PiDP-8/I Software

U/W FOCAL Manual Supplement for the PiDP-8/I
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This document is a supplement to the U/W FOCAL Manual. ("The Manual") Although it is not a complete FOCAL tutorial — much less a reference guide! — we suggest that you start learning about our distribution of FOCAL by skimming through this document first, then proceeding to the Manual, since this document will alert you to the areas of the Manual that are simply incorrect for the PiDP-8/I distribution of U/W FOCAL. Having gotten through the Manual, come back here and re-read this supplement more carefully; you will get more out of this supplement on that second pass with the context from the Manual.

Other helpful sources are the U/W FOCAL reference cards, the U/W FOCAL DECUS submission, the DECUS and OMSI manuals for PS/8 FOCAL, 1971, and the DEC FOCAL-8 Manual. To a first approximation, those are ordered in decreasing degree of application to our distribution of U/W FOCAL. The final document in that series is still quite useful for understanding U/W FOCAL, however.

See below for the reasons why we felt it was necessary to write this document.

Starting and Stopping U/W FOCAL

The section "Starting the Program" in the Manual is entirely concerned with loading U/W FOCAL from paper tape using the front panel and the BIN loader.

The PiDP-8/I software project does not currently ship U/W FOCAL in SIMH paper tape image form. Instead, it's installed by default on the OS/8 system disk, which greatly simplifies starting it:

 .R UWF16K

Yes, that's all. You're welcome. :)

To get back to OS/8, just hit Ctrl-C.

Loading and Saving Programs

There are many ways to get program text into U/W FOCAL other than simply typing it in. This section gives several methods, because each may be of use to you in different circumstances. Some of them may not be of direct use to you, but may open your eyes to techniques that may be useful to you in other contexts, so we encourage you to read this entire section.

Pasting Text in from a Terminal Emulator

The Naïve Way

If you are SSHing into your PiDP-8/I, you might think to write your FOCAL programs in your favorite text editor on your client PC then copy and paste that text into U/W FOCAL over SSH. That won't work. We believe it is because of the way U/W FOCAL handles terminal I/O and interrupts, being written with the assumption that such input is coming from a 110 bps Teletype or at most a 300 bps “high-speed” paper tape reader. If you try this over a modern gigabit class SSH connection, the input ends up trashed in FOCAL.

The Way That Works

"But I really really want to write my FOCAL programs in my favorite text editor and paste them into my PiDP-8/I," I hear you say. Dispair not. There is a path. Follow.

The problem affecting U/W FOCAL which prevents it from handling input at modern paste-through-SSH speeds doesn't affect OS/8 itself, so we'll use it as an intermediary:

*HELLO.DA<TTY:                  ⇠ use default extension for O I
01.10 TYPE "Hello, world!"!     ⇠ paste your program text here
^Z                              ⇠ hit Ctrl-Z to tell PIP it's reached end-of-file (EOF)
*$                              ⇠ hit Escape to return to OS/8 from PIP ($ = Esc)
.R UWF16K                       ⇠ run U/W FOCAL
*O I HELLO                      ⇠ open text file for input; "types" pgm in for us
_G                              ⇠ underscore = EOF seen, G starts program
Hello, world!                   ⇠ and it runs!

That is, we use OS/8's PIP command to accept text input from the terminal (a.k.a. TTY = teletype) and write it to a text file. Then we load that text in as program input using commands we'll explain in detail below.

The PUNCH Command

When the Manual talks about loading and saving programs, it is in terms of the PUNCH command, which punches the current program out on paper tape, because the Manual was written for the paper tape based version of U/W FOCAL.

The PiDP-8/I software project ships the OS/8 version of U/W FOCAL instead, which doesn't even have a PUNCH command. (We get the PLOT command instead.)

Even if it did work, mounting and unmounting simulated paper tapes under SIMH is a bit of a hassle. We can do better.

