Mainframe computers were very large indeed,often occupying w

Mainframe computers were very large indeed,often occupying whole air-conditioned rooms,employing full-time technicians and run on specially-writtten software.
这个句子里occupying employing run 都做Mainframe的定语..
为什么前两个是现在分词..run是过去分词..
他们怎么看出来是并列的啊..
如果run和were并列..那么occupy和employ也可以看作是和were并列的了么