Microarchitecture of the Pentium Pro Processor

The retire unit is also checking the status of uops in the instruction pool - it is looking for uops that have executed and can be removed from the pool. Once removed, the uops' original architectural target is written as per the original IA instruction. The retirement unit must not only notice which uops are complete, it must also re-impose the original program order on them. It must also do this in the face of interrupts, traps, faults, breakpoints and mis- predictions.
There are two clock cycles devoted to the retirement process. The retirement unit must first read the instruction pool to find the potential candidates for retirement and determine which of these candidates are next in the original program order. Then it writes the results of this cycle's retirements to both the Instruction Pool and the RRF. The retirement unit is capable of retiring 3 uops per clock.
Tour stop #4: BUS INTERFACE unit
Figure 7 shows a more detailed view of the bus interface unit:
There are two types of memory access: loads and stores. Loads only need to specify the memory address to be accessed, the width of the data being retrieved, and the destination register. Loads are encoded into a single uop. Stores need to provide a memory address, a data width, and the data to be written. Stores therefore require two uops, one to generate the address, one to generate the data. These uops are scheduled independently to maximize their concurrency, but must re-combine in the store buffer for the store to complete.
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