GameBoy ROM Tutorial

Howdy y'all,

This is a quick little tutorial on mask ROM recovery, in which we'll begin with photographs of the mask ROM from Nintendo's GameBoy and end up with a ROM file that can be disassembled or emulated.

The GameBoy is a good target for this because it uses what's called a Via ROM, meaning that metal Vias between layers encode the individual bits and those bits can be read from the surface of the chip. The ROM is also small enough that I can include it in a Github repository and that you won't spend weeks working out minor bit errors.

Cheers from Knoxville,

--Travis Goodspeed

Photography

We'll begin with dmg01cpurom.bmp, which I photographed in my home lab after decapsulating a chip in HNO3 and cleaning it in an ultrasonic bath of acetone. This chip does not require delayering with hydrofluoric acid or bit staining with a Dash Etch solution of HNO3, HF, and acetic acid.

The photo was produced in my metallurgical microscope as twenty-two frames under 50x magnification, which were then stitched together in Hugin. A biological microscope will not work, because the silicon substrate is opaqe to visible light; in a metallurgical microscope, the light reflects off of the sample rather than transmitting through it.

My original photo was too large for Github, so the version that you will work with is reduced in resolution. Lossy compression is avoided because it can confuse the image recognition.

If you produce your own photo of the DMG-01-CPU chip, the instructions here will work just as well.

Bit Extraction

In this tutorial, we'll be extracting the bits with MaskRomTool, a CAD program of mine for rapid bit extraction.

Begin by compiling MaskRomTool and installing it. If you are comfortable with the unix command-line, you will want the executables maskromtool and gatorom to be in your $PATH. If not, don't worry about that part, as all the features are also available from the GUI.

On Windows and macOS, you can avoid compiling the code by using a prebuilt release. These are compiled every few months when the code feels particularly stable.

After you have it running, use MaskRomTool to open dmg01cpurom.bmp.

Notice how a grey crosshair follows your mouse. They begin straight, but will soon tilt to match the row and column lines that you place on the ROM. This allows you to predict where repeated rows and columns of the same angle will fall, as most images are tilted but most bits are in perfect lines.

Notice also that the bits in the ROM come in pairs of regular rows and groups of eight regular columns. In MaskRomTool, you'll be placing rows and columns, but there are no strict limits about matching the grouping of the original image. If you find that you can reliably place a massive row that crosses the entire width of the image, or a massive column that crosses the entire height of the image, feel free to do so. If the image is crooked and you need a very short line to work around an error, that's also fine.

Navigation

To move around the image, you will use either your mouse or your laptop's trackpad. Mice are preferred because right clicking is a little easier.

Drag the background with the middle mouse button to scroll a little bit, or use two fingers on the trackpad to pan around the screen. If you hold the shift key, this panning will be much faster.

Zoom is very important for these projects. The QAZ keys handle zooming, with Q snapping to native resolution, A and Z zooming in and out. On a mouse, holding ctrl and spinning the scroll wheel will also adjust the zoom. With a trackpad, you might pinch to zoom, but only if you operating system supports it.

Spend a moment moving around the ROM photograph, guessing at what different structures are. After that, we'll start marking up the image.

Placing Rows

We'll begin with a few short rows and try long ones later. First, click a little to the left of the leftmost bit, and then move your mouse to the right but do NOT click. Instead press the R key to place a Row. A thin black line will appear between those two points.

You could repeat this to place all of the rows, but that would be labor intensive and might involve a lot of scrolling for very long ones. Instead, keep your mouse on the right side but move it down a little. When your crosshair lines up with the row, press Shift+R or the spacebar to drop another row with the same length and angle.

Repeating this across many rows should mark them out in short order. In the following screenshot, I've marked short segments of the first eight rows.

Once you get the hang of marking repeated rows from only their right-hand side, you might as well mark them across the entire image to save time and effort. If straight lines can't cross the entire image, then mark shorter lengths or better align your panorama of photographs.

If you place a row in the wrong position, there are ways to move it. First drag a box with the left mouse button to select one or more lines, then drag with the right mouse button to move them. The arrow keys will also move a selection, and the S key will set the position of a single line to a new end point.

Placing Columns

While you've marked some rows, the software still doesn't know where your bits are because you haven't yet marked any columns.

To mark you first column, first click above the bit in your first column. Then, as with a row, move your mouse beneath the last bit of the column and hit C to place a column line. (Do NOT click a second time.)

Repeat this by moving your mouse beneath the other last bits and pressing Shift+C. As each column line is dropped across the row lines, blue squares will appear over each bit. The software now knows where the bits are, and in the next step we'll teach it to know the difference between a one and a zero.

Please double-check that bits are in reasonable positions. If any of the bit boxes are not directly over the bit, select the offending row or column and drag with the right mouse button to correct its position. If the angle is wrong, use D to erase the line and then draw a fresh one.

Recognizing Bits

To identify bits, the software needs not just the bit positions that you've provided, but also a threshold and color channel to distinguish the bits. Click Edit and then Choose Bit Threshold to see the following graph and choose your own threshold.

