If you are using the Keil C51 compiler clearly the target processor is an 8051 type part. If you read up on the architecture of this part you will find that there are four register banks.

There are two places that you can go to read up about the USING keyword, its usage, and the concepts behind register banks. The first place is the Keil C51 users guide. This manual should have come with your copy of the C51 compiler package. If you lost your C51 manuals then Keil is rather kind and offers on-line copies that you can read at their web site at http://www.keil.com. The second place to look up information on the register banks and how they work is contained in 8051 architecture documents that are commonly referred to here as the "8051 bible". You can access these on the Links section of this web site at the page http://www.8052.com/links.phtml Look to the "Interesting Sites" section.

Michael Karas