diff --git a/common/common.asm b/common/common.asm
index 45219b9..08b986a 100644
--- a/common/common.asm
+++ b/common/common.asm
@@ -242,6 +242,22 @@ _DCR	.(		; DCR r			6r		Rr <- Rr - 1.0	- decrement register
 .)
 
 _TST	.(		; TST r			7r		F <- Rr <=> 0.0	- test register
+	LDA _F
+	AND #_MSK_T	; clear TST bits
+	STA _F
+	LDA _R0+3,X	; check highest byte
+	BMI _1		; is negative
+	ORA _R0+2,X	; could be positive or zero, OR with all other bytes
+	ORA _R0+1,X
+	ORA _R0,X
+	BNE _2		; is positive
+	LDA #_F_Z	; set zero flag
+	BNE _3
+_1	LDA #_F_N	; set negative flag
+	BNE _3
+_2	LDA #_F_P	; set positive flag
+_3	ORA _F
+	STA _F
 	RTS
 .)
 
diff --git a/common/common.h b/common/common.h
index 432002c..08e464e 100644
--- a/common/common.h
+++ b/common/common.h
@@ -138,4 +138,7 @@ _MNS_1	= %11111100	; i.e. the $FC part of $FFFFFC00
 
 _MSK_O	= %11000000	; mask for overflow
 
+; mask for TST
+_MSK_T	= (_F_Z + _F_P + _F_N)^$FF
+
 #endif /* __COMMON_H */
diff --git a/common/page6.src b/common/page6.src
index 721a850..cb6cdbb 100644
--- a/common/page6.src
+++ b/common/page6.src
@@ -15,6 +15,9 @@ HDR(DEMO)
 	DCR(R3)
 	INR(R0)
 	DCR(R1)
+	TST(R4)
+	TST(R2)
+	TST(R3)
 	ESC
 	BRK