How many registers present in AT89C51 and AT89S52
as many as the datasheet says.
I do not know if Atmel make "exceptions from the bible" or "complete" datasheets, but till you are familiar with "the bible" you'll be lost anyhow.
Erik
here are the links to "the bible" Chapter 1 - 80C51 Family Architecture: www.nxp.com/acrobat_d...ARCH_1.pdf
Chapter 2 - 80C51 Family Programmer’s Guide and Instruction Set: www.nxp.com/acrobat_d...UIDE_1.pdf
Chapter 3 - 80C51 Family Hardware Description: www.nxp.com/acrobat_d...WARE_1.pdf