diff --git a/common/common.asm b/common/common.asm
index 08b986a..742c50e 100644
--- a/common/common.asm
+++ b/common/common.asm
@@ -269,12 +269,101 @@ _HEX	.(		; HEX r			9r		Rr <- hex(Rr)	- convert Rr from decimal ######.## to hex
 	RTS
 .)
 
+_GETPQ	.(		; sets X as p register and Y as q register, advances PC
+	LDY #0
+	LDA (_PC),Y	; get source registers
+	LSR
+	LSR
+	AND #_MSK_R	; p register
+	TAX
+	LDA (_PC),Y
+	ASL
+	ASL
+	AND #_MSK_R	; q register
+	TAY
+	LDA _R0+3,X
+	AND #_MSK_O	; check for existing overflow condition
+	BEQ _1		; sign and overflow are both clear
+	EOR #_MSK_O
+	BEQ _1		; sign and overflow are both set
+	BRK		; an operand is in an overflow condition, abort and call exception handler (TODO)
+_1	LDA _R0+3,Y
+	AND #_MSK_O	; check for existing overflow condition
+	BEQ _2		; sign and overflow are both clear
+	EOR #_MSK_O
+	BEQ _2		; sign and overflow are both set
+	BRK		; an operand is in an overflow condition, abort and call exception handler (TODO)
+_2	INC _PCL	; advance PC
+	BNE _3
+	INC _PCH
+_3	RTS
+.)
+
+_TRFDR	.(		; transfers RD to X as r register, updates overflow flag
+	LDA _RD		; transfer result to Rr
+	STA _R0,X
+	LDA _RD+1
+	STA _R0+1,X
+	LDA _RD+2
+	STA _R0+2,X
+	LDA _RD+3
+	STA _R0+3,X
+	AND #_MSK_O	; check for overflow
+	BEQ _4
+	EOR #_MSK_O
+	BEQ _4
+_3	LDA _F		; set overflow
+	ORA #_F_O
+	STA _F
+	BNE _5
+_4	LDA _F		; clear overflow
+	AND #_F_O^$FF
+	STA _F
+_5	RTS
+.)
+
 _ADD	.(		; ADD r pq		ar pq		Rr <- Rp + Rq	- addition
-	RTS
+	TXA
+	PHA		; save r register for later
+	JSR _GETPQ
+	CLC		; set RD to Rp + Rq
+	LDA _R0,X
+	ADC _R0,Y
+	STA _RD		
+	LDA _R0+1,X
+	ADC _R0+1,Y
+	STA _RD+1		
+	LDA _R0+2,X
+	ADC _R0+2,Y
+	STA _RD+2		
+	LDA _R0+3,X
+	ADC _R0+3,Y
+	STA _RD+3
+	PLA		; get r register
+	TAX
+	JMP _TRFDR	; transfer RD to r register, let it handle the return
 .)
 
 _SUB	.(		; SUB r pq		br pq		Rr <- Rp - Rq	- subtraction
-	RTS
+	TXA
+	PHA		; save r register for later
+	JSR _GETPQ
+	SEC		; set RD to Rp - Rq
+	LDA _R0,X
+	SBC _R0,Y
+	STA _RD		
+	LDA _R0+1,X
+	SBC _R0+1,Y
+	STA _RD+1		
+	LDA _R0+2,X
+	SBC _R0+2,Y
+	STA _RD+2		
+	LDA _R0+3,X
+	SBC _R0+3,Y
+	STA _RD+3
+	PLA		; get r register
+	TAX
+	JMP _TRFDR	; transfer RD to r register, let it handle the return
 .)
 
 _MUL	.(		; MUL r pq		cr pq		Rr <- Rp * Rq	- multiplication
diff --git a/common/common.h b/common/common.h
index c98b000..c959ce7 100644
--- a/common/common.h
+++ b/common/common.h
@@ -7,9 +7,9 @@
 ; calculation.
 
