<s>
Heathkit	O
's	O
H8	O
is	O
an	O
Intel	O
8080A-based	O
microcomputer	B-Architecture
sold	O
in	O
kit	O
form	O
starting	O
in	O
1977	O
.	O
</s>
<s>
The	O
H8	O
is	O
similar	O
to	O
the	O
S-100	B-Architecture
bus	I-Architecture
computers	O
of	O
the	O
era	O
,	O
and	O
like	O
those	O
machines	O
is	O
often	O
used	O
with	O
the	O
CP/M	B-Application
operating	I-Application
system	I-Application
on	O
floppy	B-Device
disk	I-Device
.	O
</s>
<s>
The	O
main	O
difference	O
between	O
the	O
H8	O
and	O
S-100	O
machines	O
is	O
the	O
bus	B-General_Concept
;	O
the	O
H8	O
uses	O
a	O
50-pin	O
bus	B-General_Concept
design	O
that	O
was	O
smaller	O
,	O
more	O
robust	O
and	O
better	O
engineered	O
electrically	O
.	O
</s>
<s>
The	O
machine	O
also	O
includes	O
a	O
bootstrap	B-Application
ROM	B-Device
that	O
makes	O
it	O
easier	O
to	O
start	O
up	O
,	O
including	O
code	O
for	O
running	O
basic	O
input/output	B-General_Concept
and	O
allowing	O
input	O
through	O
a	O
front-mounted	O
octal	O
keypad	O
and	O
front	B-Device
panel	I-Device
display	O
,	O
instead	O
of	O
the	O
binary	O
switches	O
and	O
lights	O
used	O
on	O
machines	O
like	O
the	O
Altair	B-Architecture
8800	I-Architecture
.	O
</s>
<s>
The	O
H8	O
requires	O
a	O
separate	O
terminal	B-General_Concept
to	O
be	O
truly	O
useful	O
;	O
Heathkit	O
introduced	O
several	O
terminals	O
as	O
well	O
.	O
</s>
<s>
A	O
successor	O
model	O
,	O
the	O
"	O
All-in-One	O
"	O
Heathkit	B-Device
H89	I-Device
,	O
combines	O
a	O
Z-80	B-General_Concept
processor	O
board	O
and	O
a	O
floppy	B-Device
disk	I-Device
drive	I-Device
into	O
the	O
cabinet	O
of	O
an	O
Heathkit	O
H19	O
terminal	B-General_Concept
.	O
</s>
<s>
These	O
were	O
later	O
sold	O
by	O
Zenith	O
Electronics	O
with	O
their	O
name	O
on	O
the	O
front	O
as	O
the	O
Zenith	B-Device
Z-89	I-Device
.	O
</s>
<s>
MITS	O
announced	O
the	O
Altair	B-Architecture
8800	I-Architecture
in	O
January	O
1975	O
and	O
started	O
selling	O
kits	O
soon	O
after	O
.	O
</s>
<s>
The	O
Altair	O
sparked	O
off	O
such	O
intense	O
interest	O
in	O
the	O
microcomputer	B-Architecture
world	O
that	O
a	O
number	O
of	O
other	O
companies	O
jumped	O
in	O
to	O
fill	O
the	O
sales	O
backlog	O
,	O
building	O
machines	O
that	O
were	O
clones	O
of	O
the	O
Altair	O
.	O
</s>
<s>
The	O
primary	O
component	O
of	O
this	O
design	O
is	O
the	O
S-100	B-Architecture
bus	I-Architecture
,	O
so	O
named	O
because	O
it	O
uses	O
a	O
100-pin	O
edge	O
connector	O
that	O
MITS	O
found	O
at	O
bargain	O
prices	O
when	O
they	O
were	O
designing	O
the	O
machine	O
.	O
</s>
<s>
The	O
introduction	O
of	O
