TI 990 architecture / was Re: TI-99/4A Floppies

Allison ajp166 at bellatlantic.net
Tue Oct 2 09:10:56 CDT 2007

>Subject: TI 990 architecture / was Re: TI-99/4A Floppies
>   From: Brent Hilpert <hilpert at cs.ubc.ca>
>   Date: Tue, 02 Oct 2007 00:35:01 -0700
>     To: General at priv-edtnaa06.telusplanet.net,
> "Discussion at priv-edtnaa06.telusplanet.net":On-Topic and Off-Topic Posts
> <cctalk at classiccmp.org>
>Chuck Guzis wrote:
>> On 2 Oct 2007 at 2:16, Liam Proven wrote:
>> > They were pretty much the first ever 16-bit home micro, but it was a
>> > crippled 16-bit chip - as detailed in another message in this thread.
>> > They did have good keyboards, were solidly built and I believe the
>> > graphics chip was, for its time, decent and capable.
>> Tossing all of the other chips and CROMs and other stuff out, how
>> compatible was the TMS9900 with the TI 990 mini?  The same or
>> considerablly different?
>I can't state so categorically but my understanding is that they were very
>much the same.
>Going back to a conversation of a few weeks ago, when we were
>developing/running Verex/Thoth at UBC ca 1980 it was on a 990/10. The next
>major step in the project was to develop a distributed kernel for multiple
>processors. To this end, 3 bare single board computers based on the 9900 chip
>were ordered and received from TI. Something makes me think they were called
>"990/5"s. I remember making up a front panel for the 3 of them with reset
>buttons and a few status LEDs to go in the rack with the /10. The idea, of
>course, was to use the 9900s because we already had the compilers,etc.
>generating code for the 990/10.

Actually 1980 was mid to late in the life of the TI9900 chip. The first
one I worked with was on a Technico Superstarter System, TI9900, 2k ram,
1k prom (monitor ans line by line asm) and a 2708 eprom programmer on
one board. I still have it.  I purchased it at PCC '78 in in memory 
serves Philly.  Fir the amount of resource on the board it was pretty
capable for systems of that day.

>(Cheriton left before we actually got into using them at the software level
>and the distributed kernel would become the VKernel at Stanford on other hardware).
>Also, the description of the 9900 in Osborne's "An Introduction to
>Microcomputers, Vol II" ('76) fits well with my recollections of the 990/10.
>To my knowledge the 9900 chip was not crippled; rather (going from what others
>have described) the design and implementation of the 99/4 home computer failed
>to make effective use of it. I didn't follow micros too much in the early 80s
>but I remember wondering at the time why the 99/4 was doing so poorly when it
>had that great processor in it.

There are really three 99/4 home computers, original with chiclet keys, the 
second and most common with a really nice keyboard and the whie version that
is really the same thing with a few board level cost reductions.

The 9900 chips is not crippled, for 1976 three voltage NMOS its about as fast
as the technology of the time could go.  The TI99/4 did however do a nasty to 
it. One is they muxed the bus down to 8bits wide and that does slow the system
some. There were 128 words of ram (6810s) that if you execute there the speed
is noticeable.  The other is the GROM (sort of an interpreted language with a
register point to next instruction) is a bottleneck as well.  There was a 
later 9980 and the 9985 which were a 8bit bus interface and were somewhat 
crippled but I'd never seen one in a TI99/4A.

>I quite liked the 990 architecture with the workspace pointer. Yes, there was
>the overhead of accessing registers in memory, but there were also savings.
>The workspace pointer essentially became the stack/frame pointer. Procedure
>calls, interrupts and process switches were quick because there were only 3
>machine registers to save (PC,WSP,PSW). Stack variables and parameters were
>referenced in instructions as registers, thus saving on instruction
>length/memory accesses to retrieve addresses/offsets, etc. For use with
>"modern software design", i.e.: stack-oriented high-level languages, I thought
>it was a quite effective architecture.

It was a very minicomputer in look and feel and the addressing modes were on 
par with PDP11 and other CISC machines.  


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