Purchase and Install an Ultimate 1MB Board for Your Atari 800XL (1-3 hours)

Task: Purchase and Install an Ultimate 1MB Board for Your Atari 800XL

Needed: Web browser, soldering iron, drill, U1MB board

Time: 1-3 hours


Modifications or mods are a great way to expand what your Atari 8-bit computer can do. The Ultimate 1MB (U1MB) mod from Lotharek and Sebastian Bartkowicz (aka Candle`o`Sinis) is one of the more popular mods as it provides a good number of desirable features. First, as the name implies, this board expands the RAM to 1MB greatly extending the capabilities of the 8-bit computers. Second, this board replaces the provided operating system (OS) ROM with several different OS options including XL, XEGS, and Altirra. Third, it provides a clock with the date and time. Fourth, it has SpartaDos X built-in with a boot option. Fifth, it provides several programming languages including Atari BASIC (rev. C), Altirra BASIC, Synassembler, and Atari Assembler Editor. These can all be toggled and saved as defaults from a nice menu interface. There are some more advanced options for the BIOS and SIO drivers.

I recently purchased and installed the U1MB kit and did the installation. As far as mods go, this one is pretty straightforward and well worth it if you find the features listed above appealing. Below are some tips on completing this project.


The first step is to purchase the U1MB board. At this time, they list for a little over $70. There is also a version which includes a special firmware from Flashjazzcat. This one runs about $96. Details of the extra features can found at Flashjazzcat’s web page. This is the one I ordered. The kit comes with everything you need (pictured below) including the board, a set of wires, connection cables, and risers for keeping the board above the main circuit board. Of course, these ship from Poland and you will need to pay oversees postage if you don’t live in Europe. You can also order these from The Brewing Academy for $100. Note they only sell the Flashjazzcat version. Might be a cheaper option for those in the U.S.

U1MB Kit
U1MB Kit

The second step is to identify the Atari 800XL you want to mod. Some 800XLs have chips in sockets making them really easy to remove. This is the one you want for this mod as it will save a bunch of time. The socketed 800XLs have a label on the bottom of the case that says they were made in Hong Kong. If you use a non-socketed 800XL you will need to de-solder the chips and replace them with sockets. This is doable but you will need a system for desoldering and you will need sockets.

The third step is the preparation. You will need a soldering iron to connect four wires to the motherboard. You will also need a drill to drill two holes in the motherboard. Simply match the drill bit to the size of the screw.

The fourth step is to open up the 800XL, remove the motherboard, and remove the RF shield. Remove the six screws from the bottom of the case and gently remove the top being careful to disconnect the keyboard ribbon from the motherboard. There are several screws holding the motherboard on the bottom part of the case. Remove these and then remove the motherboard from the case. This needs to be done at a bit of an angle due to the joystick ports on the side. Once the motherboard it out you can remove the RF shield. On my 800XL, the top shield and the bottom shield were held together with screws (bottom) and tiny nuts (top). Some may have tabs you need to twist. You can discard the shields as they won’t fit with the U1MB board in place.

The fifth step is to perform the U1MB installation. I recommend first watching and studying the video by Flashjazzcat on how to do this. Here are the steps I completed. Refer to the labeled motherboard image below.

  1. Remove the OS and MMU chips. You will not need these further. Install sockets if you need them.
  2. Drill two holes where indicated by the orange arrows below. Make sure they match the distance of the holes in the U1MB which will sit on top of the OS chip with the holes at the top. Match the drill bit to the size of the screws. The video says 3mm but that was way too big for the screws in my kit. Note that you could use one of the RF shield holes for the riser thus eliminating the need to drill.
  3. Solder the four colored wires to the trace positions shown in the image below. Match the color of the wire to the arrow. Wires 1, 2, and 3 connect to the CPU while wire 4 connects to ANTIC.
  4. Insert the two riser screws up through the holes you drilled and secure them to the motherboard with one of the supplied nuts. Place the plastic cylinder over each screw. This will hold up the board.
  5. Connect the bigger of the cables to the OS socket. Note the position of pin one is to the lower left. Fold the ribbon over toward the back of the board (top of photo). The U1MB board will sit on top of this. Note that Flashjazzcat recommends shortening the ribbon cables. I didn’t need to do this.
  6. Connect the smaller cable to the MMU socket. Also note the position of pin one is lower left.
  7. Place the U1MB board on the two screws and secure with the remaining nuts.
  8. Twist the OS cable up and over the U1MB and insert the ribbon connector into the board on the far right side. Make sure the red edge of the ribbon is toward the back of the board.
  9. Twist the MMU connector and insert into the U1MB board on the left side. Make sure the red edge of the ribbon is toward the back of the board.
  10. Connect the wires to the four pins sticking up on the U1MB board just to the right of where you connected the MMU.
  11. Reassemble the motherboard and the case taking special care with the keyboard connector. The case can be a little difficult to reassemble because of how high the U1MB sits up. It is tight but should fit with some jiggling and tightening of the screws. Make sure the wires are clear of the cartridge slot.
800XL Motherboard with Labels
800XL Motherboard with Labels


My final mod is shown in the image below. This mod took me about two hours total. That included watching and studying the video, setting up everything I needed, executing the install, and cleanup. I could easily do this mod again in less than an hour total now that I know what I am doing.

One of things I am looking forward to trying is using the SIDE3 cartridge with the U1MB. They are designed by the same group have some synergies. For example, it should be possible to boot ATR files from the SIDE3 with the U1MB installed. I will report back later about this.

Note that this upgrade can be performed on a 600XL with 64KB of RAM and the 65XE and 130XE computers. The placement of the wires and the chips will be a bit different in each.

Here is a post on how to upgrade the firmware.

U1MB Installed
U1MB Installed