Commodore 2001-8 User manual
•
PET
PERSO
OMPUTER
PERSONAL COMPUTER
PERSONAL COMPUTER
PERSONAL COMPUTER
PET 2001-8
PERSONAL COMPUTER
USER
MANUAL
OCTOBER 1978
The information in
this
manual has been reviewed and is believed to
be
entirely reliable. However,
no responsibility is assumed for inaccuracies. The material
in
this
manual is for information
purposes only and
is
subject
to
change without notice.
first edition
©Commodore Business Machines, tnc., 1978
"All
rights reserved"
Commodore Business Machines
901
California Avenue
Palo Alto, California 94303
Chapter
1.
Chapter
2.
Chapter
3.
Chapter4.
Chapter
5.
Chapter
6.
TA61£OPCONTENTS
WelcometoyourPETcomputer
.........................•....
UnpackingyourPETandturning
iton
.
Basickeyboardinput
,
............•.
PET
keyboard
Screen
editor
BeginningBASIC .
The PRINT statement
Variables
Direct and program
statements
Literals
Functions
Elementaryprogramming ' .
Unconditional and
conditional
looping
Dataentry
Advancedprogrammingtechniques
String variables and
functions
Subroutines
FOR
N
EXT
loops
Subscrptedvariables
1
3
11
19
32
38
•
I
Chapter 7.
ChapterS.
Chapter
9.
PETcommunicationwiththeoutsideworld
57
PET
interfacesand
lines
Commands and operations for
peripheral devices
IEEE·488bus
Machinelanguageprogramming .
.. .. .. ..
.
...
.
.. .. .. ..
......
91
Allocation
of
memory
Commands from BASIC
Machinelanguage
monitor
Errorsanddiagnostlcs.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Debug techniques
BAStCerrormessages
OS
errormessages
ii
2.1
2.2
2.3
2.4
2.5
2.6
3.1
6.1
6.2
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
7.10
7.11
7.12
LIST OF FIGURES
Rear
view
of
PET
2001
.
PET
memory
bus
.
Memory map
by
functional blocks .
ASCII code
in
main memory " .
ASCII 6bit code .
PET
graphic character codes .
PET
keyboard scan lines .
Functions expressed
in
terms of built-in BASIC functions .
Principal pointers into
PET
RAM
.
Simplified
view
of
PET
.
Edge
connectors
J1
and
J2
.
PET
IEEE connector pinout .
Receptacles
for
the
IEEE
interface .
IEEE
standard
connectors .
Parallel user
port
information .
6522 VIA addresses
in
PET
.
Parallel user
port
example .
Connector
J3
contact identification .
Second cassette interface
port
.
PET
second cassette edge connector
J3
.
Edge
connector J4 .
iii
3
4
6
7
7
8
12
43
55
57
57
58
58
59
59
61
62
62
62
63
63
Status codes for IEEE
bus..
. . . . .
.. ..
. . .
.. ..
. . . . . . .
.. .. ..
..
88
IEEE-488registeraddresses
in
PET..........................
88
Code assignments for command mode operation. ............. 89
Example Floating Point
Numbers.
.......................... 94
Machine Language Monitor Listing 100
7.13
7.14
7.15
7.16
7.17
7.18
7.19
7.20
7.21
7.22
7.23
7.24
7.25
7.26
8.1
8.2
LIST OF FIGURES(continued)
Memory expansion connector .
Multiple file structure .
OPEN
for
write
from
PET
OPEN
for
read
to
PET
.
Status
word
errors .
Default parameters .
Examples
of
default parameters .
IEEE
bus
contact identification .
Transfer
bus
handshake sequence .
Byte
transfer
from
talker
to
listener .
Signals described
by
IEEE
bus
groups
.
63
69
73
74
80
82
82
83
84
85
87
iv
LIST OF APPENDICES
A.
Detailed PET memory
map
B.
BASIC
statements
c.
BASIC
commands
D.
Expressions and operators
E.
Space and speed hints
F.
Main logic board assembly
G.
Suggested reading
v
Chapter
1.
WELCOME
TO
YOUR PET
COMPUTER
Congratulations and welcome
to
the exciting new world
of
personal computers.
By
selecting
the
PET
2001
you have
eliminated
the
problems
of
getting
apersonal
computer
system running. Your
time
is now available for learning the
functions
and
capabilities
of
your
PET.
In
fact,
if
you
follow
afew simple
procedures
outlined
in
this
manual,
you
should
be able
to
achieve
initial
operation
of
your Pet
2001
within
ashort period after unpacking the shipping container.
