There are two basic ways to build a new compiler:
If you're writing a new compiler for an established language like C, use an existing compiler from a different vendor to build your new compiler. For example, you could use the C compiler shipped with HP-UX to build gcc.
If you're writing a compiler for a new language, start by implementing a very simple compiler in a different language (the first C compiler was written in PDP-11 assembler). This initial compiler will only recognize a small subset of the target language; basically enough to do some file I/O and some simple statements. Write a new compiler in the target language subset and build it with your first compiler. Now write a slightly more capable compiler that can recognize a larger subset of the target language, and build it with the second compiler. Repeat the process until you have a compiler capable of recognizing the full target language.
