I recently noticed that my TMS99105 project at hack-a-day has more followers than my successor project to this one, so I thought I will just provide here a quick link to that project. I understand that some people may be interested in the TMS99105 project in it's own right, and I really like this project too, it came well out. 

After "completing" this project to a functional state I've worked on my VHDL implementation of the TMS9900 core, allowing the entire TI-99/4A to be re-created in the FPGA. That design is largely compatible with this one, all the peripherals like the TMS9918 and TMS9919 are the same in both projects. The source code to both projects is in the same repository,  with the TMS9900 VHDL implementation being in the "soft-cpu" branch.

Successor project reference

I may still make updates to this project too, as I really like the shiny TMS99105, and also because it is currently still more tested than the successor project. So as I make updates to the peripherals etc I will probably bring those improvements over to this project too.