The potentials
of
your
PET
are virtually limitless. This book,
by
its
very nature,
is
limited.
Questions
will
arise
that
this book has
not
covered
or
even anticipated.
Write
to
us at Commodore
with
your questions. We will answer many
that
you and other users
will
pose
with anewsletter we'll
be
sending out from time
to
time
to
users.
Commodore Systems Divisions:
901
California Avenue
Palo Alto, California 94304
USA
360
Euston Road
London NWI 381
England
3370
Pharmacy Avenue
Agincourt
Ofjario,
M1W 2k4
Canada
PET
is aPersonal Electronic Transactor. Everything is complete in one steel cabinet. It contains a
CRT
board, keyboard, computer board, and aCommodore supplied cassette. There is abuilt-in black and
white television monitor, which displays characters in aformat
that
appear
to
you
to
be forty characters
by
twenty-five lines.
At the heart
of
your
PET
2001
is
an
MCS 6502 microprocessor. This microprocessor
totally
controls
operation
of
the screen, keyboard, cassettes, and additional peripherals which can be added
to
the
PET.
The product is so construed
that
you cannot damage the
PET
from the keyboard. The operating system
cannot
be
destroyed because the computer software,
or
operating instructions are contained in afixed
memory. (Called Read-Only·Memory) This allows both the first
time
user and the sophisticated user
to
use the
PET
with impunity.
In
order to satisfy the needs
of
the serious user as well as the
first
time user
of
acomputer product, we
have used three formats in
this
manual.
Summary discussions
in
this type font are designed to answer the questions
of
aprofessional
programmer.
When
you are first using your
PET
manual, read these sections
lightly
andspend time on the
more detailedexplanations which are in the type font
of
the precedingparagraph. Afteryou have used the
PET
abit, the italicized summary sections will be useful when you want
to
review
how
a
particular
instruction works.
The third type
of
format gives adetailed description
of
how
the
PET
Implements asection. These
sections are for people who use the
PET
at the machine level. The
first
time
reader may
find
these
sections
difficult
to
follow
and we recommend helshe use them
only
on re
...
eading the material when
more familiar
with
the
PET
operating systel\1.The language which you
will
use
to
communicate
with
your
PET
is
called BASIC, an acronym for Beginners All-purpose Symbolic Instruction Code.
1
It should
be
noted that there are
other"'EY~~enCEt;
'1Ji:"teri~lwritten
aRput BASIC, with different
tYPlilS
of
usage in mind, which are available
to
the public.
We
have included a
list
of
such suggested reading
in
the appendix. Some
of
these manuals maybe more' useful
to
satisfy
aspecffic
need which
is
not covered
in
this
particular manual.
The ultimate teaching device for learning how the
PET
works
is
the
PET
itself.
In
some cases, we will
ask you to use the
PET
along with the text, rather than Just continuously reading what
to
do.
In
any
case,
this
documentation is
sufficient
to
allow you
to
get started on
an
example.
By
constructing similar
examples
of
your own, any questions which
might
arise are answered by the machine itself. Of course,
we
invite comments on
this
material and particularly on examples which you as
an
individual have used
to
resolve any unclear statement.
Lastly, since
this
manual cannot presume
to
provide all technical information
of
hardware or
programming aspects
of
the MCS 6502 microprocessor, we direct you to two other Commodore
publications: There is amanual available from your dealer called Hardware Manual which also includes
descriptions
of
the auxilliary devices which generate the control signals necessary
to
allow the
PET
to
operate. Detailed specifications
of
the computer and the language in which it operates are available
in
a
book called Programming Manual. This
is
available for purchase from your
PET
dealer
or
either can
be
purchased from Commodore directly.
NOTES
2
Chapter
2.
UNPACKtNGYOUA
PET
AN'lfTURNING
IT
ON
Please check the carton for any special unpacking instnl-cti9ns and carefully examine your
PET
for any
concealed damage. If anything
is
amiss, reportly this immediately
to
both the place
of
purchase and the
shipping agent.
Remove your
PET
from
its
protective shipping carton and place
it
on the counter, desk,
or
other suitable
surface, then plug
it
into
any standard, grounded electrical
outlet.
(In
some countries no plug is provided.)
Tv
BRIGHTNESS
ADJUSTMENT
SERIAL NUMBER
'""
ELECTRICAL SPEC.
MEMORY EXPANSION
J4 CONNECTOR
,3:~~,~,
INTERFACE
.d
I
PARAllEL
USER PORT J2
1.6A
FUSE -.,..:=/
ISlO
BW)
3-WIRE
AC
POWER
CoRO
Figure
2.1.
