The System unit is the Turbo Pascal run-time library.
■ SYSTEM.TPW Windows
■ SYSTEM.TPP DOS protected mode
■ SYSTEM.TPU DOS real mode
It implements low-level run-time support routines for all built-in features, such as file I/O, string-handling, floating point, and dynamic memory allocation.
All units and programs automatically use the System unit, so you never need to refer to it in a uses clause.