diff --git a/src/instruction.c b/src/instruction.c
index 2333349..3de98ff 100644
--- a/src/instruction.c
+++ b/src/instruction.c
@@ -325,8 +325,8 @@ instruction_data_write_1(rk65c02emu_t *e, instrdef_t *id, instruction_t *i, uint
 		 * PC which is handled within emulation of a given opcode.
 		 */
 	default:
-		rk65c02_log(LOG_ERROR,
-		    "unhandled addressing mode for opcode %x\n", i->opcode);
+		rk65c02_panic(e, "unhandled addressing mode for opcode %x\n",
+		    i->opcode);
 		break;
 	}
 }
@@ -389,8 +389,8 @@ instruction_data_read_1(rk65c02emu_t *e, instrdef_t *id, instruction_t *i)
 		 * PC which is handled within emulation of a given opcode.
 		 */
 	default:
-		rk65c02_log(LOG_ERROR,
-		    "unhandled addressing mode for opcode %x\n", i->opcode);
+		rk65c02_panic(e, "unhandled addressing mode for opcode %x\n",
+		    i->opcode);
 		break;
 	}
 
diff --git a/src/rk65c02.c b/src/rk65c02.c
index 49a2df5..ade5ae2 100644
--- a/src/rk65c02.c
+++ b/src/rk65c02.c
@@ -2,6 +2,7 @@
 #include <stdint.h>
 #include <stdlib.h>
 #include <stdbool.h>
+#include <stdarg.h>
 
 #include <errno.h>
 #include <assert.h>
@@ -129,11 +130,8 @@ rk65c02_exec(rk65c02emu_t *e)
 		 * maturity, let's catch them here to help iron out the
 		 * bugs.
 		 */
-		rk65c02_log(LOG_WARN, "invalid opcode %X @ %X\n",
+		rk65c02_panic(e, "invalid opcode %X @ %X\n",
 		    i.opcode, e->regs.PC);
-
-		e->state = STOPPED;
-		e->stopreason = EMUERROR;
 	}
 
 	if (e->trace)
@@ -235,6 +233,22 @@ rk65c02_regs_string_get(reg_state_t regs)
 
 	return str;
 }
+
+void
+rk65c02_panic(rk65c02emu_t *e, const char* fmt, ...) 
+{
+	va_list args;
+
+	va_start(args, fmt);
+	rk65c02_log(LOG_CRIT, fmt, args);
+	va_end(args);
+
+	e->state = STOPPED;
+	e->stopreason = EMUERROR;
+
+	/* TODO: run some UI callback. */
+}
+
 /*
 int
 main(void)
diff --git a/src/rk65c02.h b/src/rk65c02.h
index 259b11e..c678478 100644
--- a/src/rk65c02.h
+++ b/src/rk65c02.h
@@ -82,7 +82,8 @@ void rk65c02_step(rk65c02emu_t *, uint16_t);
 char *rk65c02_regs_string_get(reg_state_t);
 void rk65c02_dump_regs(reg_state_t);
 void rk65c02_dump_stack(rk65c02emu_t *, uint8_t);
-void rk65c02_irq(rk65c02emu_t *e);
+void rk65c02_irq(rk65c02emu_t *);
+void rk65c02_panic(rk65c02emu_t *, const char*, ...);
 
 #endif