Rear
view of
PET
2001·8 showing switch,
fuse, line cord and interfacing connectors
The power
switch
is
located in the
left
rear
of
the
PET.
Closing the switch
to
the
left
turns the
PET
on and
closing
it
to the right turns
it
off.
(There
is
awhite
dot
on the power
switch
to
indicate
it
is
in the power-on
position,
or
an
ON/OFF label.)
Immediately, when the power
switch
is
turned on, power is supplied
to
the internal circuits. There is a
time-out
circuit
in aspecial condition (reset condition) which initializes them into aknown state. If the
screen has had power immediately prior
to
this
time, you will see on
the
screen avariety
of
strange
characters which reflect the current contents
of
the computer memory which is controlling the screen.
The screen memory transfer
to
the screen is done
with
circuitry outside control
of
the main
microprocessor, and so, even when the computer is
not
operational,
the
screen always displays the
current contents
of
the screen memory.
3
At the end
of
the
power~n
cycje,
the
computer initializes the internal membrv, biank's the
s'c~reen
, . .,
.....
,.
"j
...
, , • .
.•
, , "':.16,,"
",'
',.
-
,','
.-'
,
:'.
r-
temporarily, and then displa)'s on the
sc.r13~n
am13ssage.llke the followir:tg:
u*COMMODOREBASIC***
7167
BYTES
FREE
READY,
o
The 7167 refers
to
available users'programmable memory. Abyte
Is
the fundamental data element
of
the
PET
computer and corresponds roughly
to
one letter or
digit
of
Information.The
SK
model should show-
In
theory·
"S192
bytes".
But afew hundred are used by the
PET
internally. The balance shown "7167" is
net available bytes.
If
you fail
to
get the power·up displaythe
first
time,
try
turning the power switch slowly
off,
then back on.
To get the display, four different types
of
memory
are
used:
ROM,
User ReadlWrite,
1/0
(Input/Output), and Screen Memory.
The relationship between these memories Is shown In figure 2.2.
6502
microprocessor
address and data-bus
ROM RAM TV RAM
110
14k
Bk
1k
2k
/\ I I
[\
\
II
keyboard user port
,/
IEEE·4BB
Figure 2.2.
PET
memory bus
4
ROM
(READ ONLY MEMORY)
ROM
causes the
PET
to
perform
most
of
its
operations.
In
each
PET,
14K
of
ROM
contains aseries
of
programs written
by
Commodore which scan the keyboard, print the display, control inputfoutput, count
the real
time
clock, and execute commands that the user types in.
Read
Only memories are not only
the
lowest cost memory for storing
this
data, but also give the user the
most
protection and
the
fastest
operation
of
his
machine. This is because the operating system memory Is indestructible from the
keyboard,
or
from the user's program. Notonly is the machine available
to
run basic from the moment it is
powered on, but also
the
user program cannot damage
the
basic operating system.
1/0
MEMORY
The second type
of
memory is
that
which
is
devoted
to
Input/outputoperations.This memory contains
110
devices called PIA* and
VIA**
which
allow
the
PET
to
individually control the
bits
that manipulate the
computer. Except when special
110
operations are desired, the user should not allow his program
to
interfere in any way with these areas. The operating system automatically handles these locations in
order
to
perform legitimate Input/Output operations.
USER READWRITE
MEMORY·
R.A.M.(RANDOM ACCESS MEMORY)
The third type
of
memory
is
the User Program Memory Space.
(We
will call
this
area
RAM
throughout
this
book.) In astandard 8K
PET,
it
is
located from location
$0000
to
hexidecimal
$1FFF. Adetailed map
of
all
the
memory is included in figure 2.3, showing where the
ROM,
RAM,
110,
and Screen Memory are located from aprogramming standpoint.
As
you can see
by
the
map,
the
first
1024 bytes
of
memory are reserved for the operating system
to
use
for
its
various tasks, including
the
buffering
of
data from the cassettes and other
110
devices.
The message "7167 BYTES FREE" is aresult
of
an
analysis
of
BASIC which starts at
locxation
1024
and cycles through the memory
to
determine which locations are available, thereby,
performing acheck on whether or not
the
ReadlWrite Memory is working correctly.
If the number was less than
7167,
you may have ahardware problem.
11
the number is
greater than
7167,
you probably have added your own memory. BASIC can automatically check
up
to
32K
of
RAM
as long as the added memory is continuous
to
the memory
that
comes
furnished with the
PET.
This memory
is
really the working memory in the machine;
it
is where programs
are
loaded and BASIC holds all
of
the program variables.
Later on, we
will
discuss some techniques to expand
this
memory by using tape files and
program overlays.
