diff --git a/README.md b/README.md
index 3ab4eff..eaf2675 100644
--- a/README.md
+++ b/README.md
@@ -126,6 +126,12 @@ The compiler is command line driven, and creates an executable .prg file.
 
 A list of source files can be provided.
 
+### Run time library defines
+
+* -dNOLONG : no support for long in printf
+* -dNOFLOAT : no float in printf
+* -dHEAPCHECK : check heap allocate and free and jam if heap full or free out of range
+
 #### Supported target machines
 
 * c64 : Commodore C64, (0x0800..0xa000)
diff --git a/include/crt.c b/include/crt.c
index 5bb41d8..f1a96ed 100644
--- a/include/crt.c
+++ b/include/crt.c
@@ -4214,6 +4214,9 @@ loop:
 
 hempty:	
 		// no more heap blocks
+#ifdef HEAPCHECK
+		byt $02
+#endif
 		rts
 
 avail:
@@ -4316,6 +4319,25 @@ __asm free
 		rts
 notnull:
 
+#ifdef HEAPCHECK
+		lda accu + 1
+		ldx accu
+
+		cmp #>HeapStart
+		bcc	hfail
+		bne hchk1
+		cpx #<HeapStart
+		bcs hchk1
+hfail:
+		byt $02
+hchk1:
+		cmp #>HeapEnd
+		bcc hchk2
+		bne hfail
+		cpx #<HeapEnd
+		bcs hfail
+hckh2:
+#endif
 		// cache end of block, rounding to next four byte
 		// address