I remember looking at this book at my local library when I was a kid:

https://archive.org/details/tibook_how-to-build-your-own-wor...

So I knew about the CPU before I knew about the home computer..

Oh, one other thing I remembered: there were integrated injection logic (I2L) versions of the 9900: the SBP9900 (this was bugging me because the I thought the TMS9900 was I2L, but it's NMOS):

https://www.cpushack.com/tag/sbp9900/

Here is a datasheet, the I/O structures are interesting:

https://source.z2data.com/2020/11/29/8/20/42/604/RSELSA00012...

Runs off of 500 mA at 1V.