SCREEN MEMORY
The screen memory is physically composed
of
the
same kind
of
chips that are used
to
make up the PET's standard memory. It is constantly being used by the
CRT
control electronics,
which takes
the
individual bytes
of
memory and uses them
to
address aspecial character generator
ROM,
thus displaying characters on the screen.
As
mentioned during the power-up discussion,
this
process is
totally
automatic, and
the
programmer has
no direct control over it.
*PIA -Peripheral Interface Adaptor
**VIA -Versatile Interface Adaptor
For information about these and related chips, see 6502 Hardware Manual.
5
o
1024
RAM Operating system and
BASIC working storage
8192
32768
33792
34816
35840
36864
49152
59392
61440
65536
RAM User BASIC program
-------------
User Variables
Expansion RAM area-24K
RAM TV
1---------------
Images
of
TV RAM
Images
of
TV RAM
Images
of
TV RAM
Expansion ROM area·12K
ROM
BASIC
1/0
ROM operating system
FIgure 2.3. PET memory map
On
every
cycle
of
the
TV screen
(1/60
of
asecond),
the
hardware
starts
with
the
least
address
($8000) in
the
screen
memory
and processes
the
screen
data
starting
at
the
upper left-hand
corner
of
the screen. Each character in
the
memory
is
addressed
into
the
character generator
eight
times,
giving us
an
8row
high
character
on
the
screen. The character ROM
that
is
used generates
8
dots
each
time
it
is
addressed. These
dots
are
serially
fed
to
the
screen, working from
left
to
right
and
top
to
bottom.
This gives
an
8
bit
wide 8bit
tall
character with no spaces between characters. The CRT
controller
automatically
changes
the
addressing
of
the
character generating ROM, depending
on
whether or
not
it
is
scanning
the top
line
of
acharacter,
the
second
line
of
of
a
character, etc.
There are
two
character sets stored in
the
ROM. You can change
the
character set on
the
screen by POKEing
memory
address 59468 with a14(a
12
turns
it
back) which
turns
it
to
the
second character set. After you have played
with
the
screen a
little
bit, you
may
want
to try
this
feature
to
see
if
your
PET
performs
this
way. The second character set
substitutes
lower
case letters
for
the
graphic set
that
is
available in
the
first
set.
To understand
this,
let us review
how
characters are represented in
the
PET
and in
the
memory.
6
CHARACTER
REPRESENTATION IN PET
MEMORY
The standard ASCII code is used to represent characters
in
the
main memory. (RAM)
In
the PET,
the
8th bit (bit 7) is used to signify BASIC command words or graphics characters forthe PET
screen.
~
"I
~
~ ~
~
11 1 1
1
~I
~
~
1 1
~
~
1 1
I
41
"1
~
1
~
1
~
1
;;"1~
tf\:Si::Ild
NUL
VLc.
:;~
"
~~
i,W p
1:.lC1\:S1
::>OH
1)(,;1
1f\ Qa
..
tJ\::Jlt:l
!,;IX
V("
"
~
kbr
~~'1
c.IX
VL3
~
;;
(;
:;
c s
~1~"
1:.01 .1)(,;4 $4V I dt
e1e,
I;.N~
NIlK
:<
~
c.
u"u
e11e
tH
..
':K
:;YN
..
b,Vfv
e111
~c.L
c.llJ
'(
(;
W
..
..
1e~"
lJ:;
LIlN
(
'"
HXhx
il:l'"
HI
c.~1
)
~
1yi"
1"1"
L'
bUJ;l:
•J"0z
11:111
VI
I;.~'.,;
+KL k
11CSld
H
,:;
<.
L"1
11'"
t..:H
fj~
~
'1
Jm
111"
:;0
kti
~
Nt n
1111
:;1
V:;
/'t' •
..
0
Figure 2.... ASCII ch8r8ct
...
set (7 bit
code)
Example
in
the
PET:
A
is
represented 0100 0001
Shifted
A(a
spade)
is
11000001
The
screen
memory
is
organized
with
adifferent
representatIon
from
the
main
PET
memory.
There are only
64
characters from the standard ASCII set that are normally printable.
.
lJ
"I
"•"
(j
1
~I
"•11
I
41
"1
~
,
;;<:1~
i::It:H'JI:I
"pe
e~e,
11
Q1
i::Ildl\::J
~
k
..
"
ee11
(;
:;
~
;;
e,
••
VI$4
"'1~1
c.
U
:<
~
tl11\::1
,V
..
b
~111
(;
W
'{
1,,~~
HX("
1,,~,
1Y)
~
1",e
J"•
1~11
K L +
11ee
L"
<.
