Virtual Labs

Correlating Branch Prediction with Two-Level Predictors

What does GHR stand for in the context of a correlating branch predictor?

a: General Hardware Resource Explanation

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b: Global History Register Explanation

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c: Guaranteed Hit Rate Explanation

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d: Global Hashing Routine Explanation

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In a two-level correlating predictor, what is the GHR used for?

a: To store the final prediction (Taken/Not Taken). Explanation

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b: To store the branch's target address. Explanation

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c: To provide an index into the Pattern History Table (PHT). Explanation

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d: To directly replace the program counter. Explanation

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What is stored in the Pattern History Table (PHT)?

a: A list of past program counter values. Explanation

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b: A set of 2-bit saturating counters. Explanation

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c: The assembly code for the program. Explanation

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d: The global history of all branches. Explanation

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What key problem do correlating predictors solve that simple predictors do not?

a: They can predict the outcome of a branch that has never been seen before. Explanation

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b: They are significantly cheaper to build. Explanation

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c: They use the behavior of *other* branches to predict the outcome of the current branch. Explanation

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d: They eliminate all pipeline stalls permanently. Explanation

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A (2,2) correlating predictor has a GHR with the value '10'. What index in the PHT will it use?

a: Index 0 (binary 00) Explanation

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b: Index 1 (binary 01) Explanation

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c: Index 2 (binary 10) Explanation

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d: Index 3 (binary 11) Explanation

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After a prediction is made and the actual outcome is known, which two components are updated?

a: The Program Counter and the Register File Explanation

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b: The GHR and the selected PHT entry Explanation

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c: The ALU and the Instruction Cache Explanation

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d: The PHT and the Branch Target Buffer (BTB) Explanation

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In a (2,2) predictor, the GHR is '01'. The branch is actually 'Taken' (T). What will the new state of the GHR be?

a: 00 Explanation

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b: 01 Explanation

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c: 10 Explanation

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d: 11 Explanation

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Why is a GAg predictor called a 'two-level' predictor?

a: Because it uses two bits for the GHR and two bits for the PHT counters. Explanation

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b: Because it uses two separate tables: one for history and one for patterns. Explanation

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c: Because it can only predict two branches ahead. Explanation

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d: Because it makes predictions in two stages: first by the GHR, then by the PHT. Explanation

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Consider a (2,2) predictor where GHR = '11' and PHT[3] = '01' (Weak NT). The prediction is 'Not Taken'. The branch is actually 'Taken'. What are the new states of GHR and PHT[3]?

a: GHR = '10', PHT[3] = '00' Explanation

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b: GHR = '11', PHT[3] = '10' Explanation

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c: GHR = '11', PHT[3] = '00' Explanation

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d: GHR = '10', PHT[3] = '10' Explanation

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