Comment by FarmerPotato

The design "decisions" are easy to explain. The 9985 failed. They had a development prototype with a 9900 emulating the expected CPU. The 9918 VDP was the cheapest way to add 4K later 16K of DRAM. And that was what they shipped after the 9985 was killed.

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From 1977 they expected a 9985 to succceed the cheap 40-pin 9981, both having an 8-bit external bus (1). It would have 256 bytes of RAM onboard. I speculate it would have the 9900 microcode optimizations seen in the military SBP9989.

Anecdotally, the 9985 failed seven tape-outs. It was killed. The Bedford UK team was tasked with starting over: eventually this produced the 9995.

But the Home Computer had been prototyped using a 9900 board. So that was forced into the 99/4 (not A) with some external 256 byte SRAM.

Memory was expensive. The 9918 VDP, made by a team in 1975 with junior engineer Karl Guttag, was the cheapest way to interface 4Ks DRAM which TI made and sold to itself. By the time it reached market, 16k in 8x 4116s was optimal.

Various efforts to cost-reduce and upgrade the 99/4A ran into the '82 price-war with Commodore.

Every design iteration that added more RAM (2 or 8 or 16K directly accessible from the CPU) was "paid for" by reducing the cost elsewhere (PALs for instance.) BOM was around $105. [3]

But in the price war, engineers were told to deploy the cost-savings without any new features: this was the 99/4A 2.2 or QI for quality improved. [3] The 99/4A was already a loss leader by Q4 1982 [5].

In 1981, Karl Guttag's new 9995 passed first silicon [2]. It used the new optimized 99000 CPU core which also famously passed on first tape-out. The 9995 was available in quantity in 1982 [3] when new consoles were started around it: 99/2, 99/8.

The 99/2 was supposed to be cheap enough to compete with Sinclair. [6]

The 99/8 was a technical beast for the high-end, having 64K of directly accessible RAM. Its fancy memory mapper drove 24 bit external addresses. It supported 512K off board, which the P-Box had been designed for. It had Pascal built-in. Yet there was no Advanced VDP for it: stuck with the same 9918A.

In early 1983, TI assembled a team of two dozen engineers to write software for it: Pascal applications, new LOGO, a database, new word processor, TI FORTH, and complete accounting package, and a rumored superior easy-to-use interface. Pascal was supposed to deliver many benefits. It would be a small business machine. (4)

Of course, in November 1983, all efforts ceased as Home Computer was cancelled--just as the consoles were to be unveiled at Winter CES.

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(1) An 8-bit bus was always going to be optimal--even the IBM PC 8088 saw that. 16-bit peripheral chips were never going to be made: the package size would prohibit that.

(2) Electronics Magazine and EE Times articles

(3) Internal memos of Don Bynum, program manager

(4) TI Records, DeGolyer Library, SMU : Armadillo and Pegasus

(5) "Death of a Computer", Texas Monthly, end of 1983?

(6) BYTE Magazine June 1982-ish

Based on research for my book: _Legacy: the TI Home Computer_.

From memory...