diff --git a/runtime/instruction_size.cpp b/runtime/instruction_size.cpp
new file mode 100644
index 0000000..6eba25a
--- /dev/null
+++ b/runtime/instruction_size.cpp
@@ -0,0 +1,52 @@
+#include "arch.hpp"
+#include "vm.hpp"
+
+using namespace sliger;
+
+size_t VM::instruction_size(size_t i) const
+{
+    switch (static_cast<Op>(this->program.at(i))) {
+        case Op::Nop:
+        case Op::PushNull:
+            return 1;
+        case Op::PushInt:
+        case Op::PushBool:
+            return 2;
+        case Op::PushString: {
+            auto string_length = this->program.at(i + 1);
+            return 2 + string_length;
+        }
+        case Op::PushPtr:
+            return 2;
+        case Op::Pop:
+            return 1;
+        case Op::ReserveStatic:
+        case Op::LoadStatic:
+        case Op::StoreStatic:
+        case Op::LoadLocal:
+        case Op::StoreLocal:
+        case Op::Call:
+            return 2;
+        case Op::Return:
+        case Op::Jump:
+        case Op::JumpIfTrue:
+            return 1;
+        case Op::Builtin:
+            return 2;
+        case Op::Add:
+        case Op::Subtract:
+        case Op::Multiply:
+        case Op::Divide:
+        case Op::Remainder:
+        case Op::Equal:
+        case Op::LessThan:
+        case Op::And:
+        case Op::Or:
+        case Op::Xor:
+        case Op::Not:
+            return 1;
+        case Op::SourceMap:
+            return 4;
+    }
+    return 1;
+}
diff --git a/runtime/vm.cpp b/runtime/vm.cpp
index c34a83a..adcffb1 100644
--- a/runtime/vm.cpp
+++ b/runtime/vm.cpp
@@ -451,7 +451,8 @@ void VM::run_file_builtin(Builtin builtin_id)
             auto filename = stack_pop().as_string().value;
             FILE* fp = std::fopen(filename.c_str(), mode.c_str());
             if (fp == nullptr) {
-                std::cerr << std::format("error: could not open file '{}'\n", filename);
+                std::cerr << std::format(
+                    "error: could not open file '{}'\n", filename);
                 std::exit(1);
             }
             auto file_id = this->file_id_counter;
@@ -568,6 +569,14 @@ auto VM::stack_repr_string(size_t max_items) const -> std::string
     return result;
 };
 
+void VM::assert_program_has(size_t count)
+{
+    if (this->pc + count > program.size()) {
+        std::cerr << std::format("malformed program, pc = {}", this->pc);
+        std::exit(1);
+    }
+}
+
 void VM::assert_fn_stack_has(size_t count)
 {
     if (this->stack.size() - this->bp < count) {
@@ -583,11 +592,3 @@ void VM::assert_stack_has(size_t count)
         std::exit(1);
     }
 }
-
-void VM::assert_program_has(size_t count)
-{
-    if (this->pc + count > program.size()) {
-        std::cerr << std::format("malformed program, pc = {}", this->pc);
-        std::exit(1);
-    }
-}
diff --git a/runtime/vm.hpp b/runtime/vm.hpp
index 299148b..5ed6278 100644
--- a/runtime/vm.hpp
+++ b/runtime/vm.hpp
@@ -8,6 +8,7 @@
 #include <cstdint>
 #include <cstdio>
 #include <string>
+#include <utility>
 #include <vector>
 
 namespace sliger {
@@ -108,6 +109,11 @@ struct CCPosEntry {
 
 class CodeCoverageBuilder : public json::ToAndFromJson {
 public:
+    inline void make_sure_entry_exists(SourcePos pos)
+    {
+        find_or_create_entry(pos);
+    }
+
     /// call when leaving a source location
     inline void report_cover(SourcePos pos)
     {
@@ -166,8 +172,19 @@ public:
         return json::to_json(this->flame_graph);
     }
 
-    inline auto code_coverage_json() const -> std::string
+    inline auto code_coverage_json() -> std::string
     {
+        for (size_t i = 0; i < this->program.size(); ++i) {
+            if (this->program.at(i) == std::to_underlying(Op::SourceMap)
+                && this->program.size() - 1 - i >= 3) {
+                auto index = static_cast<int32_t>(this->program.at(i + 1));
+                auto line = static_cast<int32_t>(this->program.at(i + 2));
+                auto col = static_cast<int32_t>(this->program.at(i + 3));
+                this->code_coverage.make_sure_entry_exists(
+                    { index, line, col });
+            }
+            i += instruction_size(i);
+        }
         return json::to_json(this->code_coverage);
     }
 
