fix(ALU): correct borrow flag for subtraction and handle CTR=3 stale …

[Me-Priyank]

Fixes #845

Describe the changes you have made in this PR -

This PR fixes two bugs in the ALU component:

Bug 1: Subtraction Borrow Flag Always Zero

When performing subtraction (A-B, Control Signal = 6), the Carry/Borrow output was hardcoded to 0. In real hardware, the borrow flag should be 1 when B > A to indicate that a borrow occurred.

Fix: Changed from

this.carryOut.value = 0

to

this.carryOut.value = +(this.inp1.value < this.inp2.value)

Bug 2: Control Signal 3 Outputs Stale Data

When Control Signal = 3, the ALU only updated the display message to "ALU" but did not update the output value. This caused the output to retain its previous value (stale data).

Fix: Added proper output handling that sets output to 0 and adds both output and carry to the simulation queue.

Files Modified:

• src/simulator/src/modules/ALU.js
• v1/src/simulator/src/modules/ALU.js

Screenshots of the UI changes (If any) -

Bug 1 fixed :

Bug 2 fixed :

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Code Understanding and AI Usage

Did you use AI assistance (ChatGPT, Claude, Copilot, etc.) to write any part of this code?

• No, I wrote all the code myself
• Yes, I used AI assistance (continue below)

If you used AI assistance:

• I have reviewed every single line of the AI-generated code
• I can explain the purpose and logic of each function/component I added
• I have tested edge cases and understand how the code handles them
• I have modified the AI output to follow this project's coding standards and conventions

Explain your implementation approach:

Problem: The ALU component had two issues affecting simulation correctness:

1. Subtraction never set the borrow flag, making it impossible for downstream circuits to detect underflow
2. Control signal 3 left stale data on the output bus

Alternative approaches considered:

• For Bug 1: Could have used this.inp2.value > this.inp1.value but +(a < b) is more idiomatic JavaScript
• For Bug 2: Could have set output to undefined, but 0 is more consistent with hardware behavior (no operation = zero output)

Why this implementation:

• Minimal code changes (2 files, ~6 lines each)
• Follows existing patterns in the codebase (other operations use similar carry flag logic)
• Maintains backward compatibility (existing circuits will work, just with correct borrow behavior now)

Key changes:

• this.carryOut.value = +(this.inp1.value < this.inp2.value) - Sets borrow flag to 1 when B > A
• Added simulationQueue.add() calls for CTR=3 to properly propagate output changes

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Checklist before requesting a review

• I have added proper PR title and linked to the issue
• I have performed a self-review of my code
• I can explain the purpose of every function, class, and logic block I added
• I understand why my changes work and have tested them thoroughly
• I have considered potential edge cases and how my code handles them
• If it is a core feature, I have added thorough tests
• My code follows the project's style guidelines and conventions

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Note: Please check Allow edits from maintainers if you would like us to assist in the PR.

Summary by CodeRabbit

• Bug Fixes
  • Fixed the no-operation instruction to properly clear outputs and set both result and carry flags to zero.
  • Fixed the subtraction operation to correctly handle the borrow flag, now setting it to indicate when borrowing is required.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

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[coderabbitai]

Walkthrough

The ALU.js module is updated to fix two operational bugs. When control signal is 3 (no-operation), the module now sets output and carryOut to 0 and enqueues both values instead of only updating the display message. When control signal is 6 (subtraction A-B), the module now correctly sets carryOut to indicate a borrow by computing whether inp1 is less than inp2, using arithmetic coercion to convert the result to 1 or 0, rather than always setting carryOut to 0. These modifications ensure proper output handling for the no-operation mode and accurate borrow flag representation during subtraction operations.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title clearly identifies the two main bugs being fixed: borrow flag for subtraction and stale output handling for CTR=3, which directly aligns with the changeset.
Linked Issues check	✅ Passed	The PR successfully implements both required fixes from issue #845: corrects borrow flag logic for subtraction and clears stale outputs for CTR=3 control signal.
Out of Scope Changes check	✅ Passed	All changes in both ALU.js files are directly related to the two bugs specified in issue #845; no unrelated modifications detected.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[Me-Priyank]

@Nihal4777 @tachyons kindly review

[Me-Priyank]

@Nihal4777

[Me-Priyank]

@Nihal4777 can you please review