diff --git a/heap.c b/heap.c
index d4bedba..1551783 100644
--- a/heap.c
+++ b/heap.c
@@ -1,4 +1,5 @@
 /* stack grows up from the bottom and heap grows down from the top of heap space */
+#include <stdio.h>
 #include <memory.h>
 #include <assert.h>
 #include "picoc.h"
@@ -96,8 +97,11 @@ void *HeapAlloc(int Size)
     
     if (Bucket < FREELIST_BUCKETS && FreeListBucket[Bucket] != NULL)
     { /* try to allocate from a freelist bucket first */
+        printf("allocating %d(%d) from bucket", Size, AllocSize);
         NewMem = FreeListBucket[Bucket];
+        assert((unsigned long)NewMem >= (unsigned long)&HeapMemory && (unsigned char *)NewMem - &HeapMemory[0] < HEAP_SIZE);
         FreeListBucket[Bucket] = *(struct AllocNode **)NewMem;
+        assert(FreeListBucket[Bucket] == NULL || ((unsigned long)FreeListBucket[Bucket] >= (unsigned long)&HeapMemory && (unsigned char *)FreeListBucket[Bucket] - &HeapMemory[0] < HEAP_SIZE));
         NewMem->Size = AllocSize;
     }
     else if (FreeListBig != NULL)
@@ -107,14 +111,20 @@ void *HeapAlloc(int Size)
         
         if (*FreeNode != NULL)
         {
+            assert((unsigned long)*FreeNode >= (unsigned long)&HeapMemory && (unsigned char *)*FreeNode - &HeapMemory[0] < HEAP_SIZE);
+            assert((*FreeNode)->Size < HEAP_SIZE && (*FreeNode)->Size > 0);
             if ((*FreeNode)->Size < Size + SPLIT_MEM_THRESHOLD)
             { /* close in size - reduce fragmentation by not splitting */
+                printf("allocating %d(%d) from freelist, no split (%d)", Size, AllocSize, (*FreeNode)->Size);
                 NewMem = *FreeNode;
+                assert((unsigned long)NewMem >= (unsigned long)&HeapMemory && (unsigned char *)NewMem - &HeapMemory[0] < HEAP_SIZE);
                 *FreeNode = NewMem->NextFree;
             }
             else
             { /* split this big memory chunk */
+                printf("allocating %d(%d) from freelist, split chunk (%d)", Size, AllocSize, (*FreeNode)->Size);
                 NewMem = *FreeNode + (*FreeNode)->Size - AllocSize;
+                assert((unsigned long)NewMem >= (unsigned long)&HeapMemory && (unsigned char *)NewMem - &HeapMemory[0] < HEAP_SIZE);
                 (*FreeNode)->Size -= AllocSize;
                 NewMem->Size = AllocSize;
             }
@@ -123,6 +133,7 @@ void *HeapAlloc(int Size)
     
     if (NewMem == NULL)
     { /* couldn't allocate from a freelist - try to increase the size of the heap area */
+        printf("allocating %d(%d) at bottom of heap", Size, AllocSize);
         if (HeapBottom - AllocSize < StackTop)
             return NULL;
         
@@ -132,6 +143,7 @@ void *HeapAlloc(int Size)
     }
     
     memset(&NewMem->NextFree, '\0', AllocSize-sizeof(NewMem->Size));
+    printf(" = %lx\n", (unsigned long)&NewMem->NextFree);
     return (void *)&NewMem->NextFree;
 }
 
@@ -141,20 +153,27 @@ void HeapFree(void *Mem)
     struct AllocNode *MemNode = (struct AllocNode *)(Mem-sizeof(int));
     int Bucket = MemNode->Size >> 2;
     
+    assert((unsigned long)Mem >= (unsigned long)&HeapMemory && (unsigned char *)Mem - &HeapMemory[0] < HEAP_SIZE);
+    assert(MemNode->Size < HEAP_SIZE && MemNode->Size > 0);
     if (Mem == NULL)
         return;
     
     if ((void *)MemNode == HeapBottom)
     { /* pop it off the bottom of the heap, reducing the heap size */
+        printf("freeing %d from bottom of heap\n", MemNode->Size);
         HeapBottom += sizeof(int) + MemNode->Size;
     }
     else if (Bucket < FREELIST_BUCKETS)
     { /* we can fit it in a bucket */
+        printf("freeing %d to bucket\n", MemNode->Size);
+        assert(FreeListBucket[Bucket] == NULL || ((unsigned long)FreeListBucket[Bucket] >= (unsigned long)&HeapMemory && (unsigned char *)FreeListBucket[Bucket] - &HeapMemory[0] < HEAP_SIZE));
         *(struct AllocNode **)MemNode = FreeListBucket[Bucket];
         FreeListBucket[Bucket] = (struct AllocNode *)MemNode;
     }
     else
     { /* put it in the big memory freelist */
+        printf("freeing %lx:%d to freelist\n", (unsigned long)Mem, MemNode->Size);
+        assert(FreeListBig == NULL || ((unsigned long)FreeListBig >= (unsigned long)&HeapMemory && (unsigned char *)FreeListBig - &HeapMemory[0] < HEAP_SIZE));
         MemNode->NextFree = FreeListBig;
         FreeListBig = MemNode;
     }
diff --git a/picoc.h b/picoc.h
index ca2cb08..2de5890 100644
--- a/picoc.h
+++ b/picoc.h
@@ -65,7 +65,7 @@ enum LexToken
 /* used in dynamic memory allocation */
 struct AllocNode
 {
-    int Size;
+    unsigned int Size;
     struct AllocNode *NextFree;
 };