Desk accessory

A desk accessory (DA) in computing is a small transient or auxiliary application that can be run concurrently in a desktop environment with any other application on the system. Early examples, such as Sidekick and Macintosh desk accessories, used special programming models to provide a small degree of multitasking on a system that initially did not have any other multitasking ability.

Early personal information managers, such as Norton Desktop and Borland's Sidekick, provided pop-up calculator, alarm, calendar and other functions for single-tasking operating systems like MS-DOS using terminate and stay resident techniques.

Introduced in 1984, as part of the operating system for the Apple Macintosh computer, a Desk Accessory (DA) was a piece of software written as a device driver, conforming to a particular programming model. The purpose of this model was to permit very small helper-type applications to be run concurrently with any other application on the system. This provided a small degree of multitasking on a system that initially did not have any other multitasking ability.

DAs were implemented as a special class of driver. It was installed in the driver queue, and given time periodically and co-operatively as a result of the host application calling SystemTask() within its main loop. A DA was permitted to have a user interface as long as it was confined to one main window. A special window frame with black title bar and rounded corners was reserved for the use of DAs so that the user could distinguish it from the windows of the hosting application.

Typical early DAs included the Calculator and Alarm Clock. The Control panel, Chooser, and Scrapbook were initially implemented as DAs. Third-party DAs such as spelling checkers could be purchased. It was considered hard to write a DA, especially early on when there was little in the way of developer tools. However, since on the early Mac OS drivers did not have any special privileges, writing a DA was, with practice, no more difficult than any other application.

...
Wikipedia