"You can read 8051 instruction set manual if you like"

If you need to be asking this sort of question, then it is essential that you read and understand the instruction set manual!

A good source is the 80C51 Family Programmer's Guide and Instruction Set from the Philips site:
http://www.semiconductors.philips.com/acrobat/various/80C51_FAM_PROG_GUIDE_1.pdf

While you're there, you ahould also download and read the other 80C51_FAM_*.pdf documents.

See also the Intel Embedded Controllers site:
http://developer.intel.com/design/embcontrol/