diff --git a/README.md b/README.md
index 518738d..55cfea0 100644
--- a/README.md
+++ b/README.md
@@ -141,6 +141,8 @@ A list of source files can be provided.
 * -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
+* -dNOBSSCLEAR : don't clear BSS segment on startup
+* -dNOZPCLEAR : don't clear zeropage BSS segment on startup
 
 #### Supported target machines
 
diff --git a/include/crt.c b/include/crt.c
index 4f82061..24a5568 100644
--- a/include/crt.c
+++ b/include/crt.c
@@ -235,6 +235,7 @@ w0:
 
 // Clear BSS Segment
 
+#ifndef NOBSSCLEAR
 		lda #<BSSStart
 		sta ip
 		lda #>BSSStart
@@ -264,7 +265,8 @@ l2:		dey
 		sta (ip), y
 		bne l2
 w2:
-
+#endif
+#ifndef NOZPCLEAR
 		ldx #<ZeroStart
 		cpx #<ZeroEnd
 		beq	w3
@@ -273,6 +275,7 @@ l3:		sta $00, x
 		cpx #<ZeroEnd
 		bne l3
 w3:
+#endif
 		lda	#<StackEnd - 2
 		sta	sp
 		lda	#>StackEnd - 2