<s>
The	O
TRS-80	B-Device
Model	I-Device
4	I-Device
is	O
the	O
last	O
Z80-based	O
home	O
computer	O
family	O
by	O
Radio	O
Shack	O
,	O
sold	O
from	O
April	O
1983	O
through	O
late	O
1991	O
.	O
</s>
<s>
Tandy	O
Corporation	O
introduced	O
the	O
TRS-80	B-Device
Model	I-Device
4	I-Device
in	O
April	O
26th	O
,	O
1983	O
as	O
the	O
successor	O
to	O
the	O
TRS-80	O
Model	O
III	O
.	O
</s>
<s>
The	O
Model	O
4	O
has	O
a	O
faster	O
Z80A	B-General_Concept
4MHz	O
CPU	O
,	O
larger	O
video	O
display	O
of	O
80	O
columns	O
by	O
24	O
rows	O
,	O
bigger	O
keyboard	O
,	O
and	O
can	O
be	O
upgraded	O
to	O
128KB	O
of	O
RAM	O
.	O
</s>
<s>
It	O
is	O
compatible	O
with	O
Model	O
III	O
software	O
and	O
CP/M	B-Application
application	O
software	O
.	O
</s>
<s>
The	O
Model	O
4	O
was	O
announced	O
in	O
the	O
same	O
April	O
1983	O
press	O
release	O
as	O
was	O
the	O
TRS-80	B-Operating_System
Model	I-Operating_System
100	I-Operating_System
laptop	O
.	O
</s>
<s>
The	O
Model	O
4	O
uses	O
WD1770/1773	O
floppy	O
controllers	O
instead	O
of	O
the	O
WD1791	O
,	O
which	O
allows	O
for	O
a	O
larger	O
gap	O
between	O
the	O
index	O
hole	O
and	O
first	O
sector	O
;	O
later	O
releases	O
of	O
TRSDOS	B-Operating_System
and	O
LDOS	B-Operating_System
were	O
modified	O
for	O
compatibility	O
with	O
the	O
controller	O
.	O
</s>
<s>
The	O
Model	O
4	O
shipped	O
with	O
TRSDOS	B-Operating_System
6	O
,	O
identical	O
to	O
Logical	O
Systems	O
's	O
LDOS	B-Operating_System
6.00	O
third-party	O
operating	B-General_Concept
system	I-General_Concept
(	O
itself	O
an	O
enhancement	O
to	O
older	O
versions	O
of	O
TRSDOS	B-Operating_System
)	O
.	O
</s>
<s>
When	O
the	O
Model	O
4	O
boots	O
into	O
TRSDOS	B-Operating_System
6	O
,	O
the	O
video	O
display	O
switches	O
into	O
80×24	O
mode	O
and	O
the	O
entire	O
64KB	O
address	O
space	O
is	O
mapped	O
as	O
RAM	O
.	O
</s>
<s>
The	O
Model	O
4	O
can	O
run	O
CP/M	B-Application
without	O
modification	O
,	O
unlike	O
the	O
Model	O
I	O
and	O
III	O
.	O
</s>
<s>
Digital	O
Research	O
produced	O
a	O
version	O
of	O
CP/M	B-Application
3.0	O
for	O
the	O
Model	O
4	O
.	O
</s>
<s>
Montezuma	O
Micro	O
sold	O
a	O
version	O
of	O
CP/M	B-Application
2.2	O
that	O
was	O
customized	O
for	O
the	O
Model	O
4	O
's	O
hardware	O
:	O
banked	O
RAM	O
,	O
reverse	O
video	O
and	O
assignable	O
codes	O
for	O
the	O
function	O
keys	O
.	O
</s>
<s>
It	O
has	O
a	O
utility	O
for	O
reading	O
and	O
writing	O
CP/M	B-Application
disk	O
formats	O
of	O
many	O
other	O
brands	O
of	O
computer	O
.	O
</s>
<s>
Montezuma	O
sold	O
a	O
terminate-stay-resident	O
program	O
they	O
called	O
Monte	O
's	O
Window	O
,	O
which	O
provides	O
functionality	O
similar	O
to	O
Borland	B-Application
Sidekick	I-Application
.	O
</s>
<s>
This	O
is	O
one	O
of	O
the	O
first	O
task-switching	B-Operating_System
programs	O
available	O
for	O
a	O
microcomputer	O
.	O
</s>
<s>
It	O
uses	O
the	O
upper	O
64KB	O
of	O
a	O
128KB	O
machine	O
to	O
keep	O
resident	O
