CODEWITHSUNDEEP
armconditional-statementsbranch-prediction
PCRevolt
PCRevolt

Reputation: 139

ARM Branch Prediction based on code

so I'm trying to study for this test and one of the things on the study guide gives us some ARM Code and tells us to fill in a branch prediction table based on how the code runs.

I can understand how I'm supposed to do it, but with branch prediction the actual outcome comes into play, too, and I can't figure out the outcome (per cycle) from the code. The code is:

    MOV r0, #4
B1  MOV r2, #5; Branch 1
    SUB r2, r2, r0
B2  SUBS r2, r2, #1; Branch 2
    BNE B2
    SUBS r0, r0, #1
    BNE B1

What's confusing me is the BNE statements. Usually when I see one of those conditional statements there has been a CMP statement earlier in the code, and that way I can know whether to take the branch or not. But I see no compare statement anywhere in this code, so I don't know how to determine if I should take a branch or not.

Upvotes: 0

Views: 300

Answers (1)

Kyrill
Kyrill

Reputation: 3811

The SUBS performs a subtraction and a comparison with zero of the result in one instruction. The BNE conditional branches use the condition flags that were set by these SUBS instructions

Upvotes: 0

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