Intel left the opcode 0xA5 undefined in the original 8051.
Since then, various manufacturers (including Intel) have put it to various uses.

All this is chip-dependent, to you will have to read the documentation for your particular chip to determine what - if any - function has been assigned to this opcode.

Here's a few I prepared earlier:

http://www.8052.com/forum/read.phtml?id=14781

http://www.8052.com/forum/read.phtml?id=23182

http://www.8052.com/forum/read.phtml?id=22325

http://www.8052.com/forum/read.phtml?id=12617