This is showh clearly in the Philips 80C51 Family Architecture document:
http://www.semiconductors.philips.com/acrobat/various/80C51_FAM_ARCH_1.pdf

There are 3 documents in the 80C51_FAM_*.pdf series; you should download and read all three before you do anything else!
Find them at:
http://www.semiconductors.philips.com/products/all_other.html.

You must also, of course, thoroughly read & understand the specific datasheet for your particular variant.
And look for any App Notes on the vendor's website.

Have you also checked the 'Tutorials' section of this site?
See the button to the left