Editing TMS9900 (section)

==History==
[[Image:KL TI TMS9900 Black Background.jpg|right|thumb|TMS9900JL in ceramic package with gold-plated [[Lead (electronics)|pin]]s]]
The TMS9900 was designed as a single-chip version of the [[TI-990|TI 990]] [[minicomputer]] series, much like the [[Intersil 6100]] was a single chip [[PDP-8]] (12 bits), and the [[Fairchild 9440]] and [[Data General]] mN601 were both one-chip versions of [[Data General]]'s [[Data General Nova|Nova]]. Unlike multi-chip 16-bit microprocessors such as the [[National Semiconductor]] [[IMP-16]] or DEC [[LSI-11]], some of which predated the TMS9900, the 9900 was a single-chip, self-contained 16-bit microprocessor.

The minicomputer roots of the TMS9900 give rise to a number of architectural features that are not commonly found on designs that started from a blank sheet. Notable among these was the TMS9900's use of [[processor register]]s that are mapped into [[main memory]].<ref>{{cite journal |last1=Whitworth |first1=Ian |title=Review of microprocessor architecture |journal=Microprocessors and Microsystems |date=January 1979 |volume=3 |issue=1 |pages=21–28 |doi=10.1016/0141-9331(79)90088-7 }}</ref> This allows for fast [[context switching]], which can be accomplished by changing a single register, the Workspace Pointer, to point to the first entry in a list of register values. More traditional designs would require the entire set of internal registers to be stored out to memory or the stack.

The downside to this approach is that accessing these registers is much slower. In a minicomputer implementation with fast memory, the effect is relatively small and the upside in a real-time or multi-tasking environment is significant as context switches are common. In other roles, like single-user microcomputers, this tradeoff may not be worthwhile. The 40-pin implementations of the 9900 included 128 or 256 bytes of fast onboard RAM for registers.

TI used the same architecture across different divisions for corporate synergy: "one company, one computer architecture". In the late 1970s [[Walden C. Rhines]] gave a presentation of the  TMS99110, then code-named "Alpha",<ref>{{Cite web |last=Guttag |first=Karl |date=1980-04-14 |title=99000 "Alpha" Performance Comparisons |url=https://hansotten.file-hunter.com/uploads/files/99000%20(Alpha)%20Misc%20Documents.pdf |access-date=2022-10-26}}</ref> to an IBM group developing a personal computer. "We wouldn't know until 1981 just what we had lost" because IBM chose the [[Intel 8088]] for the [[IBM PC]], he recalled.  One factor was the  lack of a roadmap for accessing more than 64KB of logical memory.  The 9900 family could expand its address space to 16 MiB only by page-mapping; the 8088 could address 256K through segments.

After dropping out of the personal computer market after products such as [[TI-99/4A]], the company microprocessor division eventually switched focus to the [[Texas Instruments TMS320|TMS320]] special-purpose processor series.<ref name="rhines20170622">{{Cite magazine |last=Rhines |first=Walden C. |author-link=Walden C. Rhines |date=2017-06-22 |title=The Texas Instruments 99/4: World's First 16-Bit Home Computer |url=http://spectrum.ieee.org/geek-life/history/the-texas-instruments-994-worlds-first-16bit-computer |archive-url=https://web.archive.org/web/20170622212225/http://spectrum.ieee.org/geek-life/history/the-texas-instruments-994-worlds-first-16bit-computer |url-status=dead |archive-date=June 22, 2017 |magazine=IEEE Spectrum |language=en |access-date=2017-07-08}}</ref>

Microcomputer-on-chip implementations of variations of the 9900 in 40-pin packages included the TMS9940, TMS9980/81, and TMS9995. The SBP9900 was a ruggedized version.<ref>{{cite book |title=9900 Family Systems Design and Data Book |date=1978 |publisher=Texas Instruments |isbn=0-89512-026-7 |page=2-3 |url=http://www.bitsavers.org/components/ti/TMS9900/1978_TI_9900_Family_Systems_Design_and_Data_Book_1ed.pdf |access-date=21 November 2024}}</ref>  

The last generation was the 99000 series, created to be the CPU of the 990/10A in 1981. The TMS99105 and 110 were sold as catalog parts.<ref name=":0" />