Added error code for stack busy

src/devices/system.c

@@ -17,7 +17,9 @@ WITH REGARD TO THIS SOFTWARE.
 static const char *errors[] = {
 	"underflow",
 	"overflow",
-	"division by zero"};
+	"division by zero",
+	"busy"
+};
 
 static void
 system_print(Stack *s, char *name)

src/uxn.c

@@ -88,7 +88,7 @@ uxn_eval(Uxn *u, Uint16 pc)
 		case 0x1f: /* SFT */ POP8(a) POP(b) PUSH(src, b >> (a & 0x0f) << ((a & 0xf0) >> 4)) break;
 		}
 	}
-	return 1;
+	return (u->wst->ptr || u->rst->ptr) ? uxn_halt(u, instr, 4, pc - 1) : 1;
 err:
 	return uxn_halt(u, instr, errcode, pc - 1);
 }