floppy	B-Device
disk	I-Device
controllers	O
and	O
the	O
disk-based	O
CP/M	B-Application
operating	I-Application
system	I-Application
dramatically	O
improved	O
the	O
system	O
's	O
capabilities	O
and	O
started	O
the	O
process	O
of	O
turning	O
them	O
into	O
practical	O
small-business	O
tools	O
.	O
</s>
<s>
By	O
the	O
late	O
1970s	O
they	O
were	O
beginning	O
to	O
displace	O
minicomputers	B-Architecture
and	O
other	O
systems	O
in	O
a	O
number	O
of	O
roles	O
.	O
</s>
<s>
In	O
1977	O
they	O
decided	O
to	O
enter	O
the	O
microcomputer	B-Architecture
market	O
,	O
and	O
designed	O
the	O
H8	O
.	O
</s>
<s>
For	O
full	O
functionality	O
,	O
the	O
system	O
also	O
requires	O
a	O
4KiB	O
SRAM	B-Architecture
card	O
( $139	O
)	O
and	O
some	O
form	O
of	O
storage	O
controller	O
;	O
at	O
a	O
minimum	O
this	O
would	O
be	O
the	O
H10	O
paper	O
tape	O
punch/reader	O
or	O
the	O
H8-5	O
Serial	O
I/O	B-General_Concept
card	O
( $110	O
)	O
which	O
controls	O
a	O
cassette	B-Device
tape	I-Device
,	O
using	O
a	O
1200-baud	O
variant	O
of	O
the	O
Kansas	B-Device
City	I-Device
standard	I-Device
format	O
.	O
</s>
<s>
Another	O
common	O
accessory	O
is	O
the	O
H9	O
video	B-General_Concept
terminal	I-General_Concept
,	O
which	O
is	O
also	O
driven	O
by	O
the	O
H8-5	O
card	O
;	O
although	O
any	O
serial	B-General_Concept
terminal	I-General_Concept
will	O
suffice	O
.	O
</s>
<s>
It	O
was	O
eventually	O
superseded	O
by	O
the	O
H19	O
terminal	B-General_Concept
,	O
a	O
more	O
ergonomic	O
design	O
,	O
and	O
capable	O
of	O
lower-case	O
and	O
graphic-like	O
characters	O
.	O
</s>
<s>
The	O
H17	O
floppy	B-Device
disk	I-Device
system	O
became	O
available	O
in	O
1978	O
,	O
normally	O
sold	O
with	O
one	O
drive	O
but	O
expandable	O
with	O
a	O
second	O
(	O
and	O
later	O
to	O
three	O
)	O
.	O
</s>
<s>
The	O
H8	O
can	O
run	O
CP/M	B-Application
,	O
and	O
often	O
did	O
,	O
but	O
early	O
machines	O
require	O
either	O
a	O
special	O
version	O
of	O
CP/M	B-Application
that	O
was	O
"	O
org	O
'd	O
"	O
at	O
8KiB	O
instead	O
of	O
zero	O
,	O
or	O
a	O
small	O
hardware	O
modification	O
and	O
an	O
updated	O
ROM	B-Device
to	O
do	O
so	O
.	O
</s>
<s>
Heath	O
also	O
offered	O
HDOS	B-Application
,	O
which	O
was	O
written	O
by	O
Gordon	O
Letwin	O
.	O
</s>
<s>
Letwin	O
later	O
went	O
to	O
Microsoft	O
and	O
was	O
chief	O
architect	O
of	O
OS/2	B-Application
.	O
</s>
<s>
For	O
the	O
CP/M	B-Application
operating	I-Application
system	I-Application
,	O
Heathkit	O
provided	O
the	O
WH67	O
,	O
a	O
10	O
MB	O
eight-inch	O
hard	O
drive	O
and	O
the	O
H47	O
eight-inch	O
floppy	B-Device
