A computer used by Bill Gates in his last major work as a software developer is up for auction. The TRS-80 Model 100 was one of the earlier notebook computers.

Although Gates had performed management roles since Microsoft’s founding in 1975, he continued as an active member on development teams until 1983. While he occasionally worked on development after this, the TRS-80 Model 100 was the last project where he was officially a team member.

The computer squeezed a full keyboard and LCD display into a 30 cm x 21.5 cm device that could operate off four AA batteries. Gates worked with Jey Suzuki on the ROM firmware that allowed an instant boot with the last-used application ready for immediate use.

Gates later described this as “a cool user interface, because although most of the code is a BASIC Interpreter, we did this little file system where you never had to think about saving anything. You just had this menu where you pointed to things. “

The unit up for auction, which remains operational today, has a Post-In note with a handwritten message from Gates saying “I don’t need this anymore. It is MS property. I am going to start using the model 200.”

The online auction ends on Friday. At the time of writing, the top bid was $4,900.