TI-99/4A - test-drb@ccmp.vtda.org

29 Jan 2001

From: Ross Archer &lt;archer(a)topnow.com&gt;
...

Correct me if I'm wrong, but I've heard the 99/4A suffered something of
an unfair rap for being slow due to the fact that its TI BASIC was
dreadfully inefficient and sloppy, and that little or no blame for its
slowness rested on the actual CPU or hardware. (Windoze
users may spot a parallel here.) >;) 
Partially correct.  there were many issues.  One the 9900 is 16bit wide
data bus demuxed to 8bit (costs a lot of cycles!, penalty 1), it's run at
less than maximum speed for the time and BASIC {internal} was
interpreted as an end language {penalty 2} and the BASIC interpreter
was interpreted {penalty 3}.

Those things really hurt speed.  The other side was it did have one of
the better from a capability standpoint Basics in the standard console.

...
 The TMS 9900 CPU actually used a "window" in
RAM as its register space,
with an internal pointer register to locate its base address.  This was
actually a great idea at the time, because CPU cycles were long enough
back then it made no difference whether you stored temporary values in
internal registers or external RAM, as either could get you your data in
the 1 cycle time available. 

Actually many of the older DEC hardware also used part fo ram to
implement the registers including early PDP11, DEC-10s and previous
machines.  The idea was not new and was to save logic in the CPU
as FlipFlops (memory) were costly in hardware back then and even in
late 70s were costly on silicon.  The other factor is the 9900 was
a single chip recreation of the TI990 mini (not unlike like the
6100{pdp-8}, LSI-11 and DG MicroNova).

...
 (And interrupt latency can be really short if you can
make a "fresh"
register bank with one register load!) 
The ability to context switch fast was one of the strong points.
Compared
to Z80 or 8086 it was a nicer cpu to program save for the 32KW memory
limitation.  In many respects it was more similar to the minis like
PDP-11
or Nova than the micros of the time.  It was rich in general registers,
Addressing modes and IO.

I have a Technico Super starter board and it runs the TI9900 at 4mhz
with 16bit wide memeory and rom really fast compared to the Z80
(comparison made on both machines I still have from 1979!).  I picked up
the TI99/4a and was sadly disappointed save for the bundle of
really good and inexpensive software for it.  Still, it plays a mean game
of
Parsec!!!

Allison