disk	I-Device
system	O
.	O
</s>
<s>
In	O
1978	O
Heath	O
introduced	O
the	O
Heathkit	O
H88	O
which	O
integrated	O
the	O
H19	O
terminal	B-General_Concept
and	O
a	O
new	O
Zilog	O
Z80-based	O
single-board	O
processor	O
into	O
the	O
case	O
of	O
the	O
H19	O
.	O
</s>
<s>
A	O
version	O
with	O
a	O
disk	O
drive	O
incorporated	O
to	O
the	O
right	O
of	O
the	O
terminal	B-General_Concept
screen	O
became	O
the	O
H89	O
.	O
</s>
<s>
Soon	O
after	O
the	O
introduction	O
of	O
the	O
H89	O
,	O
Heathkit	O
was	O
purchased	O
by	O
Zenith	O
to	O
enter	O
the	O
microcomputer	B-Architecture
market	O
.	O
</s>
<s>
They	O
continued	O
sales	O
of	O
the	O
H89	O
with	O
their	O
own	O
labeling	O
on	O
the	O
front	O
as	O
the	O
Zenith	B-Device
Z89	I-Device
.	O
</s>
<s>
Eventually	O
,	O
Zenith	O
Data	O
Systems	O
(	O
Heathkit	O
plus	O
the	O
computer	O
division	O
of	O
Zenith	O
)	O
was	O
purchased	O
by	O
Bull	O
HN	O
(	O
CII	O
Bull	O
,	O
Honeywell	O
and	O
Nippon	O
Electric	O
)	O
because	O
they	O
needed	O
a	O
US	O
maker	O
of	O
microcomputers	B-Architecture
to	O
comply	O
with	O
government	O
purchase	O
requirements	O
.	O
</s>
<s>
Heath	O
chose	O
not	O
to	O
implement	O
the	O
S-100	B-Architecture
bus	I-Architecture
and	O
instead	O
created	O
their	O
own	O
,	O
known	O
as	O
"	O
Benton	O
Harbor	O
Bus	B-General_Concept
"	O
after	O
their	O
home	O
town	O
.	O
</s>
<s>
The	O
bus	B-General_Concept
is	O
based	O
on	O
a	O
50-pin	O
connector	O
and	O
laid	O
out	O
to	O
avoid	O
the	O
electrical	O
problems	O
of	O
the	O
S-100	O
system	O
(	O
like	O
+5V	O
and	O
ground	O
being	O
placed	O
beside	O
each	O
other	O
)	O
.	O
</s>
<s>
The	O
front	B-Device
panel	I-Device
plugs	O
into	O
the	O
first	O
slot	O
and	O
the	O
CPU	O
plugs	O
into	O
the	O
second	O
,	O
leaving	O
seven	O
for	O
further	O
expansion	O
.	O
</s>
<s>
Another	O
notable	O
change	O
is	O
the	O
replacement	O
of	O
the	O
front-panel	O
toggle	O
switches	O
and	O
lights	O
of	O
a	O
standard	O
early-model	O
S-100	O
system	O
with	O
a	O
keypad	O
and	O
seven-segment	O
LED	O
display	O
(	O
early	O
S-100	O
machines	O
like	O
the	O
Altair	O
or	O
IMSAI	B-Device
8080	I-Device
contain	O
no	O
ROM	B-Device
and	O
when	O
they	O
are	O
started	O
,	O
the	O
user	O
"	O
keys	O
in	O
"	O
a	O
program	O
via	O
the	O
toggle	O
switches	O
to	O
read	O
a	O
paper	O
tape	O
.	O
</s>
<s>
Once	O
this	O
"	O
loader	O
"	O
program	O
is	O
ready	O
,	O
a	O
paper	O
tape	O
containing	O
a	O
more	O
complete	O
loader	O
can	O
be	O
read	O
in	O
,	O
allowing	O
the	O
user	O
to	O
load	O
programs	O
from	O
cassette	O
or	O
floppy	B-Device
disk	I-Device
)	O
.	O
</s>
