Search the archives, as it was discussed not so long ago.

Basically, the 0xA5 opcode was undefined in the original Intel 8051.
Since then, various manufacturers have used it for various purposes in their variants & derivatives.

Triscend, among others, have used it as their Breakpoint instruction; ie, whenever the opcode 0xA5 is fetched, a Break occurs.

That's about as much as I know; for any more detail, you'll have to delve into the datasheet yourself.