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The hard way to recover the IBM 5100 Non-executable ROS

An artistic rendering of the text "IBM 5100" using the hexadeximal word images considered in this project

I was into the IBM 5100 way before John Titor, if "before" makes sense with that guy.

Sure, I never used one when they were new, but one of my favourite high school teachers egged on a budding interest in old computers by giving me some IBM 5100 junk from his undergrad engineering classes: a textbook and a DC300 tape cartridge. I finally tracked down a working 5100 a few decades later, although the tape is long gone now.

Having found one at last, it was worrying to find remarks like this online:

Image of a tweet by Twitter user @brouhaha: "Unfortunately my 5100 model C has a failed APL ROS module. I hope someday to get access to a working A or C to get a dump, so I can fabricate a replacement ROS card."

("ROS" is IBM-speak for ROM and stands for "read-only storage".)

I like to dabble in hardware, but I'm no @brouhaha. Better to get him that ROS dump now so that he'll have the replacement worked out before bit rot comes for my own machine 😃

The few times I've had the lid off of my 5100 have all been anxious moments, as I have no idea where I'd find replacements for any of the ICs or SLT modules inside the machine. I resolved early on that my recovery of the 5100's non-executable ROS---the ROS that contains the programming for the 5100's BASIC and APL interpreters---would be as minimally-invasive as possible.

In accomplishing this recovery I may have used more compute than all the IBM 5100s ever built have carried out over the past 44 years.

Table of contents

The non-executable ROS in more detail, and stage 1

For more information on what the non-executable ROS contains or the very first steps of saving it, see this document.

For those who didn't do that, the basic idea is that I can load chunks of the ROS into the 5100's RAM (or, RWS as IBM likes to call it---"read-write store"). This will be its first stopping place on a lengthy journey.

Stage 2 is video

The 5100's built-in monitor program, DCP1, lets you page the computer's RAM in hexadecimal, 32 bytes at a time. You can hold down the up and down arrow keys and watch the digits stream by. With a camera pointed at the 5100's display, I recorded the entire contents of the non-executable ROS to thirty-two fairly boring videos. Here is a still frame from one of them:

A video still of bytes $D1A0-$D1BF of the APL ROS.

and if that seems exciting to you, the entire one-minute experience of that portion of the APL ROS is on YouTube. The image shows the native 640x480 resolution of the video, since for some reason I was feeling especially frugal about SD card space at the time.

With the videos on SD cards, the non-executable ROS was rescued at last onto "modern" computer media, and so job done, I suppose. Still, it wasn't quite satisfying to leave things this way. People making emulators and hardware prostheses need bytes, not pictures, and someday I might need them, too.

Machine learning challenges

With all the data in image form, there should be no further need to open up or even turn on the 5100 again---perfect for my "minimally invasive" recovery. All I have to do is work out an automated way to turn the numbers in the video frames into binary data. As nice as it would be to feed the images into a free OCR program like Tesseract, these systems are designed to recognise natural language texts and fail on my hex dumps. I need to come up with my own solution, and since I'm not going to transcribe the images by hand (even if I felt like doing that, it would probably be prone to errors), machine learning is the only practical strategy.

If you've read anything about machine learning in the last ten years, you've probably encountered these villains:

Handwritten 2, 4, and 8 digits from the MNIST dataset.

These are three samples from the MNIST database, a collection of 70,000 handwritten digits as 28x28 greyscale images. MNIST has been called the "Hello World" of machine learning: a common starter exercise for a classification system or technique is to throw it at MNIST and see whether it can learn to recognise digits. In a way, the field of machine learning has been studying digit recognition devoutly for several decades. This is a good thing, since I have some digits, too: over three million of them. They come in sets of four, like this:

A cropped image of the hex digits 1B22 from the IBM 5100's screen

My images are smaller than MNIST: 29x16 (the pictures above are shown "to scale"). The digits themselves are much more regular than handwriting, but they're still not perfect. The old display hardware can wiggle display scanlines around a little bit, light-blocking dust is trapped between the plastic display shield and the CRT in places, and the screen geometry isn't perfectly linear in all directions. Sometimes a B can look a bit like an 8 or vice-versa. Speaking of B, there's also the fact that I need to classify the sixteen possible hexadecimal digits instead of MNIST's ten. Then there's gibberish like this:

A cropped image of four garbled hex digits from the IBM 5100's screen

Recall that my images come from videos. My camera isn't synchronised precisely with the 5100 display, and sometimes the (virtual) shutter stays open during the transition between two pages of digits. The result is a mess:

A cropped image of a page full of four garbled hex digits

In addition to recognising the sixteen hex digits 0-9A-F, I'll also need to have some process for filtering out those non-digits, or else hallucinated data derived from them will "contaminate" my recovered ROS.

Finally, speaking of contamination, a useful backup requires perfect accuracy. At time of writing, the best reported MNIST classification error fraction is 0.21%, or 21 in 10,000 (a scoreboard; some background). This is impressive, but the 5100 will not run properly if even one byte of its non-executable ROS is out of place. 100% accuracy is a tall order for machine learning, but with luck the 5100's more regular digits will give it a fighting chance.

Stage 3: image preparation

Warning! This section is even more boring than the rest of the writeup. Feel free to skip ahead.

MNIST digits may be irregularly shaped, but the images they live in are regular in important ways. The contrast and the brightness have been normalised so that pixels range in intensity from totally dark (#00) to totally bright (#FF). Even more importantly: the digits are centred. To maximise my odds of perfect classification, I need to convert my video files into very regular, tiny images of four-digit clusters like the ones shown above. Here are my steps, listed in part as commands I ran (from a bash shell) on my Linux laptop:

1. Dump the movies to per-frame .png files.

I begin with each of the movies I recorded sitting in its own subdirectory. That is, I'm assuming this arrangement:


./APL_ii/APL_LROS_ii_0000.MOV   ./APL_ii/APL_LROS_ii_2000.MOV
./APL_ii/APL_LROS_ii_4000.MOV   ./APL_ii/APL_LROS_ii_6000.MOV
./APL_ii/APL_LROS_ii_8000.MOV   ./APL_ii/APL_LROS_ii_A000.MOV
./APL_ii/APL_LROS_ii_C000.MOV   ./APL_ii/APL_LROS_ii_E000.MOV

