams, demonstrating how to calculate the total execution time of a program on a pipelined processor, taking into account the number of clock cycles for each instruction and the impact of data hazards. The lecture covers various types of instruction formats, including 3-address, 2-address, 1-address, and 0-address instructions, explaining their structure, advantages, and disadvantages. The video concludes with a discussion on the maximum number of bits required for an immediate operand in an instruction, based on the number of registers and instruction types in a processor.
Chapters
0:00 – 2:00 00:00-02:00
The video opens with a title slide for 'Session-9 Pipeline Hazards & Instruction Format' by Ekagra Sir. The instructor, Sancheet Jahn, is visible in a small window. The session begins with an introduction to the topic of pipeline hazards, setting the stage for the lecture.
2:00 – 5:00 02:00-05:00
The instructor presents a GATE 2014 problem about a five-stage pipeline. The problem describes a program with 20% branch instructions and asks for the ratio P/Q of execution times on old and new designs. The instructor begins to solve the problem by calculating the total execution time for the old design, using the formula ET = (1 + BP) * CT, where BP is the branch penalty and CT is the clock time.
5:00 – 10:00 05:00-10:00
The instructor continues solving the GATE 2014 problem. He calculates the execution time for the old design (P) as 3.08 ns. He then moves to the new design, which has eight stages, and calculates its execution time (Q) as 2.88 ns. The final answer is the ratio P/Q, which he calculates to be 1.07.
10:00 – 15:00 10:00-15:00
The instructor presents a GATE 2006 problem. The problem describes a five-stage pipeline running at 1 GHz, with a program of 10^9 instructions, 20% of which are conditional branches. The processor stops fetching new instructions after a branch until the branch is resolved. The instructor calculates the total execution time as 1.2 seconds, considering the 5 cycles of stall for each branch instruction.
15:00 – 20:00 15:00-20:00
The instructor presents a GATE 2013 problem involving a five-stage pipeline with no branch prediction. The problem asks for the total time to execute a program of 12 instructions, where instruction I4 is a branch instruction with a target of I9. The instructor uses a pipeline diagram to show the execution, noting that the branch causes a flush of 3 instructions, resulting in a total time of 165 ns.
20:00 – 25:00 20:00-25:00
The instructor discusses the concept of data hazards, specifically the Read After Write (RAW) hazard. He provides an example with two instructions, I and J, where J tries to read a register before I has written to it. He explains that this is a true data dependency and can lead to incorrect results if not handled.
25:00 – 30:00 25:00-30:00
The instructor explains the Write After Read (WAR) hazard. He gives an example where instruction J tries to write to a register before instruction I has read from it. He notes that this is an anti-data dependency and can cause incorrect results if the write happens before the read.
30:00 – 35:00 30:00-35:00
The instructor introduces the Write After Write (WAW) hazard, where two instructions write to the same register, and the second write occurs before the first. He explains that this is a true data dependency and can lead to incorrect results if the order is not preserved.
35:00 – 40:00 35:00-40:00
The instructor presents a GATE 2018 problem about a RISC processor with four stages. The problem asks for the number of clock cycles to execute 100 instructions, with different instructions taking different numbers of cycles. The instructor calculates the total cycles as 219, using the formula (k+n-1) where k is the number of stages and n is the number of instructions.
40:00 – 45:00 40:00-45:00
The instructor discusses solutions to data dependencies, such as code movement and operator forwarding. He explains that operator forwarding allows the result of an operation to be used directly by the next instruction, bypassing the need to wait for it to be written to memory.
45:00 – 50:00 45:00-50:00
The instructor presents a GATE 2010 problem about a 5-stage pipelined processor. The problem asks for the number of clock cycles to execute a sequence of 5 instructions, with different instructions taking different numbers of cycles. The instructor uses a pipeline diagram to show the execution, accounting for the 3-cycle MUL instruction, and calculates the total time as 17 cycles.
50:00 – 55:00 50:00-55:00
The instructor presents a GATE 2007 problem about a 4-stage pipelined processor. The problem asks for the number of clock cycles to execute a sequence of 3 instructions, with different instructions taking different numbers of cycles in the EX stage. The instructor uses a pipeline diagram to show the execution, accounting for the 3-cycle MUL instruction, and calculates the total time as 10 cycles.
55:00 – 60:00 55:00-60:00
The instructor presents a GATE 2015 problem about a 4-stage pipelined processor. The problem asks for the number of clock cycles to execute a sequence of 4 instructions, with different instructions taking different numbers of cycles in the PO stage. The instructor uses a pipeline diagram to show the execution, accounting for the 5-cycle DIV instruction, and calculates the total time as 13 cycles.
