I'm pretty sure the Centurion doesn't run Unix.
I thought UsagiElectric showed a case where his Centurion didn’t, but I may be misremembering.
The 9900 is a single chip implementation of the CPU board in the TI 990. They even created a dedicated memory mapper chip to go along with the product line, though it is significantly different than the one in the minicomputer line.
(edit: the 990 was first built in the early 70s, memory mappers are quite old conceptually)
(edit 2: in fact the necessity of using a memory mapper is what killed the platform, and was one of the things that made the IBM PC team decline the 9900.)
Unfortunately I don't think that there is a reasonable way to perform real hardware-level memory protection with that chip alone. I'm working on a project documenting the genesis of the 99000 chips, which include a privilege bit in the status register, from the minicomputer line.
Essay forthcoming, and probably an OS. Maybe a year...
It's arguably the only 8-bit computer which has a really different architecture from the others. You could otherwise imagine pulling the SID chip off a C-64 and putting it on a TRS-80 Color Computer etc.
Sharing the main RAM with video was a weak point in computers of that time period because the video system stole many of the memory access cycles. Some recent retrocomputers that revisit that period like
https://www.c64-wiki.com/wiki/Commander_X16
have a full-size memory bank and a video RAM memory bank which is accessed through a port which can be pretty efficient because you can auto-incremement the address register and just write 1 byte to the port to write 1 byte to video RAM and repeat.
The 990 series of minicomputers were competing with PDP-11s (Though DEC had highest market share, I believe 33% of the whole mini market?)
The 9900 was condensed in 1975 and went into the low-end 990/4. The higher end 990/9 and 990/10 were always going to be discrete TTL as the 9900 didn't support memory protection or mapping to the 2MByte total address space.
TI was always conscious of not challenging IBM head-to-head in minicomputers. Internal memos always projected TI's plan for its minis to occupy a space well below the latest IBM mainframes. From 1980, the planned 990/12 would arrive just as IBM delivered more compute power in their low-end... this was intentional, supposedly because IBM was the chief driver of TI's transistor business!
The 6502 is not very different with a very small number of registers and Zero Page being used for most of what a modern machine would use registers for. For example (unlike the Z80) there is no register-to-register add or subtract or compare -- you can only add/sub/cmp/and/or/xor a memory location to the accumulator. Also, pointers can only be done using a pair of adjacent Zero Page locations.
As long as you were using data in those in-RAM registers the TI-99/4 was around four times faster than a 1 MHz 6502 for 16 bit arithmetic -- and with a single 2-byte instruction doing what needed 7 instructions and 13 bytes of code on 6502 -- and it was also twice as fast on 8 bit arithmetic.
It was just the cheap-ass main memory (and I/O) implementation that crippled it.
