I agree with the other postings here - writing a decent compiler for the 8051 is a very big job.

If you have an application to develop, just buy a good compiler & get on with the job.

However, if you are doing this for fun (instead of for a living) don't be discouraged from having a try. Writing a simple compiler, with a small grammar, will teach you not only the basics of how compilers work, but will also require/help you to learn the instruction set of the 8051.

A simple, step by step, paper which may guide you is "Let's Build a Compiler" by Dr Jack Crenshaw. This can be found at http://compilers.iecc.com/crenshaw/

Derek