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added verilog module for LSB #621

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added verilog module for LSB #621

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26 changes: 23 additions & 3 deletions src/simulator/src/modules/LSB.js
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Original file line number Diff line number Diff line change
Expand Up @@ -24,7 +24,7 @@ export default class LSB extends CircuitElement {
this.directionFixed = true
this.bitWidth = bitWidth || parseInt(prompt('Enter bitWidth'), 10)
this.rectangleObject = false
this.intputSize = this.bitWidth
this.inputSize = this.bitWidth

this.inp1 = new Node(-10, 0, 0, this, this.inputSize)
this.output1 = new Node(20, 0, 1, this, this.bitWidth)
Expand Down Expand Up @@ -121,8 +121,28 @@ export default class LSB extends CircuitElement {
ctx.stroke()
ctx.fill()
}
generateVerilog() {
return `assign ${this.output1.verilogLabel} = (${this.enable.verilogLabel}!=0) ? ${this.inp1.verilogLabel}[0] : 0;`
static moduleVerilog() {
return `
module LSB(out, en, inp);
parameter WIDTH = 1;
output reg [WIDTH-1:0] out;
output reg en;
input [WIDTH-1:0] inp;

integer i;

always @(*) begin
out = 0;
en = 0;
for (i = WIDTH-1; i >=0; i = i - 1) begin
if (inp[i]) begin
out = WIDTH-1-i;
en = 1;
break;
end
end
end
endmodule`
}
Comment on lines +124 to 146
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@coderabbitai coderabbitai bot Jun 29, 2025 •
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⚠️ Potential issue

Critical logic error: Verilog implementation finds MSB instead of LSB.

The Verilog implementation has two critical issues:

  1. Logic Error: The for-loop iterates from MSB to LSB (WIDTH-1 down to 0), which finds the Most Significant Bit, not the Least Significant Bit. This contradicts the JavaScript implementation that correctly finds the LSB.

  2. Synthesis Issue: The break statement is not synthesizable in Verilog and will cause synthesis failures.

Apply this diff to fix both issues:

-    always @(*) begin
-        out = 0;
-        en = 0;
-        for (i = WIDTH-1; i >=0; i = i - 1) begin
-            if (inp[i]) begin
-                out = WIDTH-1-i;
-                en = 1;
-                break;
-            end
-        end
-    end
+    always @(*) begin
+        out = 0;
+        en = 0;
+        for (i = 0; i < WIDTH; i = i + 1) begin
+            if (inp[i]) begin
+                out = i;
+                en = 1;
+                i = WIDTH; // Exit loop (synthesizable alternative to break)
+            end
+        end
+    end

This ensures the module correctly finds the least significant set bit and uses synthesizable Verilog constructs.

🤖 Prompt for AI Agents 
In src/simulator/src/modules/LSB.js lines 124 to 146, the Verilog code
incorrectly searches from MSB to LSB, finding the most significant set bit
instead of the least significant one, and uses a non-synthesizable break
statement. To fix this, reverse the for-loop to iterate from 0 up to WIDTH-1 to
find the least significant set bit, and replace the break statement with a
synthesizable control structure such as a flag or conditional to exit the loop
properly without using break.

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can you resolve this

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@niladrix719, I'll resolve this issue by fixing the Verilog implementation to correctly find the LSB and use synthesizable code.

Task: Fix LSB Verilog Implementation

  • Fix the for-loop direction to iterate from LSB to MSB (0 to WIDTH-1)
  • Replace non-synthesizable break statement with synthesizable loop control
  • Ensure the output correctly represents the LSB position

The fix addresses both critical issues:

  1. Logic Error: Changed the loop to iterate from 0 to WIDTH-1 (i = 0; i < WIDTH; i = i + 1) to find the LSB correctly
  2. Synthesis Issue: Replaced the non-synthesizable break statement with i = WIDTH to exit the loop in a synthesizable way

This ensures the module correctly finds the least significant set bit position and can be synthesized properly.

🧠 Learnings used 
Learnt from: 092vk
PR: CircuitVerse/cv-frontend-vue#593
File: v1/src/simulator/src/modules/ALU.js:191-192
Timestamp: 2025-06-10T11:55:53.137Z
Learning: The ALU Verilog module in v1/src/simulator/src/modules/ALU.js currently only supports 1-bit processing. Width parameter support will be added in the future for scalability, even though the JavaScript ALU implementation already supports variable bit widths.

