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  Now we move on to the third element of ROM: the ROM BASIC. The ROM BASIC
  acts in two ways. First, it provides the core of the BASIC language, which
  includes most of the commands and the underlying foundation--such as
  memory management--that BASIC uses. The disk versions of interpreted
  BASIC, which are found in the program files BASIC.COM and BASICA.COM, are
  essentially supplements to ROM BASIC, and they rely on ROM BASIC to get
  much of their work done. The second role of ROM BASIC is to provide what
  IBM calls "cassette" BASIC--the BASIC that is activated when you start up
  your computer without a disk.

  Whenever you use any of the interpreted, disk-based BASICs, the ROM BASIC
  programs are also used--although there's nothing to make you aware of it.
  On the other hand, compiled BASIC programs don't make use of the ROM
  BASIC.