8051, 8052, 8031, 8032 are all members of the original Intel MCS-51 family.
They all have the same architecture & instruction set, just slightly different complements of on-chip memory, peripherals, etc.
See this article by Triscend for a description:
http://portal.knowledgebase.net/display/1...p?aid=9461

Look for the "bible" links on the 'Links' page here for the definitive architecture & instruction set documnents.

AT89C52 is another manufacturer's product, based on the MCS-51 architecture.

A lot of the MCS-51 derivatives by other manufacturers have enhanced performance, by reducing the number of clock cycles needed to execute instructions - but the core architecture & instruction set remain the same.

MCS-251 is Intel's "next generation" architecture - it is a development of the MCS-51 architecture, and is backward-compatible, but is not the same.

Try Intel's developer site http://developer.intel.com for info from the horse's mouth!