Learnt from: 092vk
PR: CircuitVerse/cv-frontend-vue#591
File: v1/src/simulator/src/sequential/JKflipFlop.js:159-188
Timestamp: 2025-06-10T11:26:38.102Z
Learning: In the CircuitVerse project, Verilog modules for circuit elements like JKflipFlop are designed to be consistent with the circuit simulation design, which means reset and preset signals are implemented synchronously (triggered on clock edge) rather than asynchronously, even when the signals are named as asynchronous reset/preset.

⏳ Preparing changes...

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A summary of the changes CodeRabbit can apply:

  • Correct the LSB.js Verilog module by changing the loop to count from LSB to MSB, output the index directly, and replace the non-synthesizable break statement with a synthesizable loop exit to fix the logic and ensure proper hardware synthesis.

A summary of the context that CodeRabbit is considering across the codebase:

I need to understand how the JavaScript implementation works to ensure the Verilog fix is consistent with the expected behavior.

Show context details

Generated Script:

```shell
grep -n -A 20 "resolve()" src/simulator/src/modules/LSB.js


**Shell Result:**

67: resolve() {
68- const inp = dec2bin(this.inp1.value)
69- let out = 0
70- for (let i = inp.length - 1; i >= 0; i--) {
71- if (inp[i] === '1') {
72- out = inp.length - 1 - i
73- break
74- }
75- }
76- this.output1.value = out
77- simulationArea.simulationQueue.add(this.output1)
78- if (inp != 0) {
79- this.enable.value = 1
80- } else {
81- this.enable.value = 0
82- }
83- simulationArea.simulationQueue.add(this.enable)
84- }

</details>

> <rationale>Let me verify the current Verilog implementation structure to ensure my fix targets the correct lines.</rationale>

<details>
<summary>Show context details</summary>

**Generated Script:**


grep -n -A 15 "static moduleVerilog" src/simulator/src/modules/LSB.js

**Shell Result:**

124: static moduleVerilog() {
125- return 126- module LSB(out, en, inp); 127- parameter WIDTH = 1; 128- output reg [WIDTH-1:0] out; 129- output reg en; 130- input [WIDTH-1:0] inp; 131- 132- integer i; 133- 134- always @(*) begin 135- out = 0; 136- en = 0; 137- for (i = WIDTH-1; i >=0; i = i - 1) begin 138- if (inp[i]) begin 139- out = WIDTH-1-i; 140- en = 1; 141- break; 142- end 143- end 144- end 145- endmodule
146- }

</details>

> <rationale>I should check if there are any other Verilog modules that use similar loop patterns to understand the synthesizable coding style used in this project.</rationale>

<details>
<summary>Show context details</summary>

**Generated Script:**


rg -A 15 -B 5 "for.*i.*WIDTH" src/simulator/src/modules/

**Shell Result:**

src/simulator/src/modules/LSB.js- integer i;
src/simulator/src/modules/LSB.js-
src/simulator/src/modules/LSB.js- always @(*) begin
src/simulator/src/modules/LSB.js- out = 0;
src/simulator/src/modules/LSB.js- en = 0;
src/simulator/src/modules/LSB.js: for (i = WIDTH-1; i >=0; i = i - 1) begin
src/simulator/src/modules/LSB.js- if (inp[i]) begin
src/simulator/src/modules/LSB.js- out = WIDTH-1-i;
src/simulator/src/modules/LSB.js- en = 1;
src/simulator/src/modules/LSB.js- break;
src/simulator/src/modules/LSB.js- end
src/simulator/src/modules/LSB.js- end
src/simulator/src/modules/LSB.js- end
src/simulator/src/modules/LSB.js- endmodule`
src/simulator/src/modules/LSB.js- }
src/simulator/src/modules/LSB.js-}
src/simulator/src/modules/LSB.js-
src/simulator/src/modules/LSB.js-/**
src/simulator/src/modules/LSB.js- * @memberof LSB
src/simulator/src/modules/LSB.js- * Help Tip
src/simulator/src/modules/LSB.js- * @type {string}

</details>

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- [ ] <!-- {"checkboxId": "6b3df8e2-79f5-4eb8-9d4a-c7d3a6a5f7b1", "radioGroupId": "edit-actions-group-2185921526"} --> 📋 Get copyable edits

<!-- <agentComponentType>plan-edits</agentComponentType> -->

<!-- <rawResChunk>Looking at the LSB module, I can see the critical issues mentioned in the user's request. The Verilog implementation has a logic error where it searches from MSB to LSB instead of LSB to MSB, and uses a non-synthesizable `break` statement.

<context>
<rationale>I need to understand how the JavaScript implementation works to ensure the Verilog fix is consistent with the expected behavior.</rationale>
<generated_script>
```shell
grep -n -A 20 "resolve()" src/simulator/src/modules/LSB.js

