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Inside the Commodore PET – Electronics Today

by Graham Wideman and Mark Czerwinski | Electronics Today | February 1978 Reproduced by www.commodore.ca April 13, 2002 To read the original scans (with all the images) go to our GALLERY, MAGAZINES, ET 1978

We don’t fee that “Personal Eletronic Transactor” completely  describes it, but we liked what we found in this machine.

Lcommodore-pet_cover-et_october-1978OOK WHAT  HAPPENS when an aggressive, popular-market calculator company joins forces with  a low profile, high technology micro-processor manufacturer! Commodore and their mpu wing, MOS Technology have come up with a product which while technologically  unsurprising is revolutionary from a packaging and marketing point of view.  Never before has a company tried to convince the public that a computer is an  acceptable, fun, useful, perhaps even an essential thing to have around the  house.

In the past  it’s happened with radios, TV s, automobiles, calculators, digital watches. We  really are now seeing the introduction of the personal computer to widespread  use.

We were  dying to get some hands on experience, and managed to borrow one of the few (one  or two) PETs that Commodore had in Canada. Here’s the story, with some  surprises!

IT’S  COMING, IT’S ALMOST HERE No doubt many of our readers will have heard so much about this machine that  they feel it’s a little late to do a review and call it news. Certainly  literature, advance press releases, tantalizing hints and the like have been  with us for over six months, but the real machine is still only almost just  arrived. A few words with MOS Technology in Pennsylvania put us in the picture.

By  early December about 500 to 600 PETS had rolled off the assembly line, being  produced at a rate of about 30 per day. These were almost all being shipped in  the States and without complete manuals. A temporary manual was being issued,  which contained little more than a description of BASIC, and a few elementary  explanations for how to operate the machine. There was no sign of a complete  manual in mid-December, nor much hope of its appearance within the near future.

 As for  sales of the PET in Canada, no one seems to know very much. The price fluctuates  (mostly upwards) from week to week, and availability??? The price started at  $695 and $895 (Canadian) for 4k and 8k models with delivery in January. At the  time of writing (mid-December) the price is up to $850 and $1,100 for those same  machines, and no CSA approval has been granted, which means no deliveries to  consumers. A couple of machines have appeared in very eager stores, but these  outlets can still only accept orders.

commodore-pet-fig1_et_feb-1978In view of  the low speed of production, and the incredible demand we expect will have  developed for this equipment, it is likely that the US demand will exhaust most  of the supply and Canadians will have a long wait. March is our optimistic guess  for first delivery, but then it’s just a guess.

THE  MACHINE ITSELF The actual, non-imaginary, not just promised hardware we borrowed is pictured on  the cover. We have several assurances that what’s in that one is what you’ll get  if you order one. It does however look significantly different from that  pictured in Fig. 1., chiefly in case styling, recessing of the keyboard , and  cassette deck. In addition the keyboard in Fig. 1. has completely different  graphics symbols. In any case (?), it appears that our model is closer to a  production machine, or may even be one, having regard to the fact that dates  inside indicate it was buiit in early October, and the main pcb is numbered  0109. The cassette worried us a little though (see below), So what do you get?  Basically, what you see, and a few other features.

Inside is a  microprocessor system, and it’s hooked up to an alpha-graphics keyboard,  numeric-and-more graphics keyboard, CRT display, and cassette recorder. The  microprocessor system uses the (big surprise) MOS Technology 6502 mpu, with 14k  of ROM containing everything to operate the hardware, and purportedly a very  fast interpreter for BASIC. As stated above 4k or 8k of RAM is provided for user  program and data storage.

HOW DID WE LIKE IT There is no doubt that this package of micro-pieces is, well thought out, and  designed with the idea that the user needs to know very little about computers  to get “into” the PET. “Remember the increased popularity of tape recording when  cassettes came out? Make it simpler to use, easier for the human being. Same  philosophy at work here, and that is what will sell it. Needless to say in spite  of the fact we had very little documentation, we were highly impressed. 

First of  all there isno wiringup to do. When you plug in and switch on the PET, after  only a couple of seconds it’s ready for action. No BASIC to load from tape, it’s  already in there. This is one great headache reliever If the program you’ve  written bombs out and writes over memory in selected vital (of course)  locations. No waiting for a 20 minute BASIC reload. 14k of built-in software is  worth ten times its weight in silicon.