The LIBRARY Command

The effective replacement for PUNCH in the OS/8 version of U/W FOCAL is the LIBRARY command.

If you've read the Manual, you may be wondering if it's overloaded with LINK and LOOK, but no: those commands are apparently missing from the OS/8 version. (Open question: how do you use multiple fields of core for program code with the OS/8 version, then?)

Briefly, then, I'll show how to use some of these commands:

.R UWF16K                           ⇠ start fresh
*1.10 TYPE "Hello, world!"!         ⇠ input a simple one-line program
*L S HELLO                          ⇠ write program to disk with LIBRARY SAVE
*L O HELLO                          ⇠ verify that it's really there
HELLO .FD   1                       ⇠ yup, there it is!
*E                                  ⇠ ERASE all our hard work so far
*W                                  ⇠ is it gone?
C U/W-FOCAL:  16K-V4  NO/DA/TE      ⇠ goneski
*L C HELLO                          ⇠ load it back in with LIBRARY CALL
*W                                  ⇠ did it come back?

01.10 TYPE "Hello, world!"!         ⇠ yay, there it is!
*L D HELLO                          ⇠ nuke it on disk; it's the only way to
*L O HELLO                          ⇠ sure
*                                   ⇠ Houston, we have no program

See the DECUS submission and CARD2.DA in the refcards for more examples.

The WRITE Command

U/W FOCAL's LIBRARY command saves programs as core images, which are a) non-relocatable; and b) non-portable to other versions of FOCAL. We can fix both of these problems by saving the program to an ASCII text file instead.

With a program already typed in or loaded from disk:


All of that has to be on a single line, with the semicolons. If you give these three commands separately, you end up with the WRITE command as the first line in the output file and the OUTPUT CLOSE command as the last; you must then either edit those commands out of the output file or tolerate having FOCAL run those two commands again every time you load the program from disk.

What this does is opens a data output file (extension .DA) and makes it the output destination, so that the following WRITE command sends its text there, and then it is immediately closed with O C, returning control back to the terminal.

You can then load that program back into U/W FOCAL with the same command we used above with the PIP solution:


If you TYPE that file from OS/8, you might be wondering why the banner line doesn't cause a problem on loading the file back in:


That leading C causes U/W FOCAL to treat it as a comment. Since we're in "direct mode" at that point, the comment is simply eaten.

The Paper Tape Reader

Above, I warned you off trying to save programs to simulated paper tape via the PUNCH command, but what about reading programs back in? You can do that, but it's trickier than you might guess.

First, if you've read the Manual, you may think to attach a paper tape to SIMH then use U/W FOCAL's OPEN READER command, but as with PUNCH, that command has been replaced in this version of U/W FOCAL. With the removal of paper tape support in U/W FOCAL proper, they felt free to reassign O R to OPEN RESTART/RESUME.

Thus, we again have to pop back out to OS/8 and use PIP to pull this off.

First we must create that paper tape. If you place your FOCAL source code in examples/*.fc, you can simply type make at the top level of the PiDP-8/I source tree to have it translated to bin/* with the same base name.

(This is done by Bill Cattey's txt2ptp program; there is also the inverse filter, ptp2txt. We include the -focal tag to distinguish these files from * files produced from *.pal source files by a similar process.)

We'll work with the provided examples/tratbl.fc example program, which got translated to bin/ when the PiDP-8/I software was built.

To attach that paper tape to SIMH's paper tape reader device, hit Ctrl-E to get to the SIMH command prompt, then:

sim> att ptr bin/
sim> c

On re-entering the simulator with the c ("continue") command, we can read that tape into OS/8:

*TRATBL.DA<PTR:                    ⇠ hit Esc *then* Enter

If the final command doesn't show you the program text you expect, something went wrong in the process above. There are a couple of likely possibilities:

  1. You hit Enter at the end of the PIP command, either instead of Esc or before hitting Esc.

    If you do it right, it should appear on screen as:


    with the $^ appearing when you hit Esc, signifying that it has read the paper tape and hit the end. Hitting Enter should then drop you back to the OS/8 prompt, not leave you in PIP. If you get another * prompt from PIP on hitting Enter, you fat-fingered something. Try again.