This histogram of the first 64 bits shows gaps in all three color channels. The largest gap is in the Green channel, so I would set the threshold to 172 in that channel. Notice how the bit boxes automatically adjust themselves as you change the threshold.

Although the Blue and Red channels could work for this image, we want the Green channel because it has the largest gap. When we start adding up all of the bits in the image, the larger color distances will ensure that we get fewer bit errors.

Once you've set the first sixty-four bits, click View and ASCII Preview to see them. Ain't that nifty?

11101011
01111111
00110111
01101111
10110001
01101110
01011100
10110101

More Bits

Now that you've got a taste of recognizing sixty four bits, let's recognize all of them.

First, wipe our your previous work by either deleting dmg01cpurom.bmp.json and opening dmg01cpurom.bmp in a fresh instance of Mask ROM Tool or bulk erase your lines. Bulk erasure is performed by drag-selecting lines and then pressing D to cut them out.

(Strictly speaking, you could also mark the image with many very small rows and columns. The only reason I recommend against that is that it takes forever.)

After erasing the lines, drop a row that goes from the far left of the screen to the far right. If you are starting over instead of deleting prior work, it sometimes helps to first draw a shorter row so that the angle of the cross-hairs will be tilted the same as the image. You can draw an attempt at a row with R and then delete it with D, without the software forgetting about your first starting position.

After getting the first line drawn, move your mouse down the right side of the photograph, hitting Shift+R or the space bar whenever you pass a row to drop a line. If you find that you marked the bit just a little off, you can use the S key to Set the position of the last line, moving it to the new mouse location. Arrow keys and dragging with a right click will also reposition it, with bit values changing along with the positions.

If the rows are particularly regular, you might place several at time. To do this, select some with the left mouse button and then press Shift+D to duplicate the collection. The lines can then be moved to the new position with the right click drag, while a duplicate copy will remain at the original position.

After drawing the long rows, draw the columns. Just click once above the first bit to set a start point, then hit C beneath the last bit to draw a column. Shift+C will drop a new column of the same angle at another end point, so you can step through the image and draw all of the columns in short order.

When all the bits have been set, use Edit / Choose Bit Threshold to adjust the threshold a bit. Notice how the curve is a lot smoother with so many more bits, and that 151 might be a better choice than 172.

Finding and Fixing Mistakes

It's a great feeling to have all the bits marked, but that's rarely the end of it. Before continuing on to decoding the ROM, it's a good idea to do some quick sanity checks and make sure that no mistakes were made.

The V key or DRC / Evaluate Rules to run some quick sanity checks over your design. For example, what if you placed a line wrong and the color of a bit was suspiciously close to the threshold? You might get a DRC error like this one, which you could resolve by correcting the line placement. Each Design Rule Check (DRC) violation has a position and appears as a yellow box in the GUI. The E key will jump to the next error position in the list.

While mistakes like a misplaced line are your fault, those mistakes which are not your fault must also be corrected. Sometimes a bit of dust sneaks into the photograph, obscuring a bit's value at the sample location. Sometimes a chemistry error makes the bit a little hard to see.

In situations like this, where you know better than the machine, press Shift+F with your mouse over the bit to Force its value and place a little green box around the bit. If the wrong value is applied, pressing Shift+F again will flip it.

After all of this, the bits are finally complete!

11101011111100101100101100110010011000110111110000100001011100101110000000110011110001001001001011000100001011110001101100001000
01111111011100110111111101100011001010110100111100000110101010110011001111110011001010111011000000111000001011101101011011101111
00110111110110110111011111010111011001101001011101111111110110100111000101110010010110000101011101110001111101110111101111011000
01101111111000111110111011110110001011100101011010100010001110000111100000111010011111000111001100101111010000110100111111101001
10110001001100001011100110110111000110011101100111100000111100111111011010110001111111100111011010111010000110100100001100010011
01101110011100110110100100110111011100110101101001001111111100110010111110100011001110011010011101111010000111100111001010110010
01011100011101111111110001110111101100000101101100111000011100010011000011110101001100001011111000110000100110010111011111010010
10110101110101111011101001011111001110101111101000010101111100011101011111000011111010111010101100001110011110011011011111000101
00011111000101000001101110011110101111001111000011111011011100000100011011010000110100111001100100110011110101000101101110110110
11011001000101100111100100011011001110010101010100001101111101110110010110000111010101101101111010101100101100101101111110010111
11111111111110011111101101010101001101111101000010100110101010011011010011111001101101001101010110101010010101011110010110100011
01011110111110100001111011010110001000011101000000111011001011100101001111101110100110110000101000101011100001000001111000100001
11011000100010111011110010010101001011000111000010011000111110001111011011000000100111001101010010001100111101100010100110111001
11111011001010011111101000111101001100101111100110110101011111011011110110000101001011001101000100110111001101011110110110001010
00111111111101100011101111110010001011001111010000011111101111010011011111110111001101110111010101011111111110110101011100111111
10101101111101111000110111110110100001010111100111001101101110100100111111000011011110101010001101011100100011111100111110011111

About the CLI

So far, we've been using this tool in the GUI, but when you get to larger projects you'll want to automate some things with the CLI. By default it launches the GUI, so you'll want to add -platform offscreen -e when running it standalone, to both avoid opening windows and to exit on completion.

