It might be old news but it's always a good excuse for me to beg that they open source QBasic. That would be a glorious day. (Or if Borland/whoever they ended up being opened the Turbo Pascal 7 source code.)
Turbo Pascal Version 6 source code is online and easy to find. It is not the original source code written by Anders Hejlsberg but a fantastic reverse engineering job.
Turbo Pascal 7 added just a little bit to the compiler (like "break", "continue", "inhertied") and added an optimize step to the code generator (eliminate multible "les di,…" for constant pointers). But in order to study the compiler version 6 should do it.