  2. Every time you cause the PDP-8 to read the paper tape, you must re-attach it to SIMH to read it again. Neither SIMH nor OS/8 warns you if you try to read from the paper tape reader with nothing attached; you just get no input.

    This mimics what happens with real paper tapes: once the reader has read the tape, it falls out of the machine and needs to be fed back in to be read again. The difference between the real paper tape reader and SIMH is that that repeated sequence is much more of a hassle than just sticking the tape back in the reader:

    sim> ATT PTR ...
    sim> C

    That TTY: based PIP method above will start to look awfully attractive after a while...

Once you make it through that gauntlet, loading the ASCII program text into U/W FOCAL is just as above: O I TRATBL.

Lowercase Input

The version of U/W FOCAL we include by default on the PiDP-8/I's OS/8 system disk copes with lowercase input only within a fairly narrow scope. The fact that it copes with lowercase input at all is likely due to the fact that the version we ship was released late in the commercial life of OS/8, by which time lowercase terminals were much more common than at the beginning of OS/8's lifetime.

The examples in the Manual are given in all-uppercase, which means there is no reason you would immediately understand how U/W FOCAL deals with lowercase input, having no examples to build a mental model from. If you just guess, chances are that you will be wrong sooner or later, because U/W FOCAL's behavior in this area can be surprising!

The two main rules to keep in mind are:

  1. U/W FOCAL is case-sensitive for variable and built-in function names, but it is case-insensitive for command names.

  2. U/W FOCAL doesn't support lowercase variable and function names. It may sometimes appear to work, but internally, U/W FOCAL isn't doing what you want it to.

The following gives incorrect output because of a violation of rule 1:

*type fsin(pi/2)!

The correct answer is 1. It fails because there is no built-in function called fsin nor a built-in constant pi.

FOCAL gives an answer here instead of detecting our failure to call things by their right names because it is falling back to its rule to use a value of 0 where no value or function is available to do what you asked. Zero divided by 2 is 0; then it tries to subscript a nonexistent fsin variable with index 0, so it punts and gives the answer you see above, zero.

A better language would have detected your errors and given a diagnostic, but U/W FOCAL is implemented in less than a page of PDP-8 core memory, roughly the same number of bytes as Clang gives when compiling an empty C program on the machine I'm typing this on. The fact that U/W FOCAL detects errors at all is somewhat impressive.

To get the expected result, call the FSIN function and use the PI constant, which are very much not the same thing as fsin and pi to FOCAL:

*type FSIN(PI/2)!

U/W FOCAL doesn't care that you gave the type command in lowercase, but it does care about the case of the function and variable names.

U/W FOCAL's tolerance of lowercase in command names doesn't extend to arguments. In particular, the OPEN command's argument must be uppercase: o o doesn't work, nor does O o, but o O does.

Violating rule 2 can be even more surprising:

.R UWF16K               ⇠ We need a fresh environment for this demo.
*s a=1                  ⇠ What, no error?  I thought you said...
*s b=2
*s c=3
*type $ !

No, that transcript isn't cut off at the end: the TYPE command simply doesn't give any output! Why?

The reason is that U/W FOCAL can't [currently] cope with lowercase variable names.

But wait, it gets weird:

*s A=1
*s foo=42
*type $ !
A (+00) 1.000000000E+00
&/(+00) 4.200000001E+01

We now see output for our uppercased A variable, but what is that &/ noise? Apparently "foo" somehow gets mangled into &/ by FOCAL's command parser.