60:00 – 65:00 60:00-65:00
The instructor presents a GATE 2021 problem about a 5-stage pipelined processor. The problem asks for the speedup of executing a sequence of 8 instructions with data dependencies, comparing execution with and without operand forwarding. The instructor uses a pipeline diagram to show the execution, accounting for the stalls caused by data dependencies, and calculates the speedup as 2.0.
65:00 – 70:00 65:00-70:00
The instructor begins a new topic on instruction formats. He explains that instructions can be classified by the number of operands they reference. He introduces the concept of 3-address instructions, which involve three operands: two source and one destination.
70:00 – 75:00 70:00-75:00
The instructor continues discussing 3-address instructions, explaining their structure and advantages. He notes that they can handle complex operations in a single step but require more bits to encode, increasing hardware complexity. He provides examples like ADD R1, A, B and MUL X, R1, R2.
75:00 – 80:00 75:00-80:00
The instructor introduces 2-address instructions. He explains that these instructions use two addresses, where one address serves as both a source and the destination. He discusses the advantages, such as smaller instruction size and efficiency, and the disadvantage of optimization difficulty due to the dual use of an address.
80:00 – 85:00 80:00-85:00
The instructor discusses 1-address instructions. He explains that these instructions use an implied accumulator register for data manipulation. One operand is stored in the accumulator, and the other is retrieved from a register or memory. He notes the advantages of simplicity and efficiency.
85:00 – 90:00 85:00-90:00
The instructor introduces 0-address instructions. He explains that these instructions use no explicit operands and are common in stack-based architectures. The result of an operation is implied to be at the top of the stack. He provides examples like PUSH, ADD, and MUL.
90:00 – 95:00 90:00-95:00
The instructor presents a GATE 2025 problem. The problem asks for the maximum number of bits that can be used to store the immediate operand in an instruction, given a processor with 64 general-purpose registers and 50 distinct instruction types. The instructor begins to solve the problem by analyzing the instruction format.
95:00 – 100:00 95:00-100:00
The instructor continues solving the GATE 2025 problem. He calculates the number of bits required for the opcode (6 bits for 50 instructions) and the register field (6 bits for 64 registers). He then subtracts these from the total instruction length to find the number of bits available for the immediate operand, which he calculates as 24 bits.
100:00 – 105:00 100:00-105:00
The instructor presents a GATE 2021 problem about a 5-stage pipelined processor. The problem asks for the speedup of executing a sequence of 8 instructions with data dependencies, comparing execution with and without operand forwarding. The instructor uses a pipeline diagram to show the execution, accounting for the stalls caused by data dependencies, and calculates the speedup as 2.0.
105:00 – 110:00 105:00-110:00
The instructor presents a GATE 2010 problem about a 5-stage pipelined processor. The problem asks for the number of clock cycles to execute a sequence of 5 instructions, with different instructions taking different numbers of cycles. The instructor uses a pipeline diagram to show the execution, accounting for the 3-cycle MUL instruction, and calculates the total time as 17 cycles.
110:00 – 115:00 110:00-115:00
The instructor presents a GATE 2007 problem about a 4-stage pipelined processor. The problem asks for the number of clock cycles to execute a sequence of 3 instructions, with different instructions taking different numbers of cycles in the EX stage. The instructor uses a pipeline diagram to show the execution, accounting for the 3-cycle MUL instruction, and calculates the total time as 10 cycles.
115:00 – 120:00 115:00-120:00
The instructor presents a GATE 2015 problem about a 4-stage pipelined processor. The problem asks for the number of clock cycles to execute a sequence of 4 instructions, with different instructions taking different numbers of cycles in the PO stage. The instructor uses a pipeline diagram to show the execution, accounting for the 5-cycle DIV instruction, and calculates the total time as 13 cycles.
120:00 – 123:25 120:00-123:25
The instructor concludes the lecture by summarizing the key concepts discussed, including pipeline hazards and instruction formats. He emphasizes the importance of understanding these concepts for solving GATE exam problems. The video ends with a brief shot of the instructor.
This video provides a comprehensive review of key concepts in computer architecture, focusing on instruction pipelining. The lecture systematically progresses from the fundamental problem of pipeline hazards, defining and illustrating the three main types (RAW, WAR, WAW), to practical applications through a series of solved GATE exam problems. These problems demonstrate how to calculate execution time, clock cycles, and speedup, taking into account data dependencies and pipeline stalls. The second half of the video shifts to instruction formats, classifying them by the number of operands (3-address, 2-address, 1-address, 0-address) and explaining their structure, advantages, and disadvantages. The synthesis of these topics provides a solid foundation for understanding how processor design impacts program execution and performance.