FPU Write-Back Stage

The WB-stage captures each Single Precision or Double Precision result as they exit the execute stage in dedicated registers. FPU instruction execution time is variable, but only one instruction is permitted to be in the WB-stage at any one time. If more than one instruction has completed execution and is in a position to retire, the pipeline will determine which instruction to retire to maintain instruction execution order and eliminate any WAW hazards. The instruction will then complete the write-back in one cycle during the WB-stage before being retired. The Instruction/Hazard Tracker logic will ensure instructions enter the WB-stage in the same order as they were issued.

Prior to writing the result, if FTZ mode is enabled (see Flush-To-Zero (FTZ)), the result is modified accordingly if subnormal. This final value is also passed onto the RAW hazard mitigation forwarding logic.