diff --git a/common/common.asm b/common/common.asm
index 808e002..f607661 100644
--- a/common/common.asm
+++ b/common/common.asm
@@ -118,7 +118,9 @@ _1	RTS		; done
 
 _POP	.(		; POP r			2r		Rr <- RS	- pop from stack
 	LDY _RSI	; get register stack index
-	DEY		; transfer four bytes over
+	BNE _1		; all good, something can be popped off the stack
+	BRK		; next pop will cause a stack underflow, abort and call exception handler (TODO)
+_1	DEY		; transfer four bytes over
 	LDA _RS,Y
 	STA _R0+3,X
 	DEY
@@ -136,7 +138,10 @@ _POP	.(		; POP r			2r		Rr <- RS	- pop from stack
 
 _PSH	.(		; PSH r			3r		RS <- Rr	- push onto stack
 	LDY _RSI	; get register stack index
-	LDA _R0,X	; transfer four bytes over
+	CPY #_RSS	; compare against limit
+	BCC _1		; still room, all okay
+	BRK		; next push will cause a stack overflow, abort and call exception handler (TODO)
+_1	LDA _R0,X	; transfer four bytes over
 	STA _RS,Y
 	INY
 	LDA _R0+1,X
diff --git a/common/common.h b/common/common.h
index fa732bb..c91bb2b 100644
--- a/common/common.h
+++ b/common/common.h
@@ -89,11 +89,12 @@ _IDX	= _ACC + 1	; saved index X to restore
 _IDY	= _IDX + 1	; saved index Y to restore
 _PS	= _IDY + 1	; saved processor status to restore
 
-; 256 bytes of page two
+; 224 bytes of page two
 _RS	= $200		; register stack
+_RSS	= (FN_FX - _RS)	; register stack size
 
-; 64 bytes of page three
-FN_FX	= $300		; list of system and user functions
+; last 32 bytes of page two
+FN_FX	= $2E0		; list of system and user functions
 
 ; function constants
 _ESC_C	= $00