./BCom/BCom_0000.MOV   ./BCom/BCom_2000.MOV
./BCom/BCom_4000.MOV   ./BCom/BCom_6000.MOV
./BCom/BCom_8000.MOV   ./BCom/BCom_A000.MOV
./BCom/BCom_C000.MOV   ./BCom/BCom_E000.MOV

./BCom_ii/BCom_ii_0000.MOV   ./BCom_ii/BCom_ii_2000.MOV
./BCom_ii/BCom_ii_4000.MOV   ./BCom_ii/BCom_ii_6000.MOV
./BCom_ii/BCom_ii_8000.MOV   ./BCom_ii/BCom_ii_A000.MOV
./BCom_ii/BCom_ii_C000.MOV   ./BCom_ii/BCom_ii_E000.MOV


# In each movie subdirectory
mkdir 00_originals
# In 00_originals, e.g.
ffmpeg -i ../../APL_LROS_0000.MOV %04d.png

2. Crop the per-frame .png files.

This step produces cropped images like the multi-digit "mess" shown just above. Note the conversion from colour to greyscale. The cropping rectangle (the argument to the -crop flag) was identified by hand.

# In each movie subdirectory:
mkdir 01_cropped
# In 00_originals:
for i in *.png; do convert -crop 350x75+39+151 +repage -colorspace RGB -colorspace Gray $i ../01_cropped/$i; done

3. Trim the per-frame .png files to useful frames.

The beginning and ending of each movie have extra frames that were recorded while I moved my hands between the camera and the 5100's keyboard. These are not of much use. I went into each 01_cropped subdirectory and deleted the extras by hand, leaving only about five to ten beginning and ending frames.

4. Crop individual 4-digit words.

This step does most of the work. From the coarsely-cropped images in step 2, a Python program uses a crude gradient descent optimisation procedure to gradually adjust the locations of tiny cropping windows placed around each of the 4-digit words.

The technique sounds fancier than it is. The objective is to centre the windows on the words as precisely as possible---in fact, with subpixel precision (that is, we'd like to specify the locations of the windows in fractional pixels). To do this, we iterate the following procedure for each window:

  1. Use image interpolation to sample a picture of the 4-digit word within the window.
  2. Treating pixel intensities much like physical weights arranged on a grid, find the "centre of gravity" of the sampled picture.
  3. Shift the window slightly in the direction of the centre of gravity---if it is left-of-centre in the sampled picture, for example, nudge the window leftward.

Although effective, this procedure isn't perfect. For starters, it takes a long time: on my slightly-old laptop, the better part of a day to crop all 1,010,070 word-images in the coarse crops from step 3. A more important issue comes from the "centre of gravity" strategy in the first place: some digits like 8 or B have more "on" pixels than others like 0 or 1, and so 008B cropped will not quite line up with B800:

An image of the digits 008B above the digits B800, showing how they are centred differently by the procedure

In the end I chose to move on and hope for the best. If I were starting over from scratch, I might try a different technique that makes use of the morphological operators and connected components-based methods I used in recovering the 5100's executable ROS.

Code and data files used by this step:

  • library for cropping single digit images.
  • program for cropping multiple digit images.
  • crop_list.csv: initial window locations, prior to optimisation; these were created by hand.
  • shell script for applying the digit-image cropping program to all coarsely-cropped images.

And on the command line:

# In the top-level directory, make more subdirs:
for i in `find . -name 01_cropped`; do j=`echo $i|sed s/1_cropped/2_words/`; mkdir $j; done
# Now run the cropping script and waaaaaaaaait.

It's worth mentioning that also attempts to adjust image brightness and contrast, which varies in the original images owing to banding from camera shutter/5100 display synchronisation issues. You can see this banding for yourself at the YouTube link from earlier.

Stage 4 means training data

Training data is the only essential ingredient for successful machine learning. All other details are basically fashion decisions.

For a classifier like the one I want to make, training data amounts to pictures of 4-digit words (the input to the classifier) paired with the correct numerical label (the four-digit output we'd like the classifier to produce). For example:

(Image of digits 22A0, 22A0)
(Image of digits 92DC, 92DC)
(Image of digits 955A, 955A)
(Image of digits 6000, 6000)
(Image of digits 4770, 4770)

With loads of training data, you can recognise anything simply: just scan all of your pairs like an enormous identity parade (police lineup) and emit the label associated with the closest match. This turns out to be a real machine learning technique.

As it will turn out, 7,046 of the words in the non-executable ROS---over 4% of them---will appear there only once. Unless all of these singletons wind up in the training data, they won't be identifiable via simple matching. More sophisticated classification methods learn to generalise from training examples in sophisticated ways, allowing them to decode input that they've never seen before. This is most of the learning in "machine learning". The best techniques are distinguished by how much generalisation they can do.

So, if I use a good technique, how much training data will I need? It's difficult to know. In machine learning research, fully 60,000 of the 70,000 MNIST digits are commonly reserved as training data---over 85% of the entire dataset. Hoping that I won't need such an extreme ratio, and knowing that my dataset isn't MNIST, I choose an arbitrary minimum number of training data examples: 100,000.

The first 11,000 by hand: a quiet New Year's week

I've got some good friends who've helped me out on various projects over the years, but I think I'm on my own for the tedious task of labelling all of this training data. The least I can do is try to make it less painful for myself.

Jotting down labels into a text editor would be pretty slow; instead, I decide to make a custom program that presents word images and collects digits as fast as I can type them. It looks like this:

A screenshot of the word-image labelling program. It resembles an ordinary text-mode terminal program with word-images appearing between text prompts for labels.

This is an ordinary command-line program with interspersed graphics, made possible by using a terminal emulator that supports Sixel graphics. This method for showing screen images inline with text was introduced in the early 1980s by Digital to provide graphics capabilities for their VT240 and VT241 terminals.

