This short procedure reads unit from file to symbol tables. If header is not valid it reports error.

This is where the actual compilation starts. Turbo Pascal first clears and prepares some variables and data structures and then compiles the module (unit or program). If program was compiled then compiler checks for indirect references and calls a procedure to link the code and data and to create the executable file.

Every Turbo Pascal module (program or unit) can use other units. Processing them is one of the trickiest parts of Turbo Pascal compiler.

Each used unit is checked if it is already loaded. If it is not it needs to be found and checked if recompilation is needed

This is the Turbo Pascal System unit. This unit implements Turbo Pascal’s runtime library. When compiling this unit, the compiler automatically includes a boot-strap symbol table (SYSTEM.TPS) which declares all built-in types, procedures, and functions.