He sat at the desk writing most of the day.句中writing是从句还是非谓语

He sat at the desk writing most of the day.
writing most of the day在这里做伴随状语.
可以写为
He sat at the desk.He is writing most of the day.
因为write在这里的主语是He表主动,所以后面表伴随状语加ing.