<s>
On	O
the	O
H8	O
,	O
all	O
of	O
this	O
code	O
is	O
already	O
pre-installed	O
in	O
a	O
1KiB	O
ROM	B-Device
in	O
a	O
monitor	B-Application
program	I-Application
known	O
as	O
"	O
PAM8	O
"	O
,	O
occupying	O
locations	O
0	O
through	O
3FF16	O
and	O
the	O
H17	O
disk	O
I/O	B-General_Concept
drivers	O
used	O
for	O
booting	O
,	O
occupying	O
a	O
2KiB	O
ROM	B-Device
occupying	O
locations	O
180016	O
through	O
1FFF16	O
The	O
ROM	B-Device
contains	O
code	O
to	O
control	O
the	O
keypad	O
and	O
display	O
,	O
booting	O
it	O
directly	O
into	O
an	O
operable	O
state	O
.	O
</s>
<s>
Several	O
versions	O
of	O
the	O
PAM-8	O
ROM	B-Device
were	O
sold	O
as	O
upgrades	O
;	O
at	O
one	O
point	O
Heathkit	O
switched	O
to	O
using	O
2KiB	O
ROMs	O
,	O
occupying	O
through	O
7FF16	O
and	O
subsequently	O
to	O
a	O
4KiB	O
ROM	B-Device
occupying	O
through	O
FFF16	O
.	O
</s>
<s>
The	O
ROMs	O
interfere	O
with	O
the	O
operation	O
of	O
standard	O
CP/M	B-Application
,	O
which	O
assumes	O
it	O
can	O
write	O
the	O
memory	O
near	O
location	O
0	O
,	O
in	O
particular	O
the	O
interrupt	O
handler	O
pointers	O
.	O
</s>
<s>
PAM8	O
and	O
portions	O
of	O
HDOS	B-Application
use	O
an	O
unusual	O
address	O
notation	O
called	O
"	O
split	O
octal	O
"	O
where	O
16-bit	O
numbers	O
are	O
split	O
into	O
two	O
8-bit	O
numbers	O
printed	O
in	O
octal	O
:	O
the	O
first	O
location	O
was	O
"	O
000.000	O
"	O
and	O
the	O
location	O
after	O
"	O
000.377	O
"	O
was	O
"	O
001.000	O
"	O
.	O
</s>
<s>
Most	O
mini	O
-	O
and	O
micro-computers	B-Architecture
use	O
either	O
straight	O
octal	O
(	O
377	O
was	O
followed	O
by	O
400	O
)	O
or	O
hexadecimal	O
.	O
)	O
</s>
<s>
With	O
the	O
introduction	O
of	O
the	O
optional	O
HA8-6	O
Z-80	B-General_Concept
processor	O
replacement	O
for	O
the	O
8080	B-General_Concept
board	O
,	O
the	O
front-panel	O
keyboard	O
got	O
a	O
new	O
set	O
of	O
labels	O
and	O
hexadecimal	O
notation	O
replaced	O
octal	O
.	O
</s>
<s>
The	O
50-pin	O
"	O
Benton	O
Harbor	O
Bus	B-General_Concept
"	O
was	O
considered	O
an	O
improvement	O
on	O
the	O
S-100	B-Architecture
bus	I-Architecture
.	O
</s>
<s>
The	O
50-pin	O
bus	B-General_Concept
of	O
the	O
H8	O
contains	O
sixteen	O
address	O
lines	O
,	O
eight	O
data	O
lines	O
,	O
5	O
interrupt	O
lines	O
,	O
and	O
the	O
system	O
control	O
lines	O
.	O
</s>
<s>
Like	O
the	O
S-100	B-Architecture
bus	I-Architecture
,	O
it	O
does	O
not	O
supply	O
+5V	O
;	O
each	O
card	O
is	O
expected	O
to	O
have	O
its	O
own	O
local	O
+5V	O
regulator	O
powered	O
from	O
"	O
unregulated	O
"	O
+8V	O
on	O
the	O
bus	B-General_Concept
.	O
</s>
