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).