@@ -223,6 +240,7 @@ private:
     {
         return this->stack.at(this->bp + idx);
     }
+    void assert_program_has(size_t count);
     void assert_fn_stack_has(size_t count);
     void assert_stack_has(size_t count);
     inline void stack_push(Value&& value) { this->stack.push_back(value); }
@@ -233,7 +251,7 @@ private:
         this->stack.pop_back();
         return value;
     }
-    void assert_program_has(size_t count);
+    size_t instruction_size(size_t i) const;
 
     VMOpts opts;
     uint32_t pc = 0;
diff --git a/web/public/src/code_coverage.ts b/web/public/src/code_coverage.ts
index c60a4c0..5e5b2e0 100644
--- a/web/public/src/code_coverage.ts
+++ b/web/public/src/code_coverage.ts
@@ -97,21 +97,6 @@ export function loadCodeCoverage(
         elements.push(span);
         col += 1;
     }
-    function positionInBox(
-        position: [number, number],
-        boundingRect: {
-            left: number;
-            top: number;
-            right: number;
-            bottom: number;
-        },
-    ) {
-        const [x, y] = position;
-        const outside = x < boundingRect.left ||
-            x >= boundingRect.right || y < boundingRect.top ||
-            y >= boundingRect.bottom;
-        return !outside;
-    }
     container.append(...elements);
     document.addEventListener("mousemove", (event) => {
         const [x, y] = [event.clientX, event.clientY];
@@ -146,3 +131,19 @@ export function loadCodeCoverage(
     });
     return container;
 }
+
+function positionInBox(
+    position: [number, number],
+    boundingRect: {
+        left: number;
+        top: number;
+        right: number;
+        bottom: number;
+    },
+) {
+    const [x, y] = position;
+    const outside = x < boundingRect.left ||
+        x >= boundingRect.right || y < boundingRect.top ||
+        y >= boundingRect.bottom;
+    return !outside;
+}
diff --git a/web/public/src/index.ts b/web/public/src/index.ts
index 85c7a1b..0ca2e85 100644
--- a/web/public/src/index.ts
+++ b/web/public/src/index.ts
@@ -57,6 +57,26 @@ function sourceCode(view: Element, codeData: string) {
     view.replaceChildren(outerContainer);
 }
 
+function createRadio(
+    id: string,
+    content: string,
+    checked: boolean,
+): [HTMLDivElement, HTMLInputElement] {
+    const label = document.createElement("label");
+    label.htmlFor = id;
+    label.innerText = content;
+    const input = document.createElement("input");
+    input.id = id;
+    input.name = "coverage-radio";
+    input.type = "radio";
+    input.hidden = true;
+    input.checked = checked;
+    const container = document.createElement("div");
+    container.classList.add("coverage-radio-group");
+    container.append(input, label);
+    return [container, input];
+}
+
 async function codeCoverage(view: Element, codeData: string) {
     const codeCoverageData = await data.codeCoverageData();
 
@@ -66,25 +86,6 @@ async function codeCoverage(view: Element, codeData: string) {
     const innerContainer = document.createElement("div");
     innerContainer.classList.add("code-container-inner");
 
-    function createRadio(
-        id: string,
-        content: string,
-        checked: boolean,
-    ): [HTMLDivElement, HTMLInputElement] {
-        const label = document.createElement("label");
-        label.htmlFor = id;
-        label.innerText = content;
-        const input = document.createElement("input");
-        input.id = id;
-        input.name = "coverage-radio";
-        input.type = "radio";
-        input.hidden = true;
-        input.checked = checked;
-        const container = document.createElement("div");
-        container.classList.add("coverage-radio-group");
-        container.append(input, label);
-        return [container, input];
-    }
     const [perfGroup, perfInput] = createRadio(
         "performance-coverage",
         "Performance view",
diff --git a/web/public/style.css b/web/public/style.css
index 9009b0f..a3769da 100644
--- a/web/public/style.css
+++ b/web/public/style.css
@@ -159,6 +159,7 @@ main #cover {
 .coverage-radio {
     display: flex;
     flex-direction: row;
+    justify-content: center;
 }
 
 .coverage-radio-group {
@@ -166,6 +167,7 @@ main #cover {
     justify-content: center;
     flex: 1;
     padding: 2rem;
+    max-width: max-content;
 }
 
 .coverage-radio-group label {
@@ -173,6 +175,8 @@ main #cover {
     border-radius: 0.25rem;
     cursor: pointer;
     border: 2px solid var(--code-status);
+    width: 280px;
+    text-align: center;
 }
 
 .coverage-radio-group input:checked ~ label {