We have not yet tried to investigate the reason foo gets saved into a mangled variable name and a, b, and c above do not, because the workaround is simple: keep CAPS LOCK engaged while typing FOCAL programs except when typing text you want FOCAL to send back out to the terminal:

*1.1 TYPE "Hello, world!"!
Hello, world!

See the Variables section of CARD2.DA for more information on variable naming.

Default Output Format

FOCAL is primarily a scientific programming language. That, coupled with the small memory size of the PDP-8 family and the slow terminals of the day mean its default output format might not be what you initially expect. Consider these two examples pulled from the U/W FOCAL Manual:

 1.000000000E+00 0.000000000E+00-1.000000000E+00
*TYPE 180*FATN(-1)/PI !

This may raise several questions in your mind, such as:

  1. Why is there no space between the second and third outputs of the first command?

  2. Why does the ouptut of the first command begin in the second column and the second begin at the left margin?

  3. Is the second command giving an answer of -4.5°?

If you've read the U/W FOCAL Manual carefully, you know the answer to all three of these questions, but those used to modern programming environments might have skimmed those sections and thus be surprised by the above outputs.

The first two questions have the same answer: U/W FOCAL reserves space for the sign in its numeric outputs even if it doesn't end up being needed. This was done, no doubt, so that columns of positive and negative numbers line up nicely. It might help to see what's going on if you mentally replace the spaces in that first output line above with + signs.

This then explains the apparent discrepancy between the first and second commands' outputs: the first output of the first command is positive, while the second command's output is negative, so there is a space at the beginning of the first output for the implicit + sign.

As for the third question, the default output format is in scientific notation with full precision displayed: 4.5×10¹ = 45 degrees, the correct answer.

Improving the Output Format

The following changes to the examples as given in the Manual show how you can get output more suitable to your purposes:

 1    0    -1

That sets the output precision to 1 significant digit, which is all we need for the expected {-1, 0, -1} ouptut set. The tabstop formatting options (:5) put space between the answers, but there is a trick which gives similar output with a shorter command:

     1     0    -1

That tells it to use 5 significant digits with zero decimal digits. Since the answers have only one significant digit, FOCAL right-justifies each output with 4 spaces. There are 5 spaces between the 1 and 0 outputs because of that pesky implicit + sign, though.

The second example above can be improved thus:

*TYPE %3.2, 180*FATN(-1)/PI !

That tells FOCAL to display 3 significant digits, and to include up to 2 decimal places even if the traling one(s) would be 0, thus showing all 3 significant digits in an answer expected in degrees. If you'd wanted 4 significant digits with any trailing zeroes instead, you'd give %4.3 instead. If you'd given %3, the output would be -45, the trailing zero being deemed unnecessary.

ASCII Character & Key Names

Many of the common names for keys and their ASCII character equivalents have shifted over the years, and indeed they shifted considerably even during the time when the PDP-8 was a commercially viable machine. The following table maps names used in the Manual to their decimal ASCII codes and their common meaning today.

Old Name ASCII Current Name
RUBOUT 127 Delete or Del
BACKARROW 95 Underscore
UNDERLINE 95 Underscore

Beware that the ASCII values above differ from the values given in the U/W FOCAL Manual Appendix "Decimal Values for All Character Codes." FOCAL sets the 8th bit on ASCII characters for reasons unimportant here. Just add 128 to the values above if you need to get the FOCAL equivalent.

Some terminals and terminal emulator software may remap Backspace and Delete, either making one equivalent to the other or swapping them. Without such remapping, if you hit the key most commonly marked Backspace on modern keyboards, U/W FOCAL will just insert an ASCII character 8 at that point in the program entry, almost certainly not what you want. You either need to remap Backspace to Delete or hit the key most commonly marked Del.

The U/W FOCAL Manual also references keys that used to appear on some terminals, most especially teletypes, which no longer appear on modern keyboards:

Teletype Key Modern Equivalent

Command Overloading

The Manual tells you right up front that U/W FOCAL only pays attention to the first letter of each command when trying to decide what you're asking it to do, which is why we have strange commands like YNCREMENT: I was already taken by IF.