11~'
NJ
~
11W
N
,.
>
1111
0~/"t'
figure 2.5.
ASCII
64
ch.,.cler
.'1
(6
bll code)
7
These are the same characters that are directly available on the
PET
keyboard.
The representation in screen memory
is
derived from the standard ASCII set by dropping bit
6;
giving us asix bit code for the keyboard characters.
The graphic,
or
shifted characters, set
Is
represented by a 1
In
bit six
of
the screen memory, giving an
additional
64
displayable characters.
This gives the following table for
PET
displayable characters.
It
should
be
noted that all
of
the graphics
characters are organized so that they are
just
a
shift
from the normal keyboard character.
Ii
01
~ ~
~
~
1111
I
~I
~
~
1 1
~
~
11
I
41
~
1
~
1
~
1
~
1
~,1i:::J
~
......
~
p
..
1,
I:H:U::H
AQ1•••
...
,
."1"
Ii
k"
~
I•T
1l~11
(;
S"
~
"
~
• 1
....
j)
1•4I I
111
..
,t. U"
~
rII
1111
..
tV
..
0
)<.
..
I
11111 GW
'(
I0I
111
....
HX<
II
I
4-
-
•
111111
IV)Y,1
~
-
1
..
,
..
J"*,•I
.J
11111 KL+J+••
lUIl::J
L,(L1•,
11111 MJ=\I,"
111
..
N1)/
..
,
•,
1111
0
+-
/"I"-
'.
FIgure 2.8.
PET
graphic charaet...
Nt
(7 bit
code)
Example: This gives us the following conversions:
Character
In
main memory
In
screen memory
A01000001 00000001
+1100
0001
00000oo1
100110001 00110001
~
10110001 01110001
Note the reduction from seven
bit
ASCII
to
six bit gives the effect
of
changing the order
of
Aand
1.
In
screen memory, the 8th bit
is
used
to
store reverse field. The reverse
field consists
of
taking the dot pattern from the character generator and reversing It, replacing
awhite
dot
with black and ablack dot with awhite.
If
the operating system
is
used, it automatically translates the values from ASCII Into the screen
memory representation. Both PRINT and direct Input from the keyboard result in automatic
translation between the screen memory and the main memory.
8
USE OF THE SCREEN MEMORY,
r"
There are three ways
to
get data
into
the screen memory. The
first
of
these;s
to
POKE into
the appropriate memory addreSs
the
desireci'translatedc"fiara:cter. This is programmed only when
normal updating
of
the
screen
is
too
slow.
As
long
as
the
PET
directly
controls
the screen, there
is
no
apparent
effect
from the fact
that
the screen
and the
PET
are contending
for
access
to
the memory. The routines In
the
PET
change the screen
memory only during
times
when
the
screen memory
is
not being used for display. This slows
the use
of
the screen
memory
down
to
about
40
percent
of
the
speed
obtainable
with
aPOKE. The POKE,
however, gives avisual effect
of
flashing dots, because the screen Is displaying the character
that
Is being passed from the
PET
to
the
screen memory, rather
than
the character
that
should
be
displayed at
that
particular
position. When aprogram pokes
to
the screen,
the
faster
it
runs the more flashing there will
be.
The second way
to
get data
onto
the screen is
the
keyboard. During a
time
when keyboard input
is
enabled, the character being struck on the keyboard is automatically displayed on the screen.
The third approach
is
by use
of
the PRINT command
in
BASIC. When
..
PRINT
"ABC"
is typed
to
BASIC,
it
results in the next line being printed as:
ABC
This
is
aprint
of
aliteral field in which all characters between the quotes are printed.
The next position at which acharacter
will
be displayed
if
typed on the keyboard is indicated
by aflashing signal called acursor. The cursor is avisual indication
to
the user
of
the next
print position in screen memory.
What Is
physically
happening
in
the
machine Is
that
every
time
the screen
is
recycled, about
1/60th
of
asecond, an
interrupt
to
the
PET
is
generated. This generates areal-time
clock
on the
computer
(the PET) and steps a
blinker
counter. When
this
counter reads
37,
the character referenced
by
the screen memory
pointer
Is reversed in the 8th
bit.
This causes the reference character
to
be
shown
in
alternating normal and reverse
field,
giving as visual
effect
of
blinking.
By moving
the
pointer,
we can
print
output
any place on the screen. This
is
done by using a
combination
of
the
keyboard and some software
called
the screen editor,
which
manipulates
screen memory under
control
of
the
keyboard.
9
Chapter
3.