Usage: maskromtool [options] image json
Mask ROM Tool

Options:
  -h, --help                 Displays help on commandline options.
  --help-all                 Displays help including Qt specific options.
  -v, --version              Displays version information.
  -V, --verbose              Print verbose debugging messages.
  --stress                   Stress test bit marking.
  -e, --exit                 Exit after processing arguments.
  --disable-opengl           Disable OpenGL.
  --enable-opengl            Enable OpenGL.
  -d, --drc                  Run default Design Rule Checks.
  -D, --DRC                  Run all Design Rule Checks.
  --sampler <Default>        Bit Sampling Algorithm.
  --diff-ascii <file>        Compares against ASCII art, for finding errors.
  -a, --export-ascii <file>  Export ASCII bits.
  -o, --export <file>        Export ROM bytes.
  --export-histogram <file>  Export histogram.
  --export-csv <file>        Export CSV bits for use in Matlab or Excel.
  --export-json <file>       Export JSON bit positions.
  --export-python <file>     Export Python arrays.
  --export-photo <file>      Export a photograph.

Arguments:
  image                      ROM photograph to open.
  json                       JSON lines to open.

Supposing that we have the ROM as dmg01cpurom.bmp and its matching .json file, we can export the bits from the CLI like this in macOS or Linux. In Windows, we call maskromtoolcli.exe instead of maskromtool so that the console connection is maintained.

dell% maskromtool -platform offscreen -e dmg01cpurom.bmp -a DMG_ROM.txt
Allocation limit was  128 MB
Disabling allocation limit.
Loaded background image of  9000 , 2249
Done loading, now marking bits.
Exporting to ASCII.
dell% 

Decoding a ROM File

You now have the bits of your project in physical order, but this is quite different from the logically ordered bytes that a disassembler or emulator would prefer. We'll first do this graphically, and then also see how to do it from the Unix command line.

Before we begin, please install MAME and make sure that its disassembler, unidasm, is within the PATH. If you fail to do this, the solver will still work, but you'll miss out on seeing the disassembled code.

Decoding Graphically

You can play around with decoding settings manually with Edit/Decoding. Begin by setting the disassembly architecture to LR35902 and the wordsize to 8. The flips, rotation, and banking will be solved for you in a bit, and the list of Flags at the bottom shows you the options that would be passed to GatoROM for decoding on the command line.

You can solve for particular byte sequences or Yara rules with View/Solver. Double-clicking a solution will reconfigure the decoder and update the hex and disassembly views, so that they can be quickly searched.

Begin by using the Bytes tap of the Solver to solve for 0:31,1:fe,2:ff, which means that the first three bytes will be 31 fe ff. This is the LR35902 machine code for setting the stack pointer to 0xfffe. Double clicking on the solution -z --decode-cols-downr -i -r 180 --flipx will apply those settings to the decoder.

View/HexPreview will show the decoding live in hexadecimal. After selecting some bytes, you can also highlight them with View/HighlightHexSelection to see where those bits are located in your project file.

Decoding with GatoROM

GatoROM is a bit decoder that ships with MaskRomTool. It runs from the command line, but under the hood it is powered by the same libraries that run in the GUI solver.

First we need an ASCII file of the ROM bits. You can generate this with File / Export / ASCII in MaskRomTool's GUI or from the CLI with maskromtool -platform offscreen dmg01cpurom.bmp -a DMG_ROM.txt -e.

Here's how to solve for the ROM knowing that the first two bytes are 31 and fe. The -z flag tells it that we want Zorrom compatibility mode, and it accurately identifies three potential decodings before writing the correct one to DMG_ROM.bin.

dell% gatorom DMG_ROM.txt --solve --solve-bytes "0:31,1:fe" -z -o DMG_ROM.bin
Grade 50        31 11 47 fe 3e f9 1e 0e         -z --decode-cols-left -i -r 180 --flipx 
Grade 50        8a fe a8 01 d4 52 b0 a4         -z --decode-squeeze-lr -r 180 --flipx 
Grade 100       31 fe ff af 21 ff 9f 32         -z --decode-cols-downr -i -r 180 --flipx 
Exporting       -z --decode-cols-downr -i -r 180 --flipx 
dell% 

We can also search in other ways. What if we know that 31 fe ff exists somewhere in the image, but we don't know exactly where? This yields three potential solutions, and we can explore the different ones if needed by dumping them to files.

dell% gatorom DMG_ROM.txt --solve --solve-string "31,fe,ff" 
Grade 100       f5 06 19 78 86 23 05 20         --decode-cols-downl -i -r 0 --flipx 
Grade 100       31 fe ff af 21 ff 9f 32         --decode-cols-downr -i -r 0 --flipx 
Grade 100       f5 06 19 78 86 23 05 20         --decode-cols-downl-swap -i -r 0 --flipx 
dell%