I fixed and added a bunch of things:

• Added GROM support (currently 64K GROM region supported)
  • GROM address write
  • GROM address read
  • GROM data read - address mapping to SRAM at 192K
• Added border color support to TMS9918 graphics mode 1 clone
• Added VDP read back feature, including address auto increment for reads (it was there already for writes)
• Fixed video active region bugs
• Fixed character cell too narrow bug
• Added write protection to bottom 8K of RAM to make it look like ROM.
  • Contents loaded from PC, TMS99105 writes to that area are ignored. TI-99/4A code corrupts the ROM if writes pass through...

With those changes I was able to successfully boot the system with TI's original ROM and GROM contents. Yippee! The picture below shows that. I don't know how much this works, since there is no user input support yet here... Need to add some sort of keyboard emulation next.