BASIC KEYBOARD UNIT
Whenever
the
blinking
cursor
appears on
the
screen,
the
computer
transfers
data
from
the
keyboard
to
the
screen memory.
Keyboard
data
;s transferred by the
interrupt
routine
to
the
screen memory each
time
anew
key is struck. Only after acarriage return is
the
keyboard
data
transferred
to
the operating
program, and then a
whole
line
is transferred at once.
There are
two
exceptions
to
this,
neither one
of
which
causes
the
cursor
to
blink. One
of
them
is
the
use
of
GET,
which
will
be
discussed
in a
later
section,
and
the
other one is when
the keyboard
data
is
accessed
directly
using
machine
language programs.
The
PET
keyboard
has
been optimized for use as a
computer
keyboard,
though
the
organization
,
is
similar
to
that
of
a
typewriter
so a
touch
typist
does
not
feel
totally
out
of
place.
However, some
important
changes have been made:
1.
Because
of
the
high
use
of
numbers and
calculations
with
the
computer, acalculator-like
number
pad
has
been added to
the
right
of
the
main keyboard.
2.
The
number
pad
has
all
of
the
mathematical
operators in aform
that
is normal for BASIC.
3.
The various keys
for
screen movement and
editing
are
located
on
the
numeric pad.
4.
The characters
which
are normally
the
shift
of
the
numbers on astandard keyboard no
longer require
shifting.
These characters are
quite
often
used in BASIC, and
it
is
convenient
to have them available
without
shifting.
5.
All standard characters are
unshifted,
so
that
a
complete
64-character
graphics
set is
available by use
of
the
shift
keys. These
graphics
give
the
PET
significant
line
drawing
ability.
PET
KEYBOARD
The keyboard
consists
of
73 keys,
including
two
shift
keys,
either
one
of
which
may
be pressed
to
cause
the
upper or
shifted
characters
displayed
on
the
keyboard
to
be
operational.
Lower characters
are always used
unless
one
of
the
two
shift
keys is pressed
simultaneously.
Each key has
a
thin,
transparent
plastic
film
covering
the
key
top
which should be removed. This
protection
was
left in place
to
protect
the
keys against
scratching
dUring
shipping.
To
remove
the
film,
carefully
peel
it
off
by uSing
the
sticky
side
of
apiece
of
masking tape so as
to
avoid
scratching
the
keytops.
There are 64 printed characters on
the
keyboard with 64 upper case,
or
shifted
characters on
the
same keys. The rest
of
the
keyboard
consists
of
function
characters. Some
of
the
functions
are obvious:
like
character
return or
cursor
right and
left.
Reverse on
allows
all subsequent characters to
be diplayed in reverse
field·
black
on
white.
The reverse key ;s operational
on
amemory basis. From
time
to
time
the
key
is
struck,
the
function
is operational until
it
is terminated by aRETURN pressed or printed, or by pressing reverse-off
(the
shifted
reverse key). This
concept
of
reversal
of
function,
up and down, right and
left
is carried
through
to
the
function
keys, so
that
the
complementary
functions
are usually combined, with one being
the
shift
of
the
other.
The keyboard
is
scanned
using
a6520 PIA, afour
line
to
ten
line
decoder
and
the
interrupt
routine
from
the
CRT
controller.
Each
time
the
Interrupt
occurs
from
the
CRT,
the
keyboard
is
scanned
using a
left
to
right
scan. The keyboard
is
organized
on
a 2 x 5
row
matrix
with
the
matrix
being repeated 8
times
across
the
keyboard. To
implement
noise
protection
and Nkey
roll
over,
the
keyboard scan
routine
keeps
the
final
value
of
the
last
scan
in abuffer.
Until
that
key
is
released,
no
other
keyboard
scans
are acknowledged
unless
a
later
scanned key
Is struck. The
later
scanned key
is
then
considered
to
be
the
next
key
closure.
The
algorithm
does
not
11
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12
give classical Nkey roll over
but
does
allow
for
legitimate
rejection
of
noise and trapping
of
the
keys
in
the order
that
they are struck.
The keyboard Is
left
scanning the
last
row, which contains
the
stop
key. This
allows
the routine in
BASIC,
that
checks for
the
stop key
to
sample the
input
1/0 device, Without having
to
perform any
of
the normal
functions
of
scanning. The user can take advantage
of
this
by reading the
input
character
for
that
row.
The
shift
key
is
aspecial mUltiple key closure and
is
treated separately. If
either
of
the
two
shift
keys
Is pressed, the software sets aspecial
shift
switch
which
is
used
to
change the decode
of
the key.