What it doesn't make as clear is how the creators of U/W FOCAL began adding a second layer of overloading to this scheme after they began running out of letters in the English alphabet.

Early examples of FOCAL's command overloading scheme are the ON, OPEN and OUTPUT commands. By looking at the first argument to the command, FOCAL can tell which of the three you mean. If you give a valid FOCAL expression in parentheses, it is clearly an ON command. If you give anything beginning with a letter, FOCAL decides whether it's an OPEN or OUTPUT command based on which letter that is; you will notice that the first letter of no OPEN sub-command is the same as the first letter of any OUTPUT sub-command.

In later versions of U/W FOCAL, they added a third level to some commands. We have OPEN RESTART READ and OPEN RESUME INPUT, for example. It may help to abbreviate the commands, as the first letter of each word is all that really matters: O R R is clearly not the same as O R I.

There are other examples of this, such as LIBRARY and LIST. It is the first letter of the first argument to these commands that determines what FOCAL does.

In at least one case, you can see this feature of FOCAL used to talk about a single command under different names. Some FOCAL documents talk about a ZVR command, meaning Zero VaRiable. It's just another way of spelling the ZERO command: it does the same thing.

FOCAL only checks the first letter of the command and any necessary disambiguating arguments. The following is therefore a perfectly legal FOCAL program:

01.10 SPEW I = 0

It does exactly the same thing as a program we will encounter shortly.


The OS/8 version of U/W FOCAL includes a "library" feature modeled on a similar feature in OMSI PS/8 FOCAL. These features collectively allow the user access to FOCAL program files stored in the OS/8 file system from within a U/W FOCAL program or as immediate commands to U/W FOCAL.

We showed how you can use some of these commands to save and load programs to disk above, but there's a lot more to it. Coverage of this begins on page 34 of the DECUS submission.


These commands allow you to get OS/8 directory listings from within U/W FOCAL:


Our distribution of U/W FOCAL includes the LOGICAL BRANCH feature, which is not documented in the Manual, but it is documented on page 37 of the DECUS submission. Briefly, it acts like a GOTO command where the jump is only made if keyboard input is detected.

The following program counts upward continuously until a key is pressed, then it prints how many iterations it ran:

01.10 SET I = 0
01.20 YNCR I
01.30 LOGICAL BRANCH 01.20 ; TYPE I !

The DECUS document suggests using this feature to turn the keyboard into a giant "switch register," allowing the user to control the behavior of a long-running program without stopping to wait for user input. Can you say "video games"?


Another addition to our version of U/W FOCAL as compared to the version documented in the Manual is LOGICAL EXIT which immediately exits U/W FOCAL and returns you to OS/8, just as if you had hit Ctrl-C.

FOCAL Statement Functions

This appears to be the same thing the Manual calls Program Defined Functions. Therefore, you may look on the material beginning on page 63 of the DECUS submission as just another take on the same issue. Some of the examples are more enlightening than the one in the manual. The first example in the DECUS submission, F(EXP), is more robustly coded than the same function in the Manual; comparing the two is instructive.

FRA Built-In Function

This function is not documented in the Manual, but it is documented on page 60 of the DECUS submission. It allows you to set up random access to a file, storing numbers in various raw binary formats directly to the file as if it were a large in-memory array.

If you've been trying to use the FCOM function but have run into the limit on the size of a PDP-8 core memory field, switching to FRA is the logical next step.

Front Panel Differences

Whenever the Manual refers to the PDP-8's front panel, it is speaking generically of all the models it ran on as of October 1978. The PDP-8 models introduced in the decade following the introduction of the PDP-8/I differ in many ways, and one of the greatest areas of difference is in their front panel controls and indicators. We do not intend to fully document all of the differences here, but only to clarify the differences brought up by the U/W FOCAL Manual.

