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Terminology

This terminology page contains terms/concepts that are unique to the BOOM core that may/may not match with other out-of-order terminology.

.. glossary::

    Fetch Packet
        A bundle returned by the Front-end which contains
        some set of consecutive instructions with a mask
        denoting which instructions are valid, amongst
        other meta-data related to instruction fetch and
        branch prediction. The Fetch PC will point
        to the first valid instruction in the
        Fetch Packet, as it is the PC used by the
        Front End to fetch the Fetch Packet.

    Fetch PC
        The PC corresponding to the head of a
        Fetch Packet instruction group.

    Fetch Buffer
        Buffer that holds Fetch Packets that are sent to the
        Back-end.

    TAGE Predictor
        A high performance branch predictor. For more information
        read the paper "A case for (partially) tagged geometric history length predictors".

    GShare Predictor
        A simpler branch predictor that uses a global history to index into a set of
        counters.

    Bi-Modal Table (BIM)
        A counter table.

    Micro-Op (UOP)
        Element sent throughout the pipeline holding information about the type of
        Micro-Op, its PC, pointers to the FTQ, ROB, LDQ, STQs, and more.

    Front-end
        The Fetch and Branch Prediction portions of the pipeline that fetch instructions
        from the i-cache.

    Back-end
        The stages starting from Dispatch to Writeback. Here instructions
        are executed, dependencies resolved, branches resolved, etc.

    Fetch Boundary
        The bytes at the end of a i-cache response that might be half of an instruction
        used in RVC.

    Fetch Target Queue (FTQ)
        Queue used to track the branch prediction information for inflight Micro-Ops.
        This is dequeued once all instructions in its Fetch Packet entry are
        committed.

    Next-Line Predictor (NLP)
        Consists of a Branch Target Buffer (BTB),
        Return Address Stack (RAS) and Bi-Modal Table (BIM).
        This is used to make quick predictions to redirect the Front-end

    Backing predictor (BPD)
        Slower but more complicated predictor used to track longer
        histories. In BOOM you can have multiple different types of
        a Backing predictor (TAGE, GShare...).

    Branch Target Buffer (BTB)
        Tagged entry table in which a PC is used to find a matching
        target. Thus, if there is a hit, the specified target is used
        to redirect the pipeline.

    Return Address Stack (RAS)
        Stack used to track function calls. It is pushed with a PC
        on a JAL or JALR and popped during a RET.

    Fetch Width
        The amount of instructions retrieved from the i-cache from the
        Front-end of the processor.

    Global History Register (GHR)
        A register holding the last N taken/not taken results of branches
        in the processor. However, in BOOM, each bit does not correspond to a
        bit of history. Instead this is a hashed history.

    Rename Snapshots
        Saved state used to reset the pipeline to a correct state after a
        misspeculation or other redirecting event.

    Branch Unit
        The functional unit that resolves a branch in the Execute Pipeline.

    Branch Rename Snapshot
        Metadata and prediction snapshots that are used to fix the branch predictor after
        mispredictions.

    Execution Unit
        A module that wraps multiple Functional Units within it.
        It is attached to one issue port only.

    Functional Unit
        A specific hardware module to compute some function (i.e. ALU, FPU, etc).