While the CPU is a better fit than the 8 bit contemporaries, the 16kb of working memory is going to be a struggle.
It's cool because the registers are all in RAM, with a "workspace pointer" on the CPU pointing at where they are. This is slow, but a context switch is just changing that pointer.
Yeah it really was an interesting choice on their part. Makes sense as a move for TI. Not the target market.
The 16K memory block is connect to the TMS9918 video chip and is accessed by port read/write. The TI-99 out of the box only has 256 bytes of RAM memory on the 16 bit buss.
UNIX99 uses an after market card with 1Mbyte of RAM on an 8bit external buss used for peripherals. This card replaces the old 32K RAM card made by TI in the 1980s.