You normally will not need to use the front panel with the OS/8 version of U/W FOCAL we distribute with the PiDP-8/I software distribution since you start and stop U/W FOCAL through OS/8 rather than the front panel. However, we thought these matters could use clarification anyway.

Beyond this point, when we refer to the PDP-8/e, we also mean the 8/f, which shared the same front panel design. We also include the 8/m, which normally came with a minimal front panel, but there was an optional upgrade for an 8/e/f style front panel. These three models are therefore interchangeable for our purposes here.


With the PDP-8/e, DEC replaced the START front panel switch of the preceding PDP-8/I with a CLEAR switch. Why did they do this?

On a PDP-8/I, the difference between START and CONTINUE is sometimes confusing to end users, since in many cases they appear to do the same thing. Why have both? The difference is that CONTINUE simply resumes operation from the current point in the program where it is stopped, whereas START resets several key registers and then continues.

The PDP-8/e change splits this operation up to avoid the confusion: the old START keypress is equivalent to CLEAR followed by CONTINUE. (This pair of switches also has a START label above them, a clear functional grouping.)

The U/W FOCAL Manual also speaks of a RESET switch in conjunction with the FOCAL starting and restarting the computer. I haven't been able to track down which PDP-8 model has such a switch yet, but for our purposes here, I can say that it just means to load the starting address and hit START on a PDP-8/I.


The PDP-8/e has many fewer switches on its front panel than the PDP-8/I, yet it is a more functional machine. One of the ways DEC achieved this is by removing the IF and DF switch groups and adding the EXTD. ADDR LOAD switch, which lets you set the IF and DF registers using the same 12-bit switch register used by the ADDR LOAD switch.

The ADDR LOAD switch on a PDP-8/e does the same thing as the Load Add switch on a PDP-8/I.

Switch Direction

DEC reversed the meaning of switch direction between the PDP-8/I and the PDP-8/e, and the Manual follows the 8/e convention: on the 8/I, up=0=off, whereas on the 8/e, up=1=on. Keep this in mind when reading the U/W FOCAL Manual's references to front panel switch settings.

Switch Ordering

When the Manual talks about the switch register (SR), it numbers the switches left to right, not by their logical bit number in the switch register. That is, "Switch 0" is the leftmost (high order bit) SR switch, not "bit 0" in the SR, which would be the rightmost SR switch.

Error Codes

The U/W FOCAL Manual gives a somewhat different error code table than the one on CARD4.DA of the U/W FOCAL reference cards. For the most part, the latter is just a simple superset of the former, and both apply. In some cases, though, the two tables differ, or one of them differs from the UWF16K program we ship on the OS/8 system disk.

?18.32 vs ?18.42FCOM index out of range

The two error code tables give different error codes for this condition. However, since I have not been able to get this error to happen, I do not know which code is correct for our current version of FOCAL.

?31.<7 — Non-existent program area called by LOOK or LINK

Our current implementation of U/W FOCAL removed those commands in favor of LIBRARY, so you can't make this one happen. An error in a LIBRARY command is most likely to give ?26.07 instead.

Irreproducible Errors

There are some errors listed in one or both tables that I have been unable to cause, though I have tried:

Code Meaning
?07.44 Operator missing or illegal use of an equal sign
?18.32 FCOM index out of range (value given in the manual)
?18.42 FCOM index out of range (value given on the refcard)
?27.90 Zero divisor

Untested Error Cases

I have not yet created programs large enough to test the "out of space" codes ?06.41 (too many variables), ?10.50 (program too large), ?13.65 (insufficient memory for BATCH operation), ?23.18 (too much space requested in OUTPUT ABORT or CLOSE), ?23.37 (output file overflow), and ?25.02 (stack overflow).

There are also some errors I simply have not yet tried to cause: ?01.03, ?01.11, ?12.10, ?12.40.