 ; Largest value:              $3fffffff or  1048575.999(5)
-; Smallest value:             $c0000001 or -1048575.998(5) <- note 998(5)
-; Largest value for DEC/HEX:  $3d08ffff or   999999.999(5)
-; Smallest value for DEC/HEX: $c2f70000 or  -999999.999(5)
+; Smallest value:             $c0000000 or -1048576.000(0)
+; Largest value for DEC/HEX:  $3d08ffff or   999999.999
+; Smallest value for DEC/HEX: $c2f70001 or  -999999.999
 
 ; Instructions
 
@@ -47,7 +47,7 @@
 ; CMR pq		0f pq		F <- Rp <=> Rq	- compare registers
 
 ; 64 bytes in page zero for common registers
-_R0	= $C0
+_R0	= $c0
 _R1	= _R0 + 4
 _R2	= _R1 + 4
 _R3	= _R2 + 4
@@ -94,7 +94,7 @@ _RS	= $200		; register stack
 _RSS	= (FN_FX - _RS)	; register stack size
 
 ; last 32 bytes of page two
-FN_FX	= $2E0		; list of system and user functions
+FN_FX	= $2e0		; list of system and user functions
 
 ; function constants
 _ESC_C	= $00
@@ -134,11 +134,12 @@ _EXT_C	= $f0
 
 ; plus and minus 1 for increment and decrement
 _PLS_1	= %00000100	; i.e. the $04 part of $00000400
-_MNS_1	= %11111100	; i.e. the $FC part of $FFFFFC00
+_MNS_1	= %11111100	; i.e. the $fc part of $fffffc00
 
 _MSK_O	= %11000000	; mask for overflow
+_MSK_R	= %00111100	; mask for registers
 
 ; mask for TST
-_MSK_T	= (_F_Z + _F_P + _F_N)^$FF
+_MSK_T	= (_F_Z + _F_P + _F_N)^$ff
 
 #endif /* __COMMON_H */
diff --git a/common/macros.h b/common/macros.h
index e3d8951..ea524a2 100644
--- a/common/macros.h
+++ b/common/macros.h
@@ -83,7 +83,7 @@
 #define TST(r)		.BYTE _TST_C + r
 #define DEC(r)		.BYTE _DEC_C + r
 #define HEX(r)		.BYTE _HEX_C + r
-#define ADD(r)		.BYTE _ADD_C + r
+#define ADD(r, p, q)	.BYTE _ADD_C + r, p * 16 + q
 #define SUB(r, p, q)	.BYTE _SUB_C + r, p * 16 + q
 #define MUL(r, p, q)	.BYTE _MUL_C + r, p * 16 + q
 #define DIV(r, p, q)	.BYTE _DIV_C + r, p * 16 + q
diff --git a/common/page6.src b/common/page6.src
index cb6cdbb..1ca4d44 100644
--- a/common/page6.src
+++ b/common/page6.src
@@ -5,19 +5,10 @@
 
 HDR(DEMO)
 	CMN
-	SET(R0, +1048575.999)
-	SET(R1, -1048575.999)
-	SET(R2, 0.0)
-	SET(R3, 0.0)
-	INR(R0)
-	DCR(R1)
-	INR(R2)
-	DCR(R3)
-	INR(R0)
-	DCR(R1)
-	TST(R4)
-	TST(R2)
-	TST(R3)
+	SET(R0, 1/1024)
+	SET(R1, -1/1024)
+	ADD(R2, R0, R1)
+	SUB(R3, R0, R1)
 	ESC
 	BRK
 
diff --git a/xa-pre-process/main.c b/xa-pre-process/main.c
index ad4d7e8..2d2f30c 100644
--- a/xa-pre-process/main.c
+++ b/xa-pre-process/main.c
@@ -24,7 +24,6 @@ int main(int argc, char **argv)
 			for (i = 0; i < count; ++i)
 			{
 				int j, sign;
-				unsigned long working;
 				const char *s = "", *p, *q;
 				switch (tokens[i].type)
 				{
@@ -41,14 +40,14 @@ int main(int argc, char **argv)
 						sign = result < 0? -1: +1;
 						if (sign < 0) result = -result;
 						result += 1 << (INT_FRAC - EXP_FRAC - 1);
-						if (sign < 0) result = -result;
 						/* Normalize */
-						working = (unsigned long)((result >> (INT_FRAC - EXP_FRAC)) % (1 << EXP_FULL));
+						result >>= (INT_FRAC - EXP_FRAC);
+						if (sign < 0) result = -result;
 						/* Output in .BYTE format */
 						for (j = 0; j < EXP_FULL; j += CHAR_BIT)
 						{
-							printf("%s$%02lX", s, working & 0xff);
-							working >>= CHAR_BIT;
+							printf("%s$%02llX", s, result & 0xff);
+							result >>= CHAR_BIT;
 							s = ", ";
 						}
 						break;