A line-drawing of a Digital VT-241 terminal
(From the VT240 owner's manual)

Since I don't have one of those, I'll use mlterm on my laptop instead. The program itself comes in two parts: the interface and a database backend library (where the "database" is really just an enormous .csv file). Both are written in ordinary Python, but the interface makes use of the UNIX-only termios and tty modules. (Windows users might be able to get it working under cygwin.) Here is the (hastily-written!) code:

To use this program, you'll need an empty database first. Assuming you've followed all of the steps so far, this one, uncompressed, will work for you. (Beware: it's over 54 MiB, but there's a lot of duplicated text, so lzip compresses it down to a measly 255 KiB.) You will need to run the "labelthon" program from a directory where all of the paths in the database are valid, like so:

./ -n 100000 --mark-apl-ros-c000-zeros database.csv

Now you and mlterm are ready to sit down for days and days of hand-labelling. You've even got a head start: you can tell by watching that all data words in APL_LROS_C000.MOV and APL_ii_LROS_C000.MOV (the second filming of the same thing) are 0000, so the --mark-apl-ros-c000-zeros flag labels all of the images from those movies accordingly. "labelthon" will select randomly from the other word images and show them to you, and you'll type in the letters you see.

Human error is a problem, so the program will want to collect two identical labellings from you before it believes the label you've chosen for a particular word image. At a minimum, then, you'll need to type in 200,000 labels, although due to the way "labelthon" chooses between showing you a new image or asking you to verify an old one (controllable by flags), you're likely to do even more than that.

If you see a "gibberish" image like the one above, type m to mark it as nonsense. If the image is clearly 0000 (many word images are, even after applying --mark-apl-ros-c000-zeros), the shortcut z saves you from typing four digits. If you get tired of this madness, type q to quit.

On New Years Day, I set to work:
Plot showing labelling progress over six days.

Blue marks show times when I'm working (the database library saves timestamped backups of the database in the background). The minor horizontal ticks are hours. The count goes down on the 4th and the 5th because I change the flags to force "verification mode" and wind up clearing out errors.

By the 6th, BBC iPlayer runs out of episodes of Ambulance Australia to watch in the background, and I'm pretty tired of typing digits. I consider crowdsourcing the 90,000 labels I had left to go, but getting reliable results from crowdsourcing can be harder than it seems, and a browse through r/HITsWorthTurkingFor suggests that Amazon's gig economy workforce probably has better things to do. Fair enough---if I don't like the job so much, why inflict it on others?

To 62,000 labels by logic analyser

What? Yes, I've had a logic analyser this whole time.

The graph above actually shows a zoom of the overall labelling progress, which looks more like this:
Plot showing overall labelling progress.

Fed up with labelling by the 7th, I'm off doing other things for a while, but before long I decide that I ought to save the 5100's executable ROS (the other important ROS in the computer). This is a different project described elsewhere, but suffice it to say that it means opening up the 5100 again after I'd hoped to keep the covers shut. That recovery was fairly straightforward, and I might have been wise to adapt those techniques to the non-executable ROS, but then again, I'd already gone to so much trouble for the existing approach.

In any case, with the covers off, it's not hard to attach my logic analyser to the appropriate bus inside the 5100 and re-run the program that loads non-executable ROS data into memory. My device, a Rigol MSO2202A-S, has plenty of memory to record the data from each load. Surely with these traces I can easily reconstruct the entire non-executable ROS and forget all about machine learning, right? So, I make my recordings, gently untangle the logic probes from the 5100's circuitry, and close its case again, hopefully now for months or even years. A look at the data afterwards reveals two issues:

First, I cleverly forgot to capture any kind of clock signal to help identify the boundaries between bytes in the eight data bit streams I'd recorded. A long stream of $00 values could be a run of five $00 bytes, or one or three or fifteen. There is no way to know for certain.

Second, though, my MSO2202A-S is terrible at saving recorded data.

My MSO2202A-S seated atop my IBM 5100, with a goofy cartoon face superimposed on top of its display.

It can store oscilloscope traces in a variety of formats, but as far as I can tell, only the .csv files it writes contain recordings from the logic probes. Obviously I have no problem with large .csv files, but the Rigol sure does, as the files it gives you are corrupt. Occasionally it loses track of the data it's writing to the file and shovels in a bunch of $00 bytes (nonprintable---cheeky for a .csv) instead in hopes that you won't notice. It's not uncommon for several hundred bytes to go missing this way. This graph shows the damage:

A chart showing that my logic analyser can sometimes skip over 30us of useful data when saving logic traces to .csv files

Each tiny bar in this bar chart represents a byte in a recorded data stream. The height of the bar is how long the byte lingered on the data lines before a different byte value replaced it. While a legitimate run of identical values (e.g. fifty $00 bytes in a row) would have a slightly taller bar, the massive spikes are gaps where the Rigol chose to drop the data.

It might have occurred to me to make multiple recordings of the same data stream (as I'd done with the videos) since the logic analyser might not have put the gaps in the same places the second time around. Sadly, this did not occur to me, and by the time it did, the 5100 was buttoned up once again.

So, to try and salvage the good parts of the recordings I had, I make another Sixel-enhanced Python program:

A screenshot of the trace alignment program. A terminal with bitmap graphics capabilities presents an image of 64 hex digits taken from a cropped video still image above 64 hex digits extracted from the logic analyser data. Below this, some positioning information, then a list of the keys the user can press to scan through images and data, perform searches for strings of hex digits, and mark when alignment is achieved.

This program allows me to align 32-byte snippets of logic analyser data with screen images from the video, with search features to make it easier to navigate through the recording. After a snippet is successfully aligned, pressing the space bar commits the data to the corresponding word images. A boring YouTube movie shows how this program works in more detail.

This way is much faster than typing hex digits by hand. After going through all of the logic analyser recordings, I recover an additional 50,000 training examples, give or take. It still takes a few days. Getting training data is a drag.

Code used by this step:

  • rigol/ A first preprocessing step for the logic analyser recordings, this script filters unprintable characters and corrupted lines from .csv files, then deletes successive repetitions of the same byte value (essentially achieving a time-based run-length encoding of the data).
  • rigol/ A simple second preprocessing step for recordings that flips all of the bits in the byte data, since the logic lines for non-executable ROS data in the 5100 are active low.
  • The interactive program for aligning preprocessed logic analyser data with video still images, then deriving training data from the aligned recordings.

