Altair 8800 User manual

ALTAIR
8800
OPERATOR~S
MANUAL
TABLE
OF
CONTENTS
PART
ONE:
Introduction
.
•.•........................
2
(Logia,
Electric
Logia,
Number
Systems,
The
Binary
System,
The
Octal System, Computer Programming, A
Simple Program, Computer Languages)
PART
TWO:
Organization
of
the
AZtair
.............
19
(Central Process.ing Unit,
Memory,
Cloak,
Input/Output)
PART
THREE:
Operation
of
the
Altair
......
......•.
28
(FPont Panel Switches and LED's, Loading a Sample Pro-
gram, Using
the
Memory,
Memory
Addressing, Operating
Hints)
PART
FOUR:
Altair
8800
Instruction
Set
.......•.••
42
(Corrunand
Instructions,
Single
Register
Instructions,
Register
Pair
Instructions,
Rotate
Aaaumulator
In-
struations)
APPENDIX:
Instruction
List
..
....................•
87
c:MITS,
Inc.,
1975
PRINTED
IN
U.S.A.
6328
LINN,
N.E., P.O. BOX 8636.,
ALBUQUERQUE,
N.M. 87108 U.S.A.
505/265-7553

2
PART
1
INTRODUCTION
Remarkable
advances in semiconductor technology
have
made
possible the development of the
ALTAIR
8800, the most eco-
nomical
computer ever
and
the
first
available
in both
kit
and
assembled
form.
The
heart of the
ALTAIR
8800
is
Intel
Corporation
1
s
Model
8080
Microcomputer, a complete Central
Processing Unit
on
a
single
silicon
chip. Fabricated with
N-channel
large
scale
integrated
circuit
(LSI) metal-oxide-
semiconductor
(MOS)
technology, Intel 1s
8080
Microcomputer
on
a chip represents a major technological breakthrough.
This operating
manual
has
been
prepared to acquaint both the
novice
and
the experienced computer user in the operation
of the
ALTAIR
8800.
The
computer
has
78
machine
language
instructions
and
is
capable of performing several important
operations not normally
available
with conventional mini-
computers. After reading
this
manual,
even
a novice will
be
able to load a
program
into
the
ALTAIR
8800.
Users of the
ALTAIR
8800 include persons with a strong
elec-
tronics
background
and
little
or
no
computer experience
and
persons with considerable
programming
experience
and
little
or
no
electronics
background. Accordingly,
this
manual
has
been
prepared with
all
types of users in
mind.
Part
1 of the
manual
prepares the user
for
better
under-
standing computer terminology, technology,
and
operation
with
an
introduction to conventional
and
electronic
logic,
a
description
of several important
number
systems, a
discus-
sion of basic programming,
and
a discourse
on
computer
lan-
guages.
Parts 2
and
3 in the
manual
describe the organization
and
operation of the
ALTAIR
8800.
Emphasis
is
placed
on
those
portions
of
the computer
most
frequently
utilized
by
the
user.
Finally,
Part
4 of the
manual
presents a
detailed
listing
of
the
ALTAIR
8BOO's
78
instructions.
An
Appendix
condenses the
instructions
into
a quick reference
listing.
Even
if
you
have
little
or
no
experience in computer oper-
ation
and
organization,
a careful reading of
this
manual
will prepare
you
for operating the
ALTAIR
8800.
As
you
gain experience with the machine,
you
will
soon
come
to un-
derstand
its
truly
incredible
versatility
and
data proces-
sing
capability.
Don't
be
discouraged
if
the
manual
seems
too complicated in places.
Just
remember
that
a computer
does only
what
its
programmer
instructs
it
to do.

A·
LOGIC
George
Boole, a ninteenth century
British
mathematician,
made
a
detailed
study
of
the
relationship
between
certain
fundamental
logical
expressions
and
their
arithmetic
coun-
terparts.
Boole did not equate mathematics with.
logic,
but
he
did
show
how
any
logical statement
can
be
analyzed with
simple
arithmetic
relationships.
In
1847,
Boole
published
a booklet
entitled
Mathematical Analysis of Logic
and
in
1854
he
published a
much
more
detailed
work
on
the
subject.
To
this
day,
all
practical
digital
computers
and
many
other
electronic
circuits
are based
upon
the logic concepts ex-
plained
by
Boole.
Boole's system
of
logic,
which
is
frequently
called
Boolean
algebra, assumes
that
a logic condition or statement
is
either
true
or
false.
It
cannot
be
both
true
and
false,
and
it
cannot
be
partially
true
or
partially
false.
For-
tunately,
electronic
circuits
are admirably
suited
for
this
type of
dual-state
operation.
If
a
circuit
in the
ON
state
is
said to
be
true
and
a
circuit
in the
OFF
state
is
said
to
be
false,
an
electronic
analogy
of
_a
logical statement
can
be
readily
synthesized.
With
this
in mind,
it
is
possible to devise
electronic
equi-
valents
for
the
three
basic logic statements:
AND,
OR
and
NOT.
The
AND
statement
is
true
if
and
only
if
either
or
all
of
its
logic conditions are
true.
A
NOT
statement merely
reverses the
meaning
of
a logic statement
so
that
a
true
statement
is
false
and
a
false
statement
is
true.
It's
easy to generate a simple equivalent of these
three
logic statements
by
using on-off switches. A switch
which
is
ON
is
said to
be
true
while a switch
which
is
OFF
is
said to
be
false.
Since a switch
which
is
OFF
will not
pass
an
electrical
current,
it
can
be
assigned a numerical
value
of
0.
Similarly,
a switch
which
is
ON
does pass
an
electrical
current
and
can
be
assigned a numerical value
of
1.
We
can
now
devise
an
electronic
equivalent
of
the
logical
AND
statement
by
examining the various permutations
for
a
two
condition
AND
statement:
3