a	O
second	O
TRSDOS	B-Operating_System
application	O
,	O
which	O
can	O
be	O
switched	O
instantly	O
with	O
another	O
application	O
loaded	O
into	O
the	O
main	O
64KB	O
.	O
</s>
<s>
A	O
third	O
partition	O
is	O
available	O
for	O
TRSDOS	B-Operating_System
library	O
commands	O
,	O
such	O
as	O
DIR	O
.	O
</s>
<s>
DoubleDuty	O
first	O
appeared	O
in	O
Radio	O
Shack	O
's	O
1985	O
Computer	O
Catalog	O
(	O
RSC-12	O
)	O
,	O
the	O
same	O
year	O
that	O
IBM	O
's	O
Topview	B-Operating_System
,	O
Apple	O
's	O
Switcher	O
,	O
and	O
Quarterdeck	O
's	O
DESQview	B-Operating_System
first	O
became	O
available	O
.	O
</s>
<s>
DoubleDuty	O
was	O
written	O
by	O
Randy	O
Cook	O
,	O
the	O
author	O
of	O
the	O
first	O
version	O
of	O
TRSDOS	B-Operating_System
for	O
the	O
original	O
Model	O
I	O
.	O
</s>
<s>
Early	O
versions	O
of	O
the	O
Model	O
4	O
mainboard	O
were	O
designed	O
to	O
accept	O
a	O
Zilog	B-Device
Z800	I-Device
16	O
bit	O
CPU	O
upgrade	O
board	O
to	O
replace	O
the	O
Z80	B-General_Concept
8	O
bit	O
CPU	O
but	O
this	O
option	O
was	O
never	O
released	O
.	O
</s>
<s>
Tech	O
produced	O
an	O
enhanced	O
CPU	O
board	O
,	O
the	O
XLR8er	B-Device
,	O
using	O
the	O
Hitachi	B-General_Concept
HD64180	I-General_Concept
Z80-compatible	O
processor	O
.	O
</s>
<s>
BYTE	O
in	O
October	O
1983	O
noted	O
the	O
lack	O
of	O
native	O
software	O
,	O
but	O
praised	O
the	O
Model	O
4	O
's	O
backwards	O
compatibility	O
and	O
TRSDOS	B-Operating_System
6	O
's	O
new	O
features	O
.	O
</s>
<s>
Creative	B-General_Concept
Computing	I-General_Concept
chose	O
the	O
Model	O
4	O
as	O
the	O
best	O
desktop	O
computer	O
under	O
$2000	O
for	O
1984	O
,	O
stating	O
that	O
the	O
$1299	O
price	O
for	O
a	O
system	O
with	O
two	O
disk	O
drives	O
was	O
"	O
a	O
real	O
bargain	O
"	O
.	O
</s>
<s>
The	O
new	O
version	O
runs	O
its	O
Z80	B-General_Concept
processor	O
at	O
a	O
true	O
clock	O
rate	O
of	O
4	O
MHz	O
without	O
wait	O
states	O
,	O
which	O
gave	O
the	O
older	O
version	O
an	O
effective	O
clock	O
rate	O
of	O
about	O
3.5	O
MHz	O
.	O
</s>
<s>
The	O
Model	O
4P	O
(	O
September	O
1983	O
,	O
Radio	O
Shack	O
catalog	O
number	O
26-1080	O
)	O
,	O
is	O
a	O
self-contained	O
luggable	B-Device
unit	O
.	O
</s>
<s>
The	O
computer	O
is	O
compatible	O
with	O
popular	O
internal	O
Model	O
4	O
peripherals	O
,	O
and	O
has	O
a	O
slot	O
for	O
an	O
internal	O
modem	B-Application
board	O
.	O
</s>
<s>
The	O
Radio	O
Shack	O
modem	B-Application
uses	O
its	O
own	O
proprietary	O
command	O
set	O
and	O
only	O
supports	O
communications	O
at	O
300	O
baud	O
.	O
</s>
<s>
Teletrends	O
produced	O
a	O
1200	O
baud	O
that	O
uses	O
the	O
Hayes	B-Device
command	I-Device
set	I-Device
.	O
</s>
<s>
The	O
DeskMate	B-Device
productivity	O
suite	O
is	O
bundled	O
with	O
the	O
4D	O
.	O
</s>
<s>
The	O
Model	O
III	O
LDOS	B-Operating_System
5.1.4	O
was	O
also	O
updated	O
to	O
version	O
5.3	O
,	O
supporting	O
the	O
same	O
feature	O
set	O
as	O
LS-DOS	O
6.3	O
.	O
</s>