All key closures are translated using aROM·based look·up table for the key. The
shift
key
is
encoded
into
bit
8
of
the ASCII character which
is
then translated
into
the
screen representation
in
the standard way.
Once the hardware translation
is
done, the encoded value
is
transferred Into a
10
character keyboard
queue. The keyboard queue is loaded every time anew key closure
is
sensed and is unloaded as soon as
characters can be transferred
to
the screen. "
ThiS
input
queue
is
scanned
by
the
GET
routine
directly
to
allow
input
without
going
to
the screen. The
input
stack
may be scanned by auser program. The user program can look at the
pointer
at
location
525
to
determine whether
or
not
it
is
greater than zero;
if
It is,
that
means
that
there
is
data in the keyboard
queue. The keyboard queue
is
located at 527·536. The
first
character may be taken
out;
all subsequent
characters moved down, and aload index pointer decremented by one.
This
is
adangerous routine, unless
written
in
a
machine
language
with
the interrupt masked, because a
new key closure
could
store anew value during atime
that
you are scanning and changing the queue.
Both the GET and keyboard
input
routine take care
of
that
automatically by only operating during the
interrupt or
with
the Interrupt masked.
Whenever the screen
editor
routine
is
operational, aspecial two·level operating system
is
in play. The
first level enables the cursor
to
flash and writes data from the keyboard
to
screen memory at the current
cursor
position.
The routine then moves
the
cursor one character further down in memory. The process
is
repeated, trying
to
keep th keyboard queue empty.
The second level flashes the cursor and translates and stores characters from the keyboard
into
the
keyboard queue. Meanwhile, the first level operating system always watches the
input
stream for a
carriage return. After the carriage return
is
printed,
this
routine automatically transfers the entire line
to
the operating system. The rest
of
the operating system does not see the characters until they have been
typed and acarriage return
is
sent. This
allows
for total
editing
of
the line, prior
to
handing
it
to
the
operating system.
An
interesting
trick
for the more advanced programmer
is
to
use the
PET
to
write
its
own programs.
By
printing
out
aline
to
the
screen, forcing acarriage return
into
the keyboard queue and then returning
control
to
BASIC, new line numbers may be entered
into
the memory. Another example
of
the use
of
the
keyboard queue
is
the
LOADJRUN
sequence which
is
implemented by the keyboard scan program when a
shift-run Is encountered, the routine
automatically
forces "LOAD, CARRIAGE RETURN,
RUN
CARRIAGE
RETURN"
into
the
keyboard queue. When control
is
returned
to
the
input
routine,the load followed by the
run
is
automatically transferred in the proper order.
It should be
noted
that
this
keyboard queue
is
only
ten characters long and
ifit
is exceeded, dramatically
bad effects can happen
to
your system. The
only
known recovery from exceeding
this
queue
is
to
power
13 j
..
..,......back
on
and
start
over. When fooling with
the
queue, remember
that
if
the userIstyping on the
--lftyboanl and you do
not
have the Interrupt turned
off,
the
operating system
Is
going
to
kill
you.
SCREEN EDITOR
Typing on the keyboard, while the cursor
is
active, transfers what is typed on the keyboard
directly
to
the screen. This function is like asimple computer terminal which requires you
to
retype awhole line
until you get
it
right, but
the
PET
lets
you edit your mistakes before you enter aline.
The
editor
is
best
understood with a
PET
to
Illustrate
it.
The user should follow discussions on his own
PET,
as many
of
the
examples are much more
difficult
to describe than
to
see.
To
follow these examples, two concepts are necessary. One is
that
when we type a?the BASICoperating
system
is
goling
to
interpret
the?
the same as PRINT.
The second concept is
that
when we follow
a?
by a",all characters after
the",
until
the
next"
is
en-
countered,
are treated by BASIC as characters
that
you will want
to
have printed onto the screen.
•
In
this
section you are operating the computer in what is known as adirect mode. (i.E. rather than
programming mode). BASIC is executing each instruction like print as soon as you type
it
into
the
system
and
hit
carriage return.
We
will
see in
the
future that
this
is not the way most programs are operated. It
does make
the
machine useful as asuper calculator.
The
first thing
that
we want
to
do is have
the
machine type asimple message.
You
should have already
done
this
with your users' guide. However,
we
hope by now that you understand a
little
better.
We
type
the line:
?"HI THERE"
R
E
T
U
R
N
You will see
that
BASIC responds by printing
HI
THERE. It should
be
noted that each time
we
struck
akey on the keyboard, the cursor moved automatically one place
to
the right, allowing
us
to
type in the
next character, and nothing else happened until after the carriage return. When the carriage return
occurred, the HI THERE appeared almost immediately on
the
screen.
