bitwise arith, more loops

include/fh_error.h

@@ -24,6 +24,7 @@ enum fh_error {
   FH_ERR_INTERNAL,
   FH_ERR_UNKNOWN_WORD,
   FH_ERR_ILLEGAL_FETCH,
+  FH_ERR_ILLEGAL_STORE,
   FH_ERR_DIV_BY_ZERO,
   FH_ERR_MAX,
 };

include/fh_runtime.h

@@ -141,7 +141,7 @@ struct fh_thread_s {
 
   /** Data heap */
   uint8_t heap[HEAP_SIZE];
-  size_t heap_top;
+  size_t heap_top; // aka "HERE"
 
   /** Compile buffer, used for both word data and literals */
   uint8_t compile[COMPILED_BUFFER_SIZE];
@@ -175,6 +175,7 @@ enum fh_error w_user_word(struct fh_thread_s *fh, const struct fh_word_s *w);
 // SFR and magic addresses are "negative"
 #define MAGICADDR_INTERACTIVE 0xFFFFFFFFULL
 #define MAGICADDR_BASE        0xFFFFBA5EULL
+#define MAGICADDR_HERE        0xFFFF4E7EULL
 #define MAGICADDR_UNRESOLVED  0xFFFFFBADULL
 
 /** Get a value rounded up to multiple of word size */