diff --git a/code/base/base.h b/code/base/base.h
index 36ca77c..9a5d733 100644
--- a/code/base/base.h
+++ b/code/base/base.h
@@ -82,6 +82,7 @@ public:
 	};
 	virtual void test_callback(const wchar_t* name/*test name*/, test_event ev, void* data, size_t flag) = 0;
 
+	//  All IO operations are blocking
 	enum usb_io_type
 	{
 		USB_IO_BULK_READ = 0,
@@ -89,8 +90,8 @@ public:
 		USB_IO_INTERRUPT_READ,
 		USB_IO_INTERRUPT_WRITE,
 	};
-	virtual int io_bulk_int(usb_io_type type, void* buf, size_t* len/*[in]-size of buf, [out]-data bytes in buf*/);
-	virtual int io_control(int type, int req, int val, int ind, void* buf, size_t* len/*[in]-in data sizef, [out]-transferred bytes*/);
+	virtual int io_bulk_int(usb_io_type type, void* buf, size_t* len/*[in]-size of buf, [out]-data bytes in buf*/) = 0;
+	virtual int io_control(int type, int req, int val, int ind, void* buf, size_t* len/*[in]-in data sizef, [out]-transferred bytes*/) = 0;
 };
 
 #ifdef TEST_DLL