In case you don't have your own MSO2202A-S but still want to use some of this code, all you need are .csv files made entirely of lines that look like this: 12345,A7, or a time in nanoseconds, a comma, and a byte in hexadecimal. You should also throw in a bunch of NUL bytes here and there for the full Rigol experience.

Finally, on the command line, assuming you've placed your .csv files in a subdirectory called rigol/00_originals, and you're using a bash-like shell:

# Preprocess the logic analyser recordings.
mkdir rigol/01_deduped
mkdir rigol/02_inverted

cd rigol/00_originals
for i in *.csv; do ../ $i > ../01_deduped/$i; done

cd ../rigol/01_deduped
for i in *.csv; do ../ $i > ../02_inverted/$i; done

# Back in the top-level directory, start aligning recordings to video images.
# Here, we'll try to match data between video and traces for the portion of
# the APL ROS starting at word address $4000.
cd ../../
./ database.csv \
    rigol/02_inverted/apl4000.csv \

Well why don't we just try classifying and call it Stage 5

I've done about as much labelling as I want to do, and I'm only 60,000 training examples in, well short of my arbitrary 100,000-example goal. I don't want to do any more hand-labelling or take the cover off of the 5100 again; crowdsourcing is out, and an earlier effort to send bytes out of the oddly-tilted I/O ports at the back of the computer didn't work.

What if I already have enough data? Why not just give classifying a try now?

With hindsight, I can compute my accuracy as I refined the dataset and tried different techniques over time:

A plot of classification accuracy over time. Dates range from 10/9 to 3/3; accuracy jumps around but ranges from 99.65% at the start to around 100% at the end.

But that's getting ahead of things a bit.

Support-vector machines (SVMs) are a classic machine learning technique, and since they had worked so well for the 5100's executable ROS, it's natural to try that first. I devise a procedure that actually trains four multi-class ("multi" since there are 16 ways to interpret a digit: 0, 1, 2, ..., F) SVM-based classifiers simultaneously: one for the first digit, one for the second, and so on. This was 9 February, and as the leftmost graph point shows, it works pretty well by machine learning standards, giving an error rate of 0.36%. Sadly, this still means over 3,000 word images are read incorrectly, and I need to work that down to 0.

At the time I don't actually know the true accuracy of my procedure, but I can hold out some of my training data as a test set, and it shows something similar. So, what follows is the classic machine learning exercise of tweaking stuff and seeing what happens. Beyond SVMs, the scikit-learn Python library offers a smörgåsbord of plug-and-play multi-class classification techniques, and each one of those comes with configuration options as well.

I can't remember everything I try, but when I see the test set accuracy start to plateau at an error rate just over 0.15%, it suggests I've hit tweaking's limits. My training data probably isn't comprehensive enough to teach these algorithms all the varied ways that digits can look, and perhaps it even has some flaws in it.

As it turns out, though, even errors in classifiers can be useful.

Code used by this step:

And some commands:

# Try classifying the data, using the "ground truth" labels in `database.csv`
# and placing the results in `database_classified_00.csv`.
./ database.csv database_classified_00.csv

# Edit the `train_classifier` function in the classification code to try
# a different type of scikit-learn classifier.

# At this point you'd repeat the above two commands, writing results to a new
# `database_classified_XX.csv` file each time. Each file will be slightly
# different due to different methods and the way the program randomly allocates
# data to the training and test sets (so, it learns from different data each
# time. In order to see how two classifications disagree (in this example, the
# sixth and the thirteenth), you can do this:
./ \
    database_classified_05.csv \
    database_classified_12.csv \

# The last flag causes the diff to ignore label images that were marked as
# nonsense in the "ground truth" database. You can use a similar invocation
# to inspect how the classifier chose to classify the ground truth data
# itself---a good way to spot the kinds of mistakes the classifier makes, or
# in some cases find errors in the ground truth labels themselves. You'll see
# this idea crop up again later, too...

Stage 6 uses disagreement to purify the data

Now and then a machine learning paper makes an impressive show of referencing one of the most famous opening lines in all of literature. Papers that do this are all alike. They observe that collections of classifiers (or other kinds of learning systems) all tend to produce the same answer when they evaluate unambiguous data, but then yield different answers when applied to messy, noisy information. By now I've got plenty of classification results from Stage 5, and thanks to the garbled digits that we saw show up during transitions between frames, I've also got plenty of messy, noisy information for the results to disagree about.

I decide to write another program that detects these disagreements and allows me to flag all word images from the corresponding video frame as ignorable nonsense. The interface looks like this:

A screenshot of the program that uses classification disagreements to identify video frames with garbled digits. A terminal with bitmap graphics capabilities presents an image of 64 garbled hex digits above some status and positioning information, as well as a list of keys the user can press to perform scanning and marking operations.

Operation is straightforward: you view each cropped video frame that yielded images that caused the classifiers to disagree, then specify whether the frame had garbled digits. I'm able to mark word images from over 7,000 video frames as invalid using this tool, flagging nearly all of the corrupt label images.

Code used by this step:


# The first argument is the database to modify, the second is the name we gave
# to subdirectories holding per-frame cropped images, and the rest list at
# least two databases of classification results that we want to compare to find
# disagreements.
./ \
    database.csv \
    01_cropped \
    database_classified_05.csv \

Perhaps a step backwards in Stage 7: masking

Marking garbled images label was a useful step (it helps ignore meaningless disagreements between classification results, for example), but it hasn't really changed the ground truth data that trains the classifier. I'm still just about as far on that front as I was after Stage 5. By now, though, I've started to think about an even fancier tweak to try: masking each digit in word images prior to any classification.

Generic classification methods like the ones I'm using don't come with sophisticated preconceived notions about the data they're analysing. Even though people know that, generally speaking, none of the other digits ever affect the fact that the second digits of 540C and 144A are both 4, my classifier has no such built-in bias. For all it knows, in the rare cases where the last three digits are 473 or 4AB, it should emit 5 instead of 4 for the second digit. Why not? The classifier has no real notion of "four" or "five" or "digit" or "second". All it sees are abstract patterns of pixels with certain measurable similarities. Why should one subset of the pixels be more important than any other subset?

