What is a Data Hazard?
In computer architecture, a data hazard refers to a type of conflict or interference that can occur between instructions in a program when there is a dependency on the data being processed. It is a situation where the order of execution of instructions can affect the result of the program.
When instructions in a program are executed, they require access to data that is stored in registers or memory. Data hazards occur when an instruction depends on the result of a previous instruction, but that previous instruction has not yet completed or its result is not yet available.
There are typically three types of data hazards:
1. Read After Write (RAW) Hazard:
This occurs when an instruction tries to read data that has been modified by a previous instruction but hasn’t been written back to the register or memory yet. The data dependency creates a hazard because the instruction that reads the data needs to wait for the previous instruction to complete before it can access the correct data.
2. Write After Read (WAR) Hazard:
In this case, an instruction tries to write data to a register or memory location that is still being read by a previous instruction. The hazard arises because the write operation can change the data before the read operation completes, leading to incorrect results.
3. Write After Write (WAW) Hazard:
When multiple instructions attempt to write data to the same register or memory location, a hazard can occur if the instructions don’t execute in the desired order. This can result in the data being overwritten before it is used by other instructions, leading to incorrect outcomes.
Data hazards are a concern in computer architecture because they can introduce delays in the execution of instructions, reducing performance. To mitigate data hazards, processors employ techniques like forwarding, where the result of an instruction is made available to subsequent instructions as soon as possible, reducing the time spent waiting for data dependencies to be resolved.
By identifying and managing data hazards, computer architects and programmers can optimize performance and ensure the correct execution of programs.