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CA & OS (Computer Architecture) - EFREI - Fall 2026
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Structural hazard - number of instructions
One of the indicators used to evaluate the efficiency of the program execution is CPI (Clock cycles Per Instruction). It gives the information about average clock cycles required to execute one instruction. For the sequential processor that has a 5-stage execution unit and each stage requires one clock cycle, CPI value is 5. The lower the CPI value is better.
CPI value can be easily calculated taking the number of instructions that program contains and the number of clock cycles required to execute that program.
How many instructions program from previous question contain?
CPI value for pipelined execution
What is CPI value for pipelined execution of the program ?
(This can be calculated manually or obtained from the statistics window of the WinMIPS64 simulator).
Structural hazard
A structural hazard occurs when two or more instructions that are in different phases of the execution request access to the same hardware resource (memory, execution unit, etc.). In MIPS it can be easily observed when you are using the floating point division unit. The program below can be used to illustrate that situation.
v: .double 0.0 y: .double 5.0 .text l.d f3,y(r0) ;f3=y div.d f5,f3,f2 ;f5=x/y div.d f6,f2,f3 ;f6=y/x add.d f8,f5,f6 ;f8=(x/y)+(y/x) add.d f4,f3,f3 mul.d f13,f2,f4 .dataA: .word 3x: .double 10.0z: .double 0.0 main: l.d f2,x(r0) ;f2=x mul.d f7,f13,f2 mul.d f9,f4,f13 s.d f8,z(r0) s.d f9,v(r0)
Analyze that program and manually calculate how many clock cycles are needed to execute it on a sequential processor. Let assume that the execution of any instruction takes 5 clock cycles,the execution of div.d instruction takes additional 23 clock cycles,mul.d takes additional 6 clock cycles, add.d takes additional 3 clock cycles.
How many clock cycles are needed to execute this program on a sequential processor?
Shortly explain why reordering instructions reduces the number of clock cycles required to execute a program?
Limiting the impact of structural hazard
Sometimes it is possible to eliminate structural stalls by changing the order in which instructions are executed. Such modifications should be made with caution, paying attention to the correct results of the program execution. One of the possible changes in the example program is to move instruction add.d f4,f3,f3 and place it after the div.d f5,f3,f2 instruction.
Modify the program as suggested and execute it.
How many clock cycles did it take to execute the modified program?
Analyze the clock cycle diagram and find the instruction that causes a structural stall (STR).
Place this instruction into answer box.
Structural hazard - CPI calculation for sequential program execution
What is CPI value for sequential program execution?
Number of structural stall cycles (STR)
Analyze clock cycle diagram an manually count number of structural stall (STR) cycles.(it must be counted manually because structural stalls are not correctly counted in statistic window of WinMIPS64).
What is the number of structural stall cycles (STR)?
Calculate or write down from statistic window CPI value.
Shortly explain why the instruction given in the answer to the previous question causes structural hazard.