Let us
talk
about the simplest function; that is, immediately correcting amistake. Retype
the
line
?HI THERE
B.
What we were trying
to
type was
HI
THERE
PET,
but we
hit
the character Bratherthan
P.
For those
of
you who are touch typists, you may have already made
this
mistake with the PET's close
keys.
In
order
to
allow
you
to
immediately correct
this
mistake, there is akey which allows
us
to
erase a
previously struck character. This key is called the delete key, located
in
the upper right-hand side
of
the
keyboard.
If
we
strike the delete key once, you
will
see that the Bhas disappeared. Typing the Presults in
an
overstrike
of
that
position.
We
can now finish typing
ET;
then
hit
carriage return, causing the
PET
to print
out HI THERE
PET,
ablank line, and READY.
The delete key is
the
fundamental editing tool which allows you
to
strike
out
as many characters
as
you
want from where you are and then retype. This is the simplest form
of
editing. It is implemented by
14
decrementing the
screenpolnter
fr9m where you are by one and striking ablank over where the screen
pointer is.
We
can go back and erase the
READY
that
is
right.in .front
of
our
cursor by
just
continuously
striking the delete key. Notice two
facts
as you are striking;
(1)
if
you strike slowly, the cursor
will
move
one character at atime, and
(2)
if
you strike fast,
the
cursor will actually move several characters
before you see
it
blink. This phenomenon occurs because
it
takes
15
times as long
to
blink
2characters
as
it
does
to
overstrike one. Also, notice
that
1.'he
PET
wraps around the screen. The screen memory is
organized so
that
deleting
the
previous character in memory moves the pointer back over
that
character.
Because
of
the
fact
that
the characters scan from right
to
left
in
40~01umn
chunks, for example deleting
the character at the beginning
of
the line, and then striking the delete
key
at the beginning
of
the
line,
deletes
the
40th character
of
the
previous line. Just keying back
40
strokes erases the READY from the
line above, however,
this
is
apretty slow way
of
editing.
There are three cursor movement keys on your
PET.
One key movesthe cursor right
or
left;
the second key
moves
it
up and down, and the third key moves
it
home (upper left-hand corner) and clears the screen.
CURSOR RIGHT AND LEFT
The cursor right key mbves the pointer one character
to
the right. If we strike
it
now five times, you
will
see
that
it
moves us five columns over. It accomplishes
this
by changing the cursor pointer in memory.
The cursor
left
key
is
on the same key as the cursor right and
is
evoked by shifting prior
to
striking. If we
type
that
four times, you will see that now we are back one character
to
the
right
of
where we started. If
we strike
it
two
more times,
it
moves us around the corner
of
the previous line. Cursor left,
of
course,
just
moves
the
cursor pointer one character less
in
memory. Going
to
the
left,
it
moves one character at a
time. Obviously, by doing this, we are able
to
edit the screen. However, faster editing can often be
achieved by use
of
the
cursor up and down keys.
CURSOR
UP
AND DOWN
The cursor down moves the pointer
40
columns to the right from
its
current position. This gives
it
the
same Visual effect as moving
it
down one line on
the
screen. For
an
example,
try
spacing over forty
positions with
the
cursor right. The cursor
is
now on the same position on the screen, but down one line.
To cause
the
cursor
to
move up, hold down the shift key while striking the cursor up/down key once;
this
gets us back
to
our original position.
Cursor up moves
the
screen memory pointer
"up"
40
characters from
its
current position, or rather,
40
characters less in screen memory than the current position.
SCREEN EDITING
We
can now use
the
cursor movement characters
to
get
up
in position on the second Hin
the
HI THERE
PET
message. Once you are there, you can now delete the Tby striking the delete key.
You
will
notice
that
all the characters
t9
the right
of
the character being deleted are moved
to
the
left
one character. You
will
now see the delete
is
actually amatter
of
mOVing
all
the
characters in memory
left
one, rather than just
substituting
ablank.
INSERTIDELETE
Before analYZing insert and delete, we should be reminded
that
the
screen memory is organized such
that
any single line may
consist
of
40
or
80
characters.
(See
section on screen memory.) Insert and delete are
concerned with
the
characters on aline. Whenever
the
delete key is struck, all
of
the characters,
starting from the position
of
the cursor,
to
the end
of
the
line, are automatically shifted one character
to
the
left,
replacing the character preceding the cursor. The cursor is then moved
to
the position
of
the
replaced character.
The last character in the line
is
automatically blanked. Insert
is
the reverse
of
this
process, If we want
to
15
This manual suits for next models
1
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