DoubleDuty is a utility that uses the full 128K of a TRS‑80 Model 4 to allow more than one program to be run at one time. Introduced by Radio Shack in 1984, DoubleDuty was written by Randy Cook, the author of Model I TRSDOS and VTOS.
Two page DoubleDuty advertisement from the January 1985 issue of 80 Micro
DoubleDuty works by creating three sections, called partitions, out of the Model 4 memory. Partitions one and two each contain a complete Model 4 TRSDOS system. The third partition is reserved for running TRSDOS library commands.
MISOSYS advertisement from the February 1990 issue of Computer News 80
Because partitions one and two exist independently, a different program can be run in each partition. For example, you can run TMaker in partition one and Scripsit Pro in partition two. Both programs operate independently and are unaware that any other program is running. Switching between partitions is possible using special key combinations:
- CAPS plus F1 switches to partition one
- CAPS plus F2 switches to partition two
- CAPS plus F3 switches to partition three (for running library commands)
- CLEAR plus CAPS plus F1 performs a soft reset on partition one
- CLEAR plus CAPS plus F2 performs a soft reset on partition two
In 1989, Roy Soltoff of MISOSYS released a updated version of DoubleDuty. That new version could use not just the 128K of memory possible with an unmodified Model 4, but also additional memory available when using one of the many memory expansions add-ons.