Few days ago, I got a problem with the 6.20 version of C51 keil Software, like you can see on http://www.keil.com/forum/msgpage.asp?MsgID=3260 . Keil support found what's append and send me a mail with a new version of OH51 (v 2.6) that correct the error due to the long NAME statement that I am using. And, thanks Andrew to help me to find my problem.
Similarly, when I downloaded "C51 v6.20," it contained C51.exe at v6.20; I have since received an update to v6.20b, and now I hear that there's a v6.20c! Where did you download this from? The only V6.20 update that's been on http://www.keil.com has included C51 V6.20c. Jon