Using a Propeller to Repair and Expand a MicroKim

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If you own a MicroKim by (or a KIM-1 by Commodore/MOS), and you would like to take it further than the various programs from The First Book of Kim or the many other resources on the web, this project is for you. It only takes a Propeller QuickStart board and 28 wires to add emulated RAM, ROM or I/O devices to your MicroKim. And a PC with a USB port.

For example, you could use the project to add Ken Wessen's Krusader ROM image and turn your MicroKim into an Apple 1 replica. Or you could instantly load the KIM-1 version of Microsoft Basic so you don't have to load it through the serial port. You can even add a (monochrome) video output! And it works on other 1MHz 6502 systems too.

DISCLAIMER: This project connects 5V signals to the Propeller, which it's not supposed to be able to handle. Don't blame me if the Magic Smoke(TM) escapes!


I've owned a MicroKim by BrielComputers for years, but it had been defective for a long time: as it turned out, the EPROM was malfunctioning. This project describes how I found out what the problem was, and how I fixed it. And hopefully it gives you some ideas on how you could use a Propeller to enhance your own (1MHz) 6502 computer.

As you may know, I've been working on two projects for a while, which I like to call "software defined computers", called Propedle and L-Star. The idea behind those projects is that a Parallax Propeller is fast enough to connect it to a 1MHz 6502 (or 65C02) and be in control of what the world looks like in the eyes of that CPU. The Propeller is connected to the address bus and data bus and makes the 6502 think that there are more devices in the system than there really are. The difference between this KimStar project and the other ones is that in this case, we start out with an already fully functional 6502-based computer and let the Propeller "get on the bus" (literally), whereas the Propeddle and L-Star are more of a "blank slate" and won't work at all without the Propeller.

KimStar is really just a Propeller on the address bus and data bus of an existing 6502 computer, and has no need for other chips such as memory chips or glue logic, unlike Propeddle and L-Star. After I used KimStar to fix my MicroKim, I imagined it could be used as a proof of concept for those who own a MicroKim and want to try expanding it in their own way with hardware that can they can buy cheaply. I used a Propeller QuickStart board and some wires to build this. If there is enough interest, I might consider designing a PCB and making it available as a kit.

Be sure to click the "Read More" link below for lots more information!

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  • 1 × MicroKim from
  • 1 × Parallax Propeller QuickStart board
  • 28 × Breadboard patch wires with one female end

  • Enhancing the MicroKim (1): 24KB RAM Extension with Preloaded Basic

    Jac Goudsmit04/24/2016 at 06:04 0 comments

    I already described how I diagnosed and fixed my MicroKim. using the KimStar subproject of L-Star: basically a Propeller QuickStart board, connected to the expansion bus of the MicroKim. I had my MicroKim working again, so I sort of shelved the KimStar project as "finished".

    Then I found out that Aart Bik (a fellow MicroKim owner, retro enthusiast and immigrant from the Netherlands) was writing some MicroKim tutorials. I felt like playing with my MicroKim so I powered it up, but it had stopped working again. Never fear, KimStar is here! This time it turned out that the Rockwell 6502 that I got with the MicroKim (produced in 2003) had given the ghost. I replaced it with an MOS 6502 from 1982 and it worked fine again.

    Then I thought: while I have KimStar and the MicroKim together again, let's make something that's truly useful. Somewhat ironically, though this version of KimStar is possibly the most useful yet, the software is also the simplest ever in the history of Software Defined 6502 Computers: The code's main purