download the free metalink assembler from http://www.semiconductors.philips.com/products/...index.html
the manual you download is very extensive

also the 8051 family programmers guide and instruction set you can find as one of the 3 80C51_FAM_*.pdf documents at:
http://www.semiconductors.philips.com/products/...other.html
will be helpful, the other 2 will help you understand the intricacies of the '51

Have fun,

Erik