4
CONDITIONS
(Inputs)
1. True AND True
2. True AND False
3. False AND True
4. False AND False
CONCLUSION
(Output)
True
False
False
False
The electronic ON-OFF switch equivalent of these permuta-
tions is simply:
CONDITIONS
(ON-OFF)
1. ---oo--.~,__----~o_..)o>----
2.
3. ~oo-----~o~)o~--
4. ~o~----v~~o---
CONCLUSION
(OUTPUT)
1
0
0
0
Similarly, the numerical equivalents of these permutations
is:
CONDITIONS
(Inputs)
1. AND 1
2. AND 0
3. 0 AND 1
4. 0 AND 0
CONCLUSION
(Output)
0
0
0
Digital design engineers refer to these table of permuta-
tions as truth tables. The truth table for the AND statement
with
two conditions is usually presented thusly:

A B
OUT
1 1 1
0 1 0
1 0 0
0 0 0
FIGURE
1-1.
AND
Function Truth Table
It
is
now
possible to derive the
truth
tables
for the
OR
and
NOT
statements, and each
is
shown
in Figures 1-2
and
1-3
respectively.
A
B
OUT
1 1
l
0
1 1
1 0 1
0
0
0
FIGURE
1-2.
OR
Function Truth Table
A
OUT
1 0
0 1
FIGURE
1-3.
NOT
Function Truth Table
5

6
B.
ELECTRONIC
LOGIC
All
three of the basic logic functions
can
be
implemented
by
relatively
simple
transistor
circuits.
By
convention,
each
circuit
has
been
assigned a
symbol
to
assist
in design-
ing
logic systems.
The
three
symbols
along with
their
re-
spective
truth
tables are
shown
in Figure
1-4.
:~OUT
A
0
0
l
l
B
0
1
0
1
OUT
0
0
0
l
A
0
0
1
l
B
0
1
0
1
OUT
0
1
l
l
A~OUT
A
0
1
OUT
1
0
FIGURE
1-4.
The
Three
Main
Logic
Symbols
The
three basic loqic
circuits
can
be
combined
with
one
an-
other to produce
still
more
logic statement analogies.
Two
of
these
circuit
combinations are
used
so
frequently
that
they are considered basic logic
circuits
and
have
been
assign-
ed
their
own
logic
symbols
and
truth
tables.
These
circuits
are the
NANO
(NOT-AND)
and
the
NOR
(NOT-OR).
Figure
1-5
shows
the logic
symbols
and
truth
tables for these
circuits.
A
OUT
OUT
B
A
B
OUT
A B
OUT
0 0
l 0 0 l
0
1 1
0
1
0
1 0 1 1
0 0
l 1 0 1
1
0
FIGURE
1-5.
The
NANO
and
NOR
Circuits

Three
or
more
logic
circuits
make
a
logic
system.
One
of
the
most
basic
logic
systems
is
the
EXCLUSIVE-OR
circuit
shown
in Figure 1-6.
FIGURE
1-6.
The
EXCLUSIVE-OR
Circuit
The
EXCLUSIVE-OR
circuit
can
be
used to implement
logical
functions,
but
it
can also
be
used
to
add
two
input condi-
tions.
Since
electronic
logic
circuits
utilize
only
two
numerical
units,
0
and
1, they are compatible with the
bi-
nary
number
system, a
number
system
which
has
only
two
digits.
For
this
reason, the
EXCLUSIVE-OR
circuit
is
often
called
a
binary adder.
Various combinations
of
logic
circuits
can
be
used to imple-
ment
numerous
electronic
functions.
For
example,
two
NANO
circuits
can
be
connected
to
form
a
bistable
circuit
called
a
flip-flop.
Since the
flip-f)op
changes
state
only
when
an
incoming signal in the
form
of
a pulse
arrives,
it
acts
as a
short
term
memory
element. Several
flip-flops
can
be
cascaded together to
form
electronic
counters
and
memory
registers.
Other
logic
circuits
can
be
connected
together
to
form
mono-
stable
and
astable
circuits.
Monostable
circuits
occupy
one
of
two
states
unless
an
incoming pulse
is
received.
They
then occupy
an
opposite
state
for
a
brief
time
and
then
resume
their
normal
state.
Astable
circuits
continually
switch
back
and
forth
between
two
states.
7

8
(.
NUMBER
SYSTEMS
Probably because
he
found
it
convenient to count with his
fingers,
early
man
devised a
number
system
which
consisted
of ten
digits.
Number
systems,
however,
can
be
based
on
any
number
of
digits.
As
we
have
already seen,
dual-state
e-
lectronic
circuits
are highly compatible with a
two
digit
number
system,
and
its
digits
are termed
bits
(binary
digits).
Systems
based
upon
eight
and
sixteen are also compatible wlth
complex
electronic
logic systems
such
as
computers since
they provide a convenient shorthand
method
for expressing
lengthy binary
numbers.