Scoreboarding - Stages

Stages

Instructions are decoded in order and go through the following four stages.

1. Issue: The system checks which registers will be read and written by this instruction. This information is remembered as it will be needed in the following stages. In order to avoid output dependencies (WAW - Write after Write) the instruction is stalled until instructions intending to write to the same register are completed. The instruction is also stalled when required functional units are currently busy.
2. Read operands: After an instruction has been issued and correctly allocated to the required hardware module, the instruction waits until all operands become available. This procedure resolves read dependencies (RAW - Read after Write) because registers which are intended to be written by another instruction are not considered available until they are actually written.
3. Execution: When all operands have been fetched, the functional unit starts its execution. After the result is ready, the scoreboard is notified.
4. Write Result: In this stage the result is about to be written to its destination register. However, this operation is delayed until earlier instructions—which intend to read registers this instruction wants to write to—have completed their read operands stage. This way, so called data dependencies (WAR - Write after Read) can be addressed.