diff --git a/core/box/ss_hot_cold_box.h b/core/box/ss_hot_cold_box.h
new file mode 100644
index 00000000..d60960bb
--- /dev/null
+++ b/core/box/ss_hot_cold_box.h
@@ -0,0 +1,96 @@
+// ss_hot_cold_box.h - Phase 3d-C: Hot/Cold Slab Split Box
+// Purpose: Cache locality optimization via hot/cold slab separation
+// License: MIT
+// Date: 2025-11-20
+
+#ifndef SS_HOT_COLD_BOX_H
+#define SS_HOT_COLD_BOX_H
+
+#include "../superslab/superslab_types.h"
+#include <stdbool.h>
+
+// ============================================================================
+// Phase 3d-C: Hot/Cold Split Box API
+// ============================================================================
+//
+// Goal: Improve L1D cache hit rate by separating hot (high utilization) and
+//       cold (low utilization) slabs within a SuperSlab.
+//
+// Strategy:
+// - Hot slabs (used > 50%): Prioritized for allocation → better cache locality
+// - Cold slabs (used ≤ 50%): Used as fallback → delayed deallocation
+//
+// Expected: +8-12% throughput from improved cache line locality
+//
+// Box Contract:
+// - ss_is_slab_hot(): Returns true if slab should be considered "hot"
+// - ss_update_hot_cold_indices(): Rebuilds hot/cold index arrays
+// - ss_init_hot_cold(): Initializes hot/cold fields on SuperSlab creation
+//
+// ============================================================================
+
+// Phase 3d-C: Hot/Cold判定閾値
+#define HOT_UTILIZATION_THRESHOLD 50  // 使用率50%以上でホット判定
+
+// Phase 3d-C: Hot判定ロジック
+// Returns: true if slab is "hot" (high utilization, should be prioritized)
+static inline bool ss_is_slab_hot(const TinySlabMeta* meta) {
+    // ヒューリスティック: 使用率 > 50% → ホット
+    // 理由: 使用率が高い = 頻繁にアクセスされている = キャッシュに載せたい
+    if (meta->capacity == 0) {
+        return false;  // Uninitialized slab
+    }
+    return (meta->used * 100 / meta->capacity) > HOT_UTILIZATION_THRESHOLD;
+}
+
+// Phase 3d-C: Hot/Cold インデックス更新
+// Rebuilds hot_indices[] and cold_indices[] arrays based on current slab state
+static inline void ss_update_hot_cold_indices(SuperSlab* ss) {
+    if (!ss) return;
+
+    ss->hot_count = 0;
+    ss->cold_count = 0;
+
+    uint32_t max_slabs = (1u << ss->lg_size) / SLAB_SIZE;
+    if (max_slabs > SLABS_PER_SUPERSLAB_MAX) {
+        max_slabs = SLABS_PER_SUPERSLAB_MAX;
+    }
+
+    // Scan active slabs and classify as hot or cold
+    for (uint32_t i = 0; i < max_slabs && i < ss->active_slabs; i++) {
+        TinySlabMeta* meta = &ss->slabs[i];
+
+        // Skip uninitialized slabs (capacity == 0)
+        if (meta->capacity == 0) {
+            continue;
+        }
+
+        if (ss_is_slab_hot(meta)) {
+            // Hot slab: high utilization
+            if (ss->hot_count < 16) {
+                ss->hot_indices[ss->hot_count++] = (uint8_t)i;
+            }
+        } else {
+            // Cold slab: low utilization
+            if (ss->cold_count < 16) {
+                ss->cold_indices[ss->cold_count++] = (uint8_t)i;
+            }
+        }
+    }
+}
+
+// Phase 3d-C: SuperSlab初期化時にhot/cold fieldsをゼロクリア
+static inline void ss_init_hot_cold(SuperSlab* ss) {
+    if (!ss) return;
+
+    ss->hot_count = 0;
+    ss->cold_count = 0;
+
+    // Initialize index arrays to 0 (defensive programming)
+    for (int i = 0; i < 16; i++) {
+        ss->hot_indices[i] = 0;
+        ss->cold_indices[i] = 0;
+    }
+}
+
+#endif // SS_HOT_COLD_BOX_H
diff --git a/core/hakmem_tiny_superslab.c b/core/hakmem_tiny_superslab.c
index b374f14d..1f4efd01 100644
--- a/core/hakmem_tiny_superslab.c
+++ b/core/hakmem_tiny_superslab.c
@@ -4,6 +4,7 @@
 // Date: 2025-10-24
 
 #include "hakmem_tiny_superslab.h"
+#include "box/ss_hot_cold_box.h"  // Phase 3d-C: Hot/Cold Split
 #include "hakmem_super_registry.h"  // Phase 1: Registry integration
 #include "hakmem_tiny.h"  // For tiny_self_u32
 #include "hakmem_tiny_config.h"  // For extern g_tiny_class_sizes
@@ -783,6 +784,14 @@ SuperSlab* superslab_allocate(uint8_t size_class) {
     ss->lru_prev = NULL;
     ss->lru_next = NULL;
 
+    // Phase 3d-C: Initialize Hot/Cold Split fields
+    ss->hot_count = 0;
+    ss->cold_count = 0;
+    for (int i = 0; i < 16; i++) {
+        ss->hot_indices[i] = 0;
+        ss->cold_indices[i] = 0;
+    }
+
     // Initialize all slab metadata (only up to max slabs for this size)
     int max_slabs = (int)(ss_size / SLAB_SIZE);
 
@@ -1116,6 +1125,9 @@ void superslab_activate_slab(SuperSlab* ss, int slab_idx) {
     if ((ss->slab_bitmap & mask) == 0) {
         ss->slab_bitmap |= mask;
         ss->active_slabs++;
+
+        // Phase 3d-C: Update hot/cold indices after activating new slab
+        ss_update_hot_cold_indices(ss);
     }
 }
 
diff --git a/core/superslab/superslab_types.h b/core/superslab/superslab_types.h
index 8bf78590..67d46f42 100644
--- a/core/superslab/superslab_types.h
+++ b/core/superslab/superslab_types.h
@@ -78,6 +78,12 @@ typedef struct SuperSlab {
     _Atomic uint32_t  remote_counts[SLABS_PER_SUPERSLAB_MAX];
     _Atomic uint32_t  slab_listed[SLABS_PER_SUPERSLAB_MAX];
 
+    // Phase 3d-C: Hot/Cold Split - Cache locality optimization
+    uint8_t  hot_count;              // Number of hot slabs (high utilization)
+    uint8_t  cold_count;             // Number of cold slabs (low utilization)
+    uint8_t  hot_indices[16];        // Indices of hot slabs (max 16)
+    uint8_t  cold_indices[16];       // Indices of cold slabs (max 16)
+
     // Per-slab metadata array
     TinySlabMeta slabs[SLABS_PER_SUPERSLAB_MAX];
 } SuperSlab;