The TMS9900 is the brain of the TI99. It is a 16-bit microprocessor and was one of the first 16-bit prococessors available in a home computer. It can address up to 65,536 bytes of memory, and performs both word and byte operations. It has 69 instructions and 7 addressing modes.

The chip on top is the older case, and can be found in the TI-99/4.

This was a very large chip and of course is where the name for the TI99 came from.

An ad for the TMS9900 which came out in 1977