The GFA BASIC interpreter is compact and reasonably fast, and was shipped with a runtime that could be distributed freely with one's programs. When a compiler was made available, execution speed could be increased by approximately a factor of 2. GFA BASIC allowed extreme optimisations for execution speed, supporting some direct assembler-level calls, and even the ability to embed sections of assembler code directly within the BASIC source code. GFA BASIC integrated neatly into GEM and TOS, the Atari ST's operating system, providing menus, dialog boxes, and mouse control. (See WIMP interface.)