diff --git a/src/emulation.c b/src/emulation.c
index 97ba34d..a342d95 100644
--- a/src/emulation.c
+++ b/src/emulation.c
@@ -280,6 +280,7 @@ void
 emul_stp(rk65c02emu_t *e, void *id, instruction_t *i)
 {
 	e->state = STOPPED;
+	e->stopreason = STP;
 }
 
 /* STA - store accumulator */
diff --git a/src/rk65c02.c b/src/rk65c02.c
index 77be08b..b2b59d7 100644
--- a/src/rk65c02.c
+++ b/src/rk65c02.c
@@ -45,6 +45,7 @@ rk65c02_start(rk65c02emu_t *e) {
 			printf("unimplemented opcode %X @ %X\n", i.opcode,
 			    e->regs.PC);
 			e->state = STOPPED;
+			e->stopreason = EMUERROR;
 		}
 	}
 }
diff --git a/src/rk65c02.h b/src/rk65c02.h
index 265080b..a21128e 100644
--- a/src/rk65c02.h
+++ b/src/rk65c02.h
@@ -6,9 +6,17 @@
 typedef enum {
 	STOPPED,
 	RUNNING,
-	STEPPING
+	STEPPING	/* XXX: how to implement? */
 } emu_state_t;
 
+typedef enum
+	STP,		/* due to 65C02 STP instruction */
+	BREAKPOINT,	/* due to breakpoint set */
+	WATCHPOINT,	/* due to watchpoint set */
+	HOST,		/* due to host stop function called */
+	EMUERROR	/* due to emulator error */
+} emu_stop_reason_t;
+
 struct reg_state {
 	uint8_t A;      /* accumulator */
 	uint8_t X;      /* index X */
@@ -41,6 +49,7 @@ struct rk65c02emu {
 	emu_state_t state;
 	bus_t *bus;
 	reg_state_t regs;
+	emu_stop_reason_t stopreason;
 };
 
 typedef struct rk65c02emu rk65c02emu_t;