diff --git a/common/common.asm b/common/common.asm
index 808e002..c8bbed9 100644
--- a/common/common.asm
+++ b/common/common.asm
@@ -116,27 +116,12 @@ _SET	.(		; SET r aabbcc.dd	1r dd cc bb aa	Rr <- aabbcc.dd	- set register
 _1	RTS		; done
 .)
 
-_POP	.(		; POP r			2r		Rr <- RS	- pop from stack
+_PSH	.(		; PSH r			2r		RS <- Rr	- push onto stack
 	LDY _RSI	; get register stack index
-	DEY		; transfer four bytes over
-	LDA _RS,Y
-	STA _R0+3,X
-	DEY
-	LDA _RS,Y
-	STA _R0+2,X
-	DEY
-	LDA _RS,Y
-	STA _R0+1,X
-	DEY
-	LDA _RS,Y
-	STA _R0,X
-	STY _RSI	; update register stack index
-	RTS
-.)
-
-_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
@@ -152,6 +137,26 @@ _PSH	.(		; PSH r			3r		RS <- Rr	- push onto stack
 	RTS
 .)
 
+_POP	.(		; POP r			3r		Rr <- RS	- pop from stack
+	LDY _RSI	; get register stack index
+	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
+	LDA _RS,Y
+	STA _R0+2,X
+	DEY
+	LDA _RS,Y
+	STA _R0+1,X
+	DEY
+	LDA _RS,Y
+	STA _R0,X
+	STY _RSI	; update register stack index
+	RTS
+.)
+
 _EXC	.(		; EXC r			4r		Rr <-> RS	- exchange Rr with stack
 	LDY _RSI	; RS to RD
 	LDA _RS-1,Y
diff --git a/common/common.h b/common/common.h
index fa732bb..432002c 100644
--- a/common/common.h
+++ b/common/common.h
@@ -14,8 +14,8 @@
 ; Instructions
 
 ; SET r aabbcc.dd	1r dd cc bb aa	Rr <- aabbccdd	- set register
-; POP r			2r		Rr <- RS	- pop from stack
-; PSH r			3r		RS <- Rr	- push onto stack
+; PSH r			2r		RS <- Rr	- push onto stack
+; POP r			3r		Rr <- RS	- pop from stack
 ; EXC r			4r		Rr <-> RS	- exchange Rr with stack
 ; INR r			5r		Rr <- Rr + 1.0	- increment register
 ; DCR r			6r		Rr <- Rr - 1.0	- decrement register
@@ -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
@@ -114,8 +115,8 @@ _SVI_C	= $0e
 _CMR_C	= $0f
 
 _SET_C	= $10
-_POP_C	= $20
-_PSH_C	= $30
+_PSH_C	= $20
+_POP_C	= $30
 _EXC_C	= $40
 _INR_C	= $50
 _DCR_C	= $60
diff --git a/common/macros.h b/common/macros.h
index fc2ad78..e3d8951 100644
--- a/common/macros.h
+++ b/common/macros.h
@@ -75,8 +75,8 @@
 #define SVI(p, q)	.BYTE _SVI_C, p * 16 + q
 #define CMR(p, q)	.BYTE _CMR_C, p * 16 + q
 #define SET(r, v)	.BYTE _SET_C + r, _SET_V(#v)
-#define POP(r)		.BYTE _POP_C + r
 #define PSH(r)		.BYTE _PSH_C + r
+#define POP(r)		.BYTE _POP_C + r
 #define EXC(r)		.BYTE _EXC_C + r
 #define INR(r)		.BYTE _INR_C + r
 #define DCR(r)		.BYTE _DCR_C + r