Missing Hardware Support

The U/W FOCAL reference cards and the DECUS submission talk about features for hardware we don't have. Either the command/feature doesn't exist at all in the version of U/W FOCAL we distribute or it doesn't do anything useful, lacking support within the version of SIMH we distribute.

Broadly, these features are for the PDP-12, the LAB-8/e, Tektronix terminals, and pen plotters. Should anyone extend SIMH with a way to control such hardware (or emulations of it) we may consider putting these features back into our distribution of U/W FOCAL.

In the meantime, the following facilities do not work:

Differences Between U/W FOCAL and Other FOCALs

The DECUS submission for U/W FOCAL lists the following advantages for the version of U/W FOCAL included with the PiDP-8/I software distribution as compared to FOCAL,1969, FOCAL-8, and OMSI PS/8 FOCAL:

  1. Extended library features with device-independent chaining and subroutine calls between programs.

  2. File reading and writing commands, 10 digit precision, full 32k memory support, 36 possible functions, 26 possible command letters.

  3. Computed line numbers and unlimited line lengths.

  4. Tabulation on output, format control for scientific notation.

  5. Double subscripting allowed.

  6. Negative exponentiation operators permitted.

  7. FLOG, FEXP, FATN, FSIN, FCOS, FITR, and FSQT rewritten for 10-digit accuracy.

  8. Character manipulations handled with FIN, FOUT, and FIND.

  9. Function return improvements:

    • FSGN(0)=0 in U/W FOCAL; =1 in FOCAL,1969
    • FOUT(A)=0 in U/W FOCAL; =A in PS/8 FOCAL
  10. n/a; see above

  11. 6 special variables are protected from the ZERO command: PI, !, ", $, %, and #.

    PI is initialized as 3.141592654.

  12. The limit on the number of variables is 676

  13. Text buffer expanded to 15 blocks

  14. Two-page handlers permitted

  15. Program and file names are wholly programmable. File size may be specified. OS/8 block numbers may be used in place of file names.

  16. The OPEN and DELETE commands can have programmed error returns.

  17. Improved distribution and random initialization of FRAN.

  18. ERASE, MODIFY, and MOVE do not erase variables.

  19. WRITE doesn't terminate a line; MODIFY doesn't echo form feed.

  20. U/W FOCAL's starting address is 100 (field 0) or 10200 (field 1).

Converting Programs from Other Versions of FOCAL

Programs saved by other versions of FOCAL generally don't have the same format as the core images used by U/W FOCAL. You must therefore use one of the text based loading methods to save your program text out of the other FOCAL and load it into U/W FOCAL.

Also, while the ERASE command may be used to zero variables in other FOCALs, you must use ZERO for that in U/W FOCAL. If your program starts off with ERASE commands to initialize its variables, there's a pretty good chance your program will just erase itself under U/W FOCAL.

Why Did We Write This?

The Manual is well written as far as it goes, but there are gaps:

  1. It is written somewhat generically for the whole PDP-8 family as of late 1978, whereas the PiDP-8/I project is focused on a single model from 1968. Those not familiar with the differences can therefore be confused by some of its directions.

  2. There are considerations in our simulated PiDP-8/I world that simply did not apply to those running U/W FOCAL on the real hardware.

  3. There are multiple versions of U/W FOCAL; the version covered by the Manual isn't the one we actually ship. Our two other primary sources also do not cover exactly the version of U/W FOCAL we ship.

  4. It inspires questions in the reader's mind without providing an answer. Whether this was intentional — with the author intending that the user answer these questions on his own — or otherwise, some of these questions we felt needed answering here within the PiDP-8/I U/W FOCAL documentation.

This document is our attempt to fill these gaps and to supplement those other documents. Extensions and corrections are welcome.


The primary sources for this supplement are:


Copyright © 2017, 2021 by Warren Young and Bill Cattey. Licensed under the terms of the SIMH license.