# UNDF: UNDF-2026-000001054
# UNDF: UNDF-2026-XXXXXXXXX
--- a/Core/debugger.c
+++ b/Core/debugger.c
@@ -2401,13 +2401,26 @@ static void test_watchpoint(GB_gameboy_t *gb, uint16_t addr, uint8_t flags, uint
 static void test_watchpoint(GB_gameboy_t *gb, uint16_t addr, uint8_t flags, uint8_t value)
 {
     if (unlikely(gb->backstep_instructions)) return;
+    /* O(1) fast-path: bail immediately if no watchpoint covers this address+flags */
+    if (!(gb->watchpoint_address_flags[addr] & flags)) return;
     uint16_t bank = bank_for_addr(gb, addr);
     for (unsigned i = 0; i < gb->n_watchpoints; i++) {
         struct GB_watchpoint_s *watchpoint = &gb->watchpoints[i];
         if (watchpoint->bank != (uint16_t)-1) {
             if (watchpoint->bank != bank) continue;
         }
         if (!(watchpoint->flags & flags)) continue;
         if (addr < watchpoint->addr) continue;
         if (addr > (uint32_t)watchpoint->addr + watchpoint->length + watchpoint->inclusive) continue;
         if (!watchpoint->condition) {
         condition_ok:
             GB_debugger_break(gb);

--- a/Core/gb.h
+++ b/Core/gb.h
@@ -768,6 +768,8 @@ struct GB_gameboy_internal_s {
         uint16_t n_watchpoints;
         struct GB_watchpoint_s *watchpoints;
+        /* Per-address flags OR'd across all watchpoints covering that address (READ|WRITE).
+           Allows O(1) early-exit in GB_read_memory / GB_write_memory hot path. */
+        uint8_t watchpoint_address_flags[0x10000];

--- a/Core/debugger.c (watchpoint add)
+++ b/Core/debugger.c (watchpoint add)
@@ -1290,6 +1290,12 @@ static bool watch(GB_gameboy_t *gb, char *arguments, char *modifiers, const debu
     gb->watchpoints[gb->n_watchpoints++] = (struct GB_watchpoint_s){
         .id = id,
         .key = key,
         .condition = condition? strdup(condition) : NULL,
         .flags = flags,
         .length = length,
         .inclusive = inclusive,
     };
+    /* Update O(1) lookup table for addresses covered by this watchpoint */
+    for (uint32_t a = result.value; a <= (uint32_t)result.value + length + inclusive; a++) {
+        gb->watchpoint_address_flags[(uint16_t)a] |= flags;
+    }

--- a/Core/debugger.c (watchpoint delete/rebuild)
+++ b/Core/debugger.c (watchpoint delete/rebuild)
@@ rebuild watchpoint_address_flags after any removal
+static void rebuild_watchpoint_flags(GB_gameboy_t *gb)
+{
+    memset(gb->watchpoint_address_flags, 0, sizeof(gb->watchpoint_address_flags));
+    for (unsigned i = 0; i < gb->n_watchpoints; i++) {
+        struct GB_watchpoint_s *wp = &gb->watchpoints[i];
+        for (uint32_t a = wp->addr; a <= (uint32_t)wp->addr + wp->length + wp->inclusive; a++) {
+            gb->watchpoint_address_flags[(uint16_t)a] |= wp->flags;
+        }
+    }
+}