So, it occurs to me that it might be beneficial to contribute my own problem understanding to the process. If the classifier for the second digit only saw images with all of the other digits censored, it wouldn't need to learn on its own to ignore the other digits, and less to learn means less training data is needed. Maybe we wouldn't need 100,000 ground truth labels after all.

Thus, from each word image, I'd like to derive four masked images like these:

A screenshot of a diagnostic program running in a terminal with bitmap graphics capabilities. This program presents an original word image of the word 10E0, then four separate images showing the individual digits 1, 0, E, 0.

My strategy for segmenting the digits is simple. Summing all of the columns of pixels in the image gives you a one-dimensional graph of peaks and valleys. The gaps between digits form the deepest valleys, since these gaps have only dark pixels and sum up to small values:

One of the word images is positioned over a graph of the sums of the pixel values in each of its column. The three deepest "valleys" in the jagged graph align with the gaps between the digits in the image.

Regrettably, this method doesn't always work. In some images, digits abut so closely that their gaps are not so dim after all:

An image like the previous, except this time it is much harder to tell which valleys are the three deepest: there appear to be two candidates for the third-deepest valley.

Here, the third-deepest valley turns out to be the middle column of the trailing C digit, just barely---so my approach can be fooled. I don't realise this right away, but my first classification results with masking enabled are so much poorer that they form their own valley: the deepest in the progress graph shown earlier. After spotting the problem, I try mitigating it with quick hacks like allowing digit splits to occur only on certain columns, and that works most of the time. Of course, sometimes it doesn't:

Another screenshot of the diagnostic program from earlier, except this time the word image 1E54 has been separated into one image showing 1E, another showing 5, and two more showing fragments of the 4 digit.

In the end, the masking tweak seems to wind up being a wash, trading familiar kinds of errors (partly attributable to the classifiers not "knowing" to limit their scope to a particular digit and nothing else) for new kinds (badly-masked images). Additionally, it's not 100% clear that the classifiers really should ignore other digits. Just as seeing the letter Q in English text makes it likely that the next letter will be a U, some patterns recur in the ROS data. If the first digit of a word is 4, the second will be 7 over 3/8ths of the time. Nearly one time in nine, the entire word will be 47f0 (an unconditional jump instruction in System/370 machine code). Our classifier might do better if it has the opportunity to learn and exploit these regularities.

Either way, after some hacking to improve the masking, classification accuracy does climb back up out of the deepest valley, but this probably has more to do with something else I'm trying at the same time...

Code used by this step:


# The new --mask-digits flag enables the most refined (i.e. tweaked) version
# of the digit masking described above.
./ --mask-digits database.csv database_classified_25.csv

# Or, to visualise digit masking at work on a word image, provided your
# terminal supports Sixel graphics:
./ APL/APL_LROS_0000/02_words/0314_0_1.png

Fine, then, neural networks for Stage 8

So far the classifier techniques I've been using are well-established, well-understood methods with interpretable inner workings and strong theoretical guarantees. Who cares about rigour, though, when there's a massively hyped method that works even better?

Simple feed-forward neural networks are explained in countless YouTube videos, so it suffices for here to say that I try them out. The scikit-learn package I've been using includes such networks in its suite of classifier methods, so putting them to work requires only an easy modification of my code. Next, since nobody knows the "right way" to design a neural network (in this case, to choose the number and size of its layers, among other hyperparameters), the only thing to do is "wing it" and try a bunch of configurations. By the time I get bored of doing that, my last network has three hidden layers of 50, 40, and 30 ReLU units. I can tell from results on the test set that performance is considerably better than what the other methods had achieved, with misclassifications now down in the single and low double digits.

I'm convinced I could improve even further, though. Virtually all neural networks that do serious image processing make use of convolutional layers, about which there are abundant further explainey YouTube videos. It has been Officially Decided that these very capable and ubiquitous network structures are out of scope for scikit-learn, though, so I switch to using Keras, a "real" library for neural networks. Once again, random tweaking is the lowest effort way to explore the space of network architectures. In later stages, I settle on a network with a single 16 3x3 filter convolutional layer, a 2x2 max-pooling layer, and then two hidden layers similar to the first two from the previous network.

It takes longer to train the new network in any configuration: much longer, nearly half a day (remember, my laptop is no speed demon). The network is more complicated this time, and it happens that I'm also processing about six times more data in training now than I was before, but the runtime increase still seems disproportionate. Oh well. When training completes at last, the performance improvement is present, but slight: perhaps one or two fewer test-set misclassifications. 100% correctness is closer than ever, but it still feels a long way off.

Code used by this step:


# Both classification programs are invoked in the usual way.
./ database.csv database_classified_30.csv
./ database.csv database_classified_31.csv

Stage 9 feeds Stage 9 into Stage 9 into Stage 9

Once again, the secret to all successful machine learning is training data. If the classifier needs improvement, get more training data.

I'm still stuck at 60,000 training examples without any appetite for making more. The results of my existing classifiers are pretty good, but still not perfect. Well, fine, what if I just use these results themselves as training examples?

Most practitioners would regard this idea as risky, generally speaking. An error in a classifier's output-turned-training-data could be amplified in an "offspring" classifier that learns to replicate the mistake; after enough generations, several of these errors might proliferate, causing the classifications to become increasingly nonsensical. There is another possibility, though: the abundance of correct labels could overwhelm the sporadic errors and draw successive generations of classifications toward a more consistent and correct final output. Mathematically, we are suggesting that this iteration of training and classification converges toward a useful fixed point. If x is the original training data, we could call the function


the output of training a classifier on the original training data and then computing its classification of the entire dataset. Next,


would be the result of training a second classifier on the result of the first and obtaining its classification of all of the data. This iteration can repeat as often as you like, so


would be eight generations away from the training data we made in Stage 4. Is this actually worthwhile? Will this process converge on anything useful? Neural networks are so complicated that no mathematics exists today that would help us answer this question using theory alone---the only way to find out is to try it and see what happens.

