<s>
The	O
TI-990	B-Device
was	O
a	O
series	O
of	O
16-bit	B-Device
minicomputers	B-Architecture
sold	O
by	O
Texas	O
Instruments	O
(	O
TI	O
)	O
in	O
the	O
1970s	O
and	O
1980s	O
.	O
</s>
<s>
The	O
TI-990	B-Device
was	O
a	O
replacement	O
for	O
TI	O
's	O
earlier	O
minicomputer	B-Architecture
systems	O
,	O
the	O
TI-960	O
and	O
the	O
TI-980	O
.	O
</s>
<s>
Among	O
its	O
core	O
concepts	O
was	O
the	O
ability	O
to	O
easily	O
support	O
multiprogramming	O
using	O
a	O
software-switchable	O
set	O
of	O
processor	B-General_Concept
registers	I-General_Concept
that	O
allowed	O
it	O
to	O
perform	O
rapid	O
context	B-Operating_System
switches	I-Operating_System
between	O
programs	O
.	O
</s>
<s>
TI	O
later	O
implemented	O
the	O
TI-990	B-Device
in	O
a	O
single-chip	O
implementation	O
,	O
the	O
TMS9900	B-General_Concept
,	O
among	O
the	O
first	O
16-bit	B-Device
microprocessors	B-Architecture
.	O
</s>
<s>
Intended	O
for	O
use	O
in	O
low-end	O
models	O
of	O
the	O
TI-990	B-Device
,	O
it	O
retained	O
the	O
990	O
's	O
memory	O
model	O
and	O
main	O
memory	B-General_Concept
register	I-General_Concept
system	O
.	O
</s>
<s>
This	O
design	O
was	O
ultimately	O
much	O
more	O
widely	O
used	O
in	O
the	O
TI-99/4A	B-Device
,	O
where	O
details	O
of	O
its	O
minicomputer-style	O
memory	O
model	O
presented	O
significant	O
disadvantages	O
.	O
</s>
<s>
On	O
the	O
TI-990	B-Device
,	O
registers	O
are	O
stored	O
in	O
memory	O
and	O
are	O
referred	O
to	O
through	O
a	O
hard	O
register	O
called	O
the	O
Workspace	O
Pointer	O
.	O
</s>
<s>
A	O
context	B-Operating_System
switch	I-Operating_System
entailed	O
the	O
saving	O
and	O
restoring	O
of	O
only	O
the	O
hard	O
registers	O
.	O
</s>
<s>
The	O
TI-990	B-Device
had	O
a	O
facility	O
to	O
allow	O
extended	O
operations	O
through	O
the	O
use	O
of	O
plug	O
in	O
hardware	O
.	O
</s>
<s>
The	O
TI-990	B-Device
used	O
a	O
fairly	O
orthogonal	B-General_Concept
instruction	I-General_Concept
set	I-General_Concept
.	O
</s>
<s>
program	O
in	O
TI-990	B-Device
assembler	O
,	O
to	O
run	O
under	O
DX10	O
:	O
</s>
<s>
You	O
can	O
try	O
out	O
the	O
above	O
for	O
yourself	O
on	O
a	O
TI-990	B-Device
simulator	O
.	O
</s>
<s>
Dave	O
Pitts	O
's	O
sim990	O
simulates	O
the	O
TI-990	B-Device
and	O
includes	O
software	O
kits	O
for	O
native	O
operating	B-General_Concept
systems	I-General_Concept
(	O
including	O
DX10	O
)	O
.	O
</s>
<s>
The	O
TI-990	B-Device
processors	O
fell	O
into	O
several	O
natural	O
groups	O
depending	O
on	O
the	O
original	O
design	O
upon	O
which	O
they	O
are	O
based	O
and	O
which	O
I/O	O
bus	O
they	O
used	O
.	O
</s>
<s>
Also	O
,	O
supported	O
on	O
higher	O
end	O
models	O
was	O
the	O
TILINE	O
I/O	O
bus	O
which	O
is	O
similar	O
to	O
DEC	O
's	O
popular	O
UNIBUS	B-Device
.	O
</s>
