AsmOS - z80 Starter OS  AsmOS is a bare-bones operating system for the TI-73, TI-83+, TI-83+ SE, TI-84+, and TI-84+ SE. It is a great starting point to develop an OS from. AsmOS will initialize hardware, boot up, and enter an infinite loop - it's your job to replace this loop with an awesome OS. It comes with a great build system that uses Spasm and Build8xu on Windows, in addition to a custom build tool.
Download it today to get started on that OS you always wanted to make!