To be honest, I'm not quite that brave. If I do exactly this iteration without any intervention, my suspicion is that the procedure will have a fixed point---a very boring one! Consider the classifier for the first digit: a relatively minor, random error might cause one generation to misclassify the first digit of a few training examples as a 3; in the next generation, the output 3 covers slightly more of the input and may cause the first digit in several more images to receive a 3 misclassification; this process repeats and reinforces itself until, many generations later, all digits are classified as 3! I never try this "pure" iteration and so don't know if this is what would really happen, but I think it's pretty likely.

Instead, I use an approach with two modifications. First, rather than train "offspring" classifiers on the entire output of their parents, I only use 60% of the parents' output. I don't bother writing a formal program for this: instead, I just run a throwaway snippet in an interpreter:

with open('database.csv', 'r') as din:
    with open('database_classified_40.csv', 'r') as dc:
        with open('database_tmp_hybrid.csv', 'w') as dth:
            while True:
                l1 = next(din)
                l2 = next(dc)
                num = int(l1.split(',')[2][1:-2]) if count > 0 else 2
                count += 1
                if num >= 2 or random.random() < 0.6:

(It's not even slightly good code! I couldn't be bothered to use itertools.izip, so the loop terminates with a StopIteration exception.)

My reason for omitting the entire dataset relates to the Anna Karenina gimmick discussed in Stage 6. "Iterated retraining" (for lack of a better term) will lead to the "generation loss"-like errors I worry about above, but my hope is that the subsampling will increase the diversity of the errors that emerge from one generation to the next, making it a bit less likely that successive generations will repeat the same mistakes.

This diversity is also helpful for the second modification to the "pure" iteration, as a prompt for manual intervention. After computing the next generation of classifier outputs, I compare it to several of the earlier outputs and manually supply the correct classification answer. I do this in hopes of stopping any remaining gradual inter-generational error proliferation right when it starts. For this slightly more complicated operation, I write another custom program:

A screenshot that looks similar to others above. A word image whose digits were classified differently is shown, along with the cropped video frame it originated from. On-screen options allow you to choose one of the classifiers' labellings or provide your own, among other actions.

This program lets me resolve disagreements between classifier outputs myself---a handy way to provide additional training data labels for word images that might be critically ambiguous. Sometimes I find more garbled digits I'd missed in Stage 6, so I can also mark entire frames as invalid.

Soon the strategy seems like it might be paying off. The number of misclassified word images in the test data falls into single digits. Busy with other things (or maybe I'm just getting bored), I spend over a month in a daily routine of resolving classification disagreements, sampling new training data, tweaking the network parameters, and kicking off a new training+classification iteration to run in the background. Errors remain few; sometimes they even fall to zero. Is this real performance, though, or is each new classifier generation increasingly delusional?

Code used by this step (besides the quick-and-dirty Python):


# The first argument is the database to modify, the second is the name we gave
# to subdirectories holding per-frame cropped images, and the rest list at
# least two databases of classification results that we want to compare to find
# disagreements.
./ \
    database.csv 01_cropped \
    database_classified_44.csv \

Ensembles: the second trick of classifying, and the first trick of Stage 10

After a month of kicking the can down the road, I can't put it off any longer. It's time to try compiling all of my classified word images into the non-executable ROS data.

Even though I'm seeing very low error rates on the test data, it seems unwise to trust the results of a single classifier run. Instead, it's probably better to collect "votes" across multiple classifications and go with the majority interpretation. In machine learning, this is a kind of ensemble method, a class of old tricks that have helped obtain contest-winning classifier performance now and then over the years. In particular, it's very similar to a method called "bagging". The main difference is the way I train new classifiers on old classifier output---in machine learning, such approaches are known as "totally weird".

Not only can I combine classifications from different classifiers, I can also combine them across multiple video frames, since each "page" in the DCP1 monitor program stays on the screen for around a tenth of a second (about three frames), and also because I recorded all the data twice for redundancy's sake. It's very helpful too that the DCP1 display always shows the memory locations of the data on the screen:

A cropped IBM 5100 screen image. On the left edge, two hexadecimal words, stacked vertically, surrounded by a magenta box. To its right, a two row, eight column matrix of hexadecimal words surrounded by a green box.

Those are the words in the magenta box. To the right of 1AE0 are the 16 bytes of data stored at that location, and likewise for 1AF0 beneath it. So, here's the plan:

  • Collect several recent classifier runs from Stage 9. Then examine each classifier run:

    • For each video frame in the classifier run, note which chunk of the ROS was being recorded (recall from Stage 1 and Stage 2 how the whole ROS had to be split across sixteen separate videos---although I have 32 videos from recording each chunk twice). Then:

      • For each data word in the frame, identify its memory location from its position in the image along with the values of the address words at left (as read by the classifier).

      • Record a "vote" for the classifier's reading of the data word as the value of the data at the deduced memory location in the current ROS chunk.

I write a program that assembles these votes into a text database and run it on the outputs of the 28 most recent classifier runs. Even though most memory locations receive comfortably more than 100 votes each, I'm surprised by the degree of consensus: there are only 137 words in the entire ROS that receive multiple interpretations, and only one of them has three readings (the rest have only two). In nearly all cases, the majority is overwhelming, with runner-up interpretations claiming only one or two votes.

Huh. With a little bit of mental arithmetic you can convince yourself that, in a sense, I've put in a lot of unnecessary effort. It must be the case that for most of my classifier runs, I could examine the run individually and, by counting votes from different frames for just that run, achieve the same majority interpretation of the entire dataset that I get from considering all of the classifier runs together. There just simply aren't enough disagreements for more than one or two of the classifier runs to have a different voting result.

Oh well. Anyway, it's 2019. Everyone knows by now that a majority vote is no guarantee of a good outcome. All of my classification results may agree, but they could still be deluded in the same way. Ponder that one, Tolstoy...

Code used by this step:


# Unlike other stages, this stage operates the program with
# a shell script. You should edit it first and specify the classifier outputs
# you'd like to poll for votes.

# Then just run the script. The result is a handful of text vote tally
# databases with names like assembly_APL_LROS_6000.txt, one for each of the
# chunks of non-executable ROS from the original video recordings.

