in the SiLabs forum someone showed SFR = -n
I have never used such and became curuous. I searched the Keil documentation but could nof find it.
You need to be careful to flag it as a C51 question - there's nothing in the forum to tell that any more, and C51 questions are very rare.
I agree, the documentation omits this:
http://www.keil.com/support/man/docs/c51/c51_le_sfr.htm
But it wouldn't make sense to have SFRs signed - as that would imply sign extension, etc.