</generated_script>
<shell_result>
67: resolve() {
68- const inp = dec2bin(this.inp1.value)
69- let out = 0
70- for (let i = inp.length - 1; i >= 0; i--) {
71- if (inp[i] === '1') {
72- out = inp.length - 1 - i
73- break
74- }
75- }
76- this.output1.value = out
77- simulationArea.simulationQueue.add(this.output1)
78- if (inp != 0) {
79- this.enable.value = 1
80- } else {
81- this.enable.value = 0
82- }
83- simulationArea.simulationQueue.add(this.enable)
84- }
</shell_result>

Let me verify the current Verilog implementation structure to ensure my fix targets the correct lines. ```shell grep -n -A 15 "static moduleVerilog" src/simulator/src/modules/LSB.js ``` 124: static moduleVerilog() { 125- return ` 126- module LSB(out, en, inp); 127- parameter WIDTH = 1; 128- output reg [WIDTH-1:0] out; 129- output reg en; 130- input [WIDTH-1:0] inp; 131- 132- integer i; 133- 134- always @(*) begin 135- out = 0; 136- en = 0; 137- for (i = WIDTH-1; i >=0; i = i - 1) begin 138- if (inp[i]) begin 139- out = WIDTH-1-i; 140- en = 1; 141- break; 142- end 143- end 144- end 145- endmodule` 146- } I should check if there are any other Verilog modules that use similar loop patterns to understand the synthesizable coding style used in this project. ```shell rg -A 15 -B 5 "for.*i.*WIDTH" src/simulator/src/modules/ ``` src/simulator/src/modules/LSB.js- integer i; src/simulator/src/modules/LSB.js- src/simulator/src/modules/LSB.js- always @(*) begin src/simulator/src/modules/LSB.js- out = 0; src/simulator/src/modules/LSB.js- en = 0; src/simulator/src/modules/LSB.js: for (i = WIDTH-1; i >=0; i = i - 1) begin src/simulator/src/modules/LSB.js- if (inp[i]) begin src/simulator/src/modules/LSB.js- out = WIDTH-1-i; src/simulator/src/modules/LSB.js- en = 1; src/simulator/src/modules/LSB.js- break; src/simulator/src/modules/LSB.js- end src/simulator/src/modules/LSB.js- end src/simulator/src/modules/LSB.js- end src/simulator/src/modules/LSB.js- endmodule` src/simulator/src/modules/LSB.js- } src/simulator/src/modules/LSB.js-} src/simulator/src/modules/LSB.js- src/simulator/src/modules/LSB.js-/** src/simulator/src/modules/LSB.js- * @memberof LSB src/simulator/src/modules/LSB.js- * Help Tip src/simulator/src/modules/LSB.js- * @type {string}

Now I understand the issue completely. The JavaScript implementation correctly finds the LSB by iterating from MSB to LSB and calculating the position as inp.length - 1 - i, but the Verilog implementation incorrectly mimics this approach without accounting for the different indexing logic needed in Verilog.

The fix should:

  1. Change the loop to iterate from 0 to WIDTH-1 to find the LSB directly
  2. Output the index i directly instead of calculating WIDTH-1-i
  3. Replace the non-synthesizable break with i = WIDTH to exit the loop
```shell # Fix the Verilog implementation in LSB.js to correctly find the LSB and use synthesizable code sed -i '137s/for (i = WIDTH-1; i >=0; i = i - 1) begin/for (i = 0; i < WIDTH; i = i + 1) begin/' src/simulator/src/modules/LSB.js sed -i '139s/out = WIDTH-1-i;/out = i;/' src/simulator/src/modules/LSB.js sed -i '141s/break;/i = WIDTH; \/\/ Exit loop (synthesizable alternative to break)/' src/simulator/src/modules/LSB.js ``` -->

}

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