The eleventh, final stage: vote, and verify

The votes are in. If the majority are correct, I might be nearly finished, so I rush to supply all of the missing pieces. First is a small program, written even more hastily than usual, to take the tally databases compiled by and emit binary data reflecting the most popular interpretations of each of the word images.

Soon I have two candidate binary ROS dump files, one for the APL interpreter and one for the BASIC interpreter and the common ROS. At 128 KiB each, both are larger than they need to be: APL should only take 96 KiB, and the BASIC/common code only uses 54 KiB. I'm still hopeful that the true data is embedded somewhere within. To find it, and to check that the decoding was correct in the first place, I'll use the same data integrity check that the IBM 5100 uses to make sure the non-executable ROS is in good shape.

When you flip the big red switch to turn on an IBM 5100, the computer growls to life and immediately begins its bring-up diagnostic. Gradually probing its internal mechanisms, starting from the most fundamental logic and memory components and working outward to the interfaces and stored programs that it uses to interact with the world, it marks progress with letters of the alphabet printed to the top-left corner of the screen. The most interesting test to us is the ninth, where the computer begins by displaying this:


The 5100 is testing the integrity of the non-executable ROS. 18 is the first portion the computer examines: it refers to a 6144-byte chunk of "common" code. Next, the bring-up counts through additional portions of the ROS depending on which language you've selected: if APL, sixteen of them numbered sequentially as 20 through 2F. BASIC uses only eight marked 10 through 17. (You can watch a video of my 5100 doing both sequences.)

The integrity check for each chunk uses a 16-bit CRC calculation (excellent video lesson). To apply the same check on a more modern computer, I first had to identify exactly what the 5100 was doing in a disassembly of the 5100's executable ROS, and after an evening of digging (aided by early hints from Christian Corti in that linked disassembly), I spotted the key loop at memory location $5F3C and the core of the calculation at $5F94. I might have carefully analysed its operations and devised an implementation of the same algorithm in a contemporary language, but honestly, it was much easier to do this:

def update_crc(byte: int, crc: Tuple[int, int]) -> Tuple[int, int]:
  """Update a 16-bit CRC given a new byte."""
  # Set up the 16 registers of the 5100.
  r = [(0, 0) for _ in range(16)]
  r[9] = (crc[0] & 0xff, crc[1] & 0xff)
  r[10] = (0, byte & 0xff)

  r[14] = (r[14][0], r[9][0])                                   # MHL R14, R9
  r[14] = (r[14][0], r[14][1] ^ r[10][1])                       # XOR R14, R10
  r[1] = r[14]                                                  # MOVE R1, R14
  r[15] = r[14]                                                 # MOVE R15, R14
  r[15] = (r[15][0], (r[15][1] << 4 & 0xff) | (r[15][1] >> 4))  # SWAP R15
  r[14] = (r[14][0], r[14][1] ^ r[15][1])                       # XOR R14, R15
  r[14] = (r[14][0], r[14][1] & 0xf0)                           # CLR R14, #$0F
  r[14] = (r[14][0], r[14][1] ^ r[9][1])                        # XOR R14, R9
  r[9] = r[1]                                                   # MOVE R9, R1
  r[15] = (r[15][0], r[15][1] & 0x0f)                           # CLR R15, #$F0
  r[9] = (r[9][0], r[9][1] ^ r[15][1])                          # XOR R9, R15
  r[1] = (r[1][0], (r[1][1] << 5 & 0xff) | (r[1][1] >> 3))      # ROR3 R1
  r[15] = r[1]                                                  # MOVE R15, R1
  r[15] = (r[15][0], r[15][1] & 0x1f)                           # CLR R15, #$E0
  r[14] = (r[14][0], r[14][1] ^ r[15][1])                       # XOR R14, R15
  r[1] = (r[1][0], r[1][1] & 0xe0)                              # CLR R1, #$1F
  r[9] = (r[9][0], r[9][1] ^ r[1][1])                           # XOR R9, R1
  r[15] = (r[15][0], (r[15][1] << 4 & 0xff) | (r[15][1] >> 4))  # SWAP R15
  r[1] = r[15]                                                  # MOVE R1, R15
  r[15] = (r[15][0], r[15][1] & 0xe0)                           # CLR R15, #$1F
  r[9] = (r[9][0], r[9][1] ^ r[15][1])                          # XOR R9, R15
  r[1] = (r[1][0], r[1][1] & 0x01)                              # CLR R1, #$FE
  r[14] = (r[14][0], r[14][1] ^ r[1][1])                        # XOR R14, R1
  r[9] = (r[14][1], r[9][1])                                    # MLH R9, R14
  return r[9]                                                   # RET R8

In other words, I set up a Python array of tuples like the 5100's registers and carefully manipulated them step-by-step in the same way the machine instructions from the CRC update routine do. I still have no idea how this code works, and there's no reason I'll ever need to learn.

Time to find out whether my decoding has succeeded:

A terminal window screenshot showing that regions $20-$2F of the recovered APL non-executable ROS---that is, all of them---pass the CRC check. The excess 32 KiB in the file form six additional regions that do not pass a check, but these do not correspond to actual data in the ROS.

Sure enough. Just like that, the IBM 5100 non-executable APL ROS is "officially" recovered as the 96 KiB at the beginning of the binary data "undumped" by our ensemble of neural networks. A slightly more complicated exploration procedure was required to understand the layout of the raw BASIC/common ROS data (see the data supplement for details), but before long this was recovered as well.

Code used by this step:


# Compile tally databases into "raw" binary data files for the APL and
# BASIC/common non-executable ROS.
./ assembly_APL_LROS_* > binary_APL_LROS_raw_undump.bin
./ assembly_BCom_* > binary_BCom_raw_undump.bin

# Extract actual ROS data from the raw files.

# Validate the contents of the final recovered data.
./ binary_APL_LROS.bin
./ binary_BCom.bin

Epilogue: what if we had tried to do all of this with IBM 5100s?

Well, it would have taken a heck of a lot longer---and about a tonne and a half of the machines.