So after  “power up” you can start programming, (more about that under SOFTWARE) or load  from a cassette the software you want. Presumably in the future, tapes for  various useful software packages will be widely available, so you will  infrequently need to write your own, but its lots of full so you’ll actually  want to.

commodore-pet-fig3_et_feb1978TAPED Upon entering the LOAD instruction,  the machine tells you to stick the tape in and press the “PLA Y” button. Here’s  where a compromise has been made. Although the PET can switch the transport  motor on and off, it tells you to do the muscle work. Commodore have saved money  by using a standard (cheap) cassette recorder, which is sensible for the  consumer market they are aiming at. In fact, as you can see in Fig. 3., our  cassette was so standard it had a grille for “condenser mike”, and the corner  had been somewhat roughly cut off for the wires to enter its body.

The  cassette feature worked quite well, although a couple of demonstration tapes did  not load properly (we suspect over-worn tapes due to heavy use). One feature we  would have liked, (which could not have been included without a large extra  cost) would be a fast forward search. Without it the machine took a long time to  find a program if the tape was started at the beginning and the program happened  to be near the end. For this reason it seems most economical (time wise) to only  use C30 cassettes (these are thicker and stronger anyway} and then record only  one or two programs near the start of each side, unless programs are recorded in  some long sequence in which they are to be used, or for storage of little used  data etc.

KEY  FEATURES Another cost saver is the keyboard,  being of calculator type construction and feel. An idea of its size and action  may be gathered from Fig. 5. All our reviewers disliked the feel (no feedback,  you have to look at the screen to see your entry) particularly since we had a  rot of problems with a reluctant “N”” On the other hand these reviewers were all  used to sitting down at expensive IBM terminals for days (and nights) on and, so  perhaps the comments are unfair.

The  keyboard arrangement has all the rows of keys fined up instead of offset as in a  normal keyboard, which would appear to be a big disadvantage to experienced  typists. However, one is unlikely to enter vast quantities of text, and this  turned out to be less annoying than anticipated.

To solve  all these problems, it would be a simple matter to replace the main keyboard  with a “proper” one. Since there are no electronics inside, it would simply be a  question of configuring the switches the same way. How long will it be until  some one comes out with a PET soup-up kit, come on, we’ve started counting?

A  CRITICAL LOOK The video monitor appears to be quite  standard; Fig. 4 is a view in the back, The 9 inch CRT provided us with high  resolution for the 25 lines of 40 characters. A brightness control is located at  the rear.

ELECTRONICS For the electronics enthusiasts  (well, you’re reading this magazine. aren’t you?) we’ve got Fig. 6. Most of the  important features are shown on this photo, from which a basic idea of the  system configuration can be gleaned.

All the big  chips are plugged into sockets, something nice to see, especially for ROMs.  (When’s APL coming???) 

Commodore  appears to be producing mostly, if not exclusively 8k models at the moment, but  the hefty ($250) price difference between 4k and 8k models means that it could  be significantly cheaper to buy a 4k model and plug your own 4k worth of RAM  into the sockets, or holes, left vacant.

No doubt 4k  buyers will be offered the option of having an extra 4k installed later, and  they may as well save by doing it themselves. In addition, a planned option is a  24k memory unit with its own power supply to connect onto the expansion  connectors.

Commodore  literature says this unit “cannot be described as static or dynamic but  somewhere between the two”. This we take to mean dynamic memory chips with  additional hardware to handle the refresh independently from the PET. This would  allow simple interface, servicing, and great savings in the power supply using  low power cheap dynamic RAMs, The 6502, like 6800 does not use the entire clock  cycle, thus allowing (through suitable synchronizing circuitry) memory refresh  to occur in the unused part of the cycle. Clever and cheap economy attainable in  such a large memory unit.

The  keyboard is accessed via a 16 line Peripheral Interface Adapter (6520 like a  6820 for Motorola fans) which we expect uses 8 outputs and 8 inputs for decoding  64 keys, and then also a couple of the PIA “handshaking” lines for the rest of  the work.1n other word most of the keyboard decoding is done by software.

The  cassette recorder as previously mentioned is a customized standard one, with a  new pcb inside full of electronics suitable for data recording.

commodore-pet-fig6_et_feb1978In fact two  cassette interfaces are included (second cassette recorder available for about  $100.00 sometime) with input, output, and motor on/off lines. This uses six  lines of the 16 on the second 6520 PIA (we assume) leaving 10 lines, 8 for  parallel 1/0, and 2 serial, at TTL levels. Both cassettes record at 1000 baud,  but using built in software any program is recorded twice in series for  reliability, cutting the effective rate to 500 baud. Thus an program takes about  2 1/2 minutes to load. In addition, the PET is probably the

first  popular micro-computer to use the IEEE 488 bus for instrumentation   communications, making it compatible with many existing and future  digital  instruments, printers etc.  Commodore themselves plan to hang on this a printer  ($1,000.00?), floppy disc (mini $1,000.00, “full size” $2,000.00??) and  telephone interface ($?) available when we don’t know, but with a standard bus  you can be sure there will be a swarm of cheap add-ons from other sources in the  not too far future, just like when the S100 bus caught on.

A final  note of the hardware. Unlike some of the early hobby computer equipment the  inside of this machine looks like it’s built for business, and for the  manufacturer to stay in business. Constructed with quality electronics, and  chassis and pcb arrangement for easy assembly and service.

SOFTWARE As stated before, one of the beauties  of the PET is its built in ROM -full of software. The 14k includes 8 k BASIC, 4k  Operating System, 1k Machine Language and 1 k Diagnostic routine, according to  Commodore’s literature. First there’s 8k of “extended” BASIC.

The correct  term is “BASIC interpreter”. In simple terms an interpreter is a machine  language program which takes your BASIC program as data, “translates” it to  machine language subroutines and then executes it. This process actually occurs  in a line-by-line translate-execute manner. The term “extended” refers to the  fact that many statements are included that are omitted in some versions of  BASIC. Using BASIC means that programming can be conducted in a civilized  manner, using statements that are almost readable in English, with base ten  numbers. The machine does the work of converting to machine language and 8 bit  binary arithmetic.

As  encouragement to skeptics we’ve included a description of the language and how  (easy it is) to use it. Commodore claim that their BASIC is significantly faster  than anybody else’s which we cannot attest to, but for general use the PET  performed admirably as compared to the reasonably typical IBM 360 and 370  interactive, multi-user systems that the reviewers were familiar with. (But we  do like APL!)

The  cassette 1/0, keyboard, video, and other functions are handled by the 8k  operating system. Lacking documentation, we can only guess at the other two ROM  items. The diagnostic routine we assume verifies the operation of the hardware,  and uses the LED as an indicator for this task.

The 1 k  machine language we guess refers to an assembly language (“machine language” is  what the mpu uses, “assembler” is one step higher, using mnemonics for each  instruction, and is generally more readable and useable). There is also a  mention of next year’s “assembler device”, to be plugged Into the expansion port  for “machine language” (?) programming.

The  advantage of machine or assembler language programming is increased efficiency  of programs over those “interpreted” from BASIC. This could be critical in case  of a routine which runs several thousand times in a loop, or for a frequently  used function. BASIC

BASIC  (INTERACTIVE) One can think of the PET as operating  in one of two modes: “calculator mode” in which the operator asks the machine to  execute and give the results of one statement; and “programmed mode” in which  the programmer enters a series of statements, then has the machine execute the  entire set at one go. The second mode requires a little more structure for  “administrative” purposes. A list of the available statements and commands with  comments may be found to the right for reference.

CALCULATOR MODE Known by this name because it is the  process of getting a quick answer from a small formula, (similar to  calculators), or telling the machine to do one thing. Example: PRINT 3+4 The machine works out 3+4 and “prints” the result oh the screen. Another  example: LOAD FRED . This causes the machine to obtain a program called “FRED”  from the cassette, including giving you instructions about what buttons to push  on the recorder.

PROGRAM  MODE Problem: Figure out the sum of the integers zero to ten.  Program: 10 J% = 0 20 FOR l% = 1 TO 10 30 J% = J% + 1% 40 NEXT 50 PRINT J% 60 END The numbers down the side are line numbers which can be any integers, and they  keep the lines in order. We have chosen multiples of ten as it makes for easy  editing at a later date by inserting line numbers in between if necessary. The  variables are J% and 1%. The % signs identify them as integers. The FOR -NEXT  pair signify a “loop” to be executed many times. The first time 1% has the value  1, the second time 2, and so on up to 10.

On each  iteration the value of 1% is added to the accumulated total J%. After the 10th iteration the program continues to line 50, prints the answer, then stops at 60.  A simple task, a simple program. With the program stored in the machine, the  operator keys in “RUN” and the answer appears. How boring. But it does  illustrate the ease of programming.


BASIC  SIDEBAR Commodore claims that their full  floating point BASIC is the fastest yet implemented on a microcomputer. Here are  the statements and features included

Standard  Dartmouth BASIC Statements:  LET,  READ, PRINT, DATA, IF, THEN. FOR. NEXT, DIM, END, GOTO

Extended  BASIC Statements:  RESTORE, REM, GET,  GOSUB, DEF, RETURN, STOP, STEP, INPUT, FN, ON, GOTO, ON, GOSUB

Scientific Functions: SGN, INT, ABS,  SOR, RND, SIN, COS, TAN, ATN, LOG, EXP, PI

Logical  Operators: AND, OR, NOT

Operation Commands: RUN, NEW, CLR,  LIST,CONT, FRE

Formatting Functions:  TAB, POSc SPC

Machine  Level Statements:  PEEK, POKE; Allow  the user to examine and store at specific memory locations. USR, SYS; Link BASIC  to machine language subroutines with parameter passing or developmental  subsystems.  WAIT; Monitors status of a memory location such as an 110 port  until specified bits are set.

String  Functions: LEFT$, RIGHT$, MID$  Returns substrings (of specified length and position) of string acted upon. CHR$,  ASC. CHR$ returns a character, give a numeric code. ASC returns a numeric, code  corresponding to a character. LEN returns the length of a string. VAL, STR$.  Convert decimal values to numeric strings and vice-versa.

Extended  I/0 Statements:  OPEN, CLOSE, Control  association of a logical file number to a physical device and, optionally, a  file name on the device. SAVE, LOAD: VERIFY. Store and retrieve a program, with  optional file name, on a physical device. Load allows for program overlay,  VERIFY compares contents of memory to stored program. PRINT#, INPUT#, GET# Allow  communication with logical device numbers other than keyboard or screen. GETII  inputs one character. CMD’ Permits communcation with multiple devices  simultaneously.

Variables: TYPES. Real, Integer (%), String ($) NAMES: Variable names are uniquely given as a letter or a letter followed by a  letter or digit.

Special  Variables: TI, TI$. Time of day, ST:  Status word for I/O operations.


Of much  more interest to people are fun programs like games and business software which  helps you make (more) money. These can also be simple, or of complexity running  to many hundred statements. These programs not only do simple calculations, but  manipulate character data, set up filing systems on tape, draw fancy graphics on  the screen and other entertaining activities. And you can exercise your  imagination writing your own programs. A computer can be a very creativity  promoting toy, requiring very little activation energy (other than original  outlay) to get you involved, a lot more healthy than a TV set, that’s certain!

GRAPHICS We suspect some pretty creative,  intelligent people sat down and worked out the graphics character set on this  machine. Figure 1 shows the keyboard and all the characters you can key in.

The display  system is based upon 8×8 dot units, which may contain 5×7 dot upper case  letters, or assorted symbols which are so designed to provide very ver5atile  diagram and graphing capabilities. Because the 8×8 blocks fit together both  horizontally and vertically, it means that adjacent lines of text look a little  crammed together, but this can be avoided by judicious line spacing if  necessary.

The  “cursor” is a flashing “element” or character that can be moved about the  screen, and it normally indicates the next space to be typed in. After typing in that space the cursor moves to the next  adjacent spot and so forth.  “RETURN” causes the  cursor to move to the first position on  the next line, just as a carriage return does on a typewriter. Additional cursor  control is provided to move it quickly up, down or sideways. The cursor is  useful also for editing and correcting programs already written on screen, with  the facility for inserting and deleting characters. When the cursor attempts to  move below the bottom of the “page”, the screen “scrolls up”. That is to say  each line moves up one, with the top line disappearing.

In addition  to the normal characters, each may be “reversed”, that is appear as black on  white, which can be used to advantage on some occasions.

Finally,  here’s our big surprise! If you’ve been counting keyboard characters, there are lot less than the 256 possible commodore-pet--fig7_et_feb1978with the character generator shown in Fig. 6.  Quite by accident we found a whole set of lower case letters!  Very interesting.  We’re not quite sure what does it but check the pictures for yourself. That  still doesn’t add up to 256 characters, so there may be yet more we don’t know  about. In fact our Fig. 1 photo, upon close scrutiny of the keyboard shows  another set of graphic symbols including assorted Greek letters, could they be inside as well? We don’t know, but it all depends on what’s in the  character generator. We wonder if Commodore has lower case letters planned as a  future “add-on option?” For that matter you could always burn your own PROM.

WHAT  MORE CAN WE SAY? It looks like computers are starting their march into the homes of the masses,  which sounds very science-fiction and scary to some. We hope that the  familiarity with computers this may bring to the man/woman/child in the street  will reverse the widespread fear of THE COMPUTER as an enemy. Perhaps man will  once again feel master over objects. There was after all a time when the  automobile was regarded as an evil fire breathing monster, until everybody got  one.


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