diff --git a/api/bdw-attrs.h b/api/bdw-attrs.h
index 19d5250f2..a22445f14 100644
--- a/api/bdw-attrs.h
+++ b/api/bdw-attrs.h
@@ -39,7 +39,10 @@ static inline uint8_t gc_allocator_alloc_table_end_pattern(void) {
 static inline enum gc_old_generation_check_kind gc_old_generation_check_kind(size_t) {
   return GC_OLD_GENERATION_CHECK_NONE;
 }
-static inline uint8_t gc_old_generation_check_alloc_table_bit_pattern(void) {
+static inline uint8_t gc_old_generation_check_alloc_table_tag_mask(void) {
+  GC_CRASH();
+}
+static inline uint8_t gc_old_generation_check_alloc_table_young_tag(void) {
   GC_CRASH();
 }
 
diff --git a/api/gc-api.h b/api/gc-api.h
index 58cd5c02d..23113060c 100644
--- a/api/gc-api.h
+++ b/api/gc-api.h
@@ -193,7 +193,9 @@ static inline int gc_object_is_old_generation(struct gc_mutator *mut,
     uintptr_t granule = (addr & (alignment - 1)) / granule_size;
     uint8_t *byte_loc = (uint8_t*)(base + granule);
     uint8_t byte = atomic_load_explicit(byte_loc, memory_order_relaxed);
-    return byte & gc_old_generation_check_alloc_table_bit_pattern();
+    uint8_t mask = gc_old_generation_check_alloc_table_tag_mask();
+    uint8_t young = gc_old_generation_check_alloc_table_young_tag();
+    return (byte & mask) != young;
   }
   case GC_OLD_GENERATION_CHECK_SMALL_OBJECT_NURSERY: {
     struct gc_heap *heap = gc_mutator_heap(mut);
diff --git a/api/gc-attrs.h b/api/gc-attrs.h
index bb563c0e2..0ef3e082b 100644
--- a/api/gc-attrs.h
+++ b/api/gc-attrs.h
@@ -34,7 +34,8 @@ enum gc_old_generation_check_kind {
 
 static inline enum gc_old_generation_check_kind gc_old_generation_check_kind(size_t obj_size) GC_ALWAYS_INLINE;
 
-static inline uint8_t gc_old_generation_check_alloc_table_bit_pattern(void) GC_ALWAYS_INLINE;
+static inline uint8_t gc_old_generation_check_alloc_table_tag_mask(void) GC_ALWAYS_INLINE;
+static inline uint8_t gc_old_generation_check_alloc_table_young_tag(void) GC_ALWAYS_INLINE;
 
 enum gc_write_barrier_kind {
   GC_WRITE_BARRIER_NONE,
diff --git a/api/mmc-attrs.h b/api/mmc-attrs.h
index fe9edb0a4..7066caf23 100644
--- a/api/mmc-attrs.h
+++ b/api/mmc-attrs.h
@@ -44,9 +44,11 @@ static inline enum gc_old_generation_check_kind gc_old_generation_check_kind(siz
   }
   return GC_OLD_GENERATION_CHECK_NONE;
 }
-static inline uint8_t gc_old_generation_check_alloc_table_bit_pattern(void) {
-  // The three mark bits.
-  return 2 + 4 + 8;
+static inline uint8_t gc_old_generation_check_alloc_table_tag_mask(void) {
+  return 7;
+}
+static inline uint8_t gc_old_generation_check_alloc_table_young_tag(void) {
+  return 1;
 }
 
 static inline enum gc_write_barrier_kind gc_write_barrier_kind(size_t obj_size) {
@@ -58,12 +60,10 @@ static inline enum gc_write_barrier_kind gc_write_barrier_kind(size_t obj_size)
   return GC_WRITE_BARRIER_NONE;
 }
 static inline size_t gc_write_barrier_card_table_alignment(void) {
-  GC_ASSERT(GC_GENERATIONAL);
-  return 4 * 1024 * 1024;
+  GC_CRASH();
 }
 static inline size_t gc_write_barrier_card_size(void) {
-  GC_ASSERT(GC_GENERATIONAL);
-  return 256;
+  GC_CRASH();
 }
 static inline size_t gc_write_barrier_field_table_alignment(void) {
   GC_ASSERT(GC_GENERATIONAL);
diff --git a/api/pcc-attrs.h b/api/pcc-attrs.h
index 913208cdd..727d134c0 100644
--- a/api/pcc-attrs.h
+++ b/api/pcc-attrs.h
@@ -46,7 +46,10 @@ static inline enum gc_old_generation_check_kind gc_old_generation_check_kind(siz
     return GC_OLD_GENERATION_CHECK_SMALL_OBJECT_NURSERY;
   return GC_OLD_GENERATION_CHECK_SLOW;
 }
-static inline uint8_t gc_old_generation_check_alloc_table_bit_pattern(void) {
+static inline uint8_t gc_old_generation_check_alloc_table_tag_mask(void) {
+  GC_CRASH();
+}
+static inline uint8_t gc_old_generation_check_alloc_table_young_tag(void) {
   GC_CRASH();
 }
 
diff --git a/api/semi-attrs.h b/api/semi-attrs.h
index b8baab693..c749fd3cf 100644
--- a/api/semi-attrs.h
+++ b/api/semi-attrs.h
@@ -41,7 +41,10 @@ static inline uint8_t gc_allocator_alloc_table_end_pattern(void) {
 static inline enum gc_old_generation_check_kind gc_old_generation_check_kind(size_t) {
   return GC_OLD_GENERATION_CHECK_NONE;
 }
-static inline uint8_t gc_old_generation_check_alloc_table_bit_pattern(void) {
+static inline uint8_t gc_old_generation_check_alloc_table_tag_mask(void) {
+  GC_CRASH();
+}
+static inline uint8_t gc_old_generation_check_alloc_table_young_tag(void) {
   GC_CRASH();
 }