To start, let's take one of the simpler neural networks used in this project---the ordinary feed-forward neural network with three hidden layers of size 50, 40, and 30. Input images have 29 * 16 = 464 pixels, so if we only count multiplications (and not additions or other operations), a single classification operation for a single digit requires

  464       50       40       30
x  50     x 40     x 30     x 16
-----     ----     ----     ----
23200  +  2000  +  1200  +   480  =  26880 floating point multiplications.

We have 1,010,070 word images in our dataset and four digits per word image. Evaluating all of the digits therefore requires the computer to multiply over 108 billion times.

The IBM 5100's processor runs at just under 2 MHz, and if we assume that each instruction takes one cycle (which I suspect is unlikely), that's 500ns per operation. The processor's arithmetic unit only handles eight-bit integers---larger ones must be processed piecemeal---and it possesses no integer multiplication or division operations. Don't even think about floating point numbers: you do all of that in software. How? Well, some Googling suggests that similarly-impoverished microcontroller programmers sometimes use a technique called Horner's method, which can do floating point multiplication and division using only additions and bit-shifts. This article shows a microcontroller implementation of multiplication in 32 operations, although it's really multiplication by one hard-coded factor---to change the factor, you would need to rewrite the code. Very well, though, let's generously consider 32 operations a rule-of-thumb (under)estimate of how many operations a 5100 spends to do a floating point multiply, or, in other words, 16 microseconds.

Under these very friendly assumptions, classification of a single digit will take 16us * 26880 = 430.1ms, a bit under half a second. To classify the entire dataset of over four million digits, then, a 5100 would take a little more than twenty days to accomplish the multiplications alone. This only applies, though, if we have a fully-trained neural network; what we also need to know is how long it must have taken our 5100 to arrive at this network in the first place. The training regime in specifies that our learning procedure carry out up to 10,000 iterations of training on 5,000-image batches sampled from the dataset, so in addition to the million word images to classify at the end, we need to think about the 50,000,000 word images the computer must process during training. That many images takes

20.11 days * 50000000 / 1010070 = 988.88 days = 2.73 years

of multiplying, although our training procedure might stop sooner if it senses that it's doing particularly well. Still, we've been pretty generous in our estimates, so we can probably hedge a bit in the opposite direction too.

So far our multiplication count has only concerned the process of evaluating an image with a classifier---what's known in neural networks as the "forward pass". To actually update the network's parameters so that it learns to make better predictions, you have to compare its classifications to the actual ground truth labels from the dataset, determine how wrong the network was in its classifications, and then go back and tweak all of the network parameters so that they won't be quite so wrong next time. This is called backpropagation or the "backward pass", and we had better account for it in our estimate, too. For this, let's just multiply our 2.73 year tally by two, giving 5.45 years. We mustn't forget to add on the final 20-day classification for all of our data, which brings us up to 5.47.

Of course, I never trained just one neural network---thanks to continued tweaking and the "inbreeding" of Stage 9, I wound up training about 49 of them. For the 5100, we're now a month and some change shy of 270 years.

Let's change units. The 5100 was introduced late in the summer of 1975---about 44 years ago. If we had bought some 5100s when Rod Stewart's "Sailing" topped the UK charts and set them all right to work, we would have needed (270 ÷ 44) just over six of them running flat-out around-the-clock to be done today.

Recall that the 49 networks weren't all like the one we've just examined---we did switch to a more complicated network and a different learning system altogether at one point, so perhaps we should update our estimate. Our new network's convolutional layer alone would have required something like

29 * 16 * 3 * 3 * 16 = 66816

multiplications on its own to process a single image. The subsequent max-pooling operation would then reduce the result to a 14 * 2 * 16 = 448-value quantity, so the new number of multiplications would become

            448       48       38
          x  48     x 38     x 16
          -----     ----     ----
66816  +  21504  +  1824  +   608  =  90752

A classification of all of the word images now takes just under 68 days on the 5100, but I guess it's worth it for the improved accuracy.

Switching to Keras for the new network meant rewriting the training code to some degree. I didn't pay very close attention as I was writing it, but where we had previously processed 50,000,000 word images during training, the new Stage 9 training methods were examining 312,000,000 of them. This would occupy the 5100 for about eleven days more than 115 years; or, for the 26 networks I trained in this way, nearly 2,991 years altogether. Or, if you preferred in 1975, you could start the work on 68 5100s running in parallel and receive the answer today. (Let's not forget to allocate an additional three 5100s to tackle the 23 earlier networks, which were mostly much simpler.)

I haven't been able to find out how many IBM 5100s were built. This timeline cites a New York Times article claiming that IBM shipped 6,000 "desktop computers" in 1980, a year before it unveiled the IBM PC and a time when multiple options like the 5110 and 5120 were available for sale. Prices were similar to the 5100's, though, so let's say that IBM achieved similar sales from '75 through '78, when the 5100 was discontinued. That's 18,000 machines total. I suspect this may be an overestimate.

If we take the 44 years spent by 71 machines and spread it out across the entire fleet, our project would require all 5100 owners to donate

44 * 365.25 * 71 / 18000 = 63.39 days

of continuous operation to our project. That's not too bad, but let's respect the eight-hour workday: a bit over 190 days, or 73% of an American work year.

How much use did an average 5100 get over its lifespan? 190 full work days sounds fairly normal; maybe an underestimate. Remember though that I've made a lot of other low estimates. If I'm under by a factor of 10 through a combination of underestimates (and I suspect the estimated number of cycles needed by floating-point multiply could be that far off on its own), then we need 7.3 work years per machine from the whole fleet, and I doubt there were more than a few hundred IBM 5100s in regular use in 1983.

Did my project to recover the non-executable ROS then use more compute (specifically, multiplications) than all the 5100s ever made have ever used in their entire history? I don't really know, but I like my odds.


  • Christian Corti's excellent IBM 5110 technical webpage (German, English) as well as helpful remarks from Christian over email.
  • The source code for Christian's IBM 5110 emulator (available at the same link, or try it in your web browser), which helpfully clarified what certain IBM 5100/5110 instructions really did.
  •, for its archived IBM 5100 and IBM 5110 technical documentation.
  • Andrew M. for technical advice.

Who, where, when

Tom Stepleton, London, 2019-06-27

Comments section

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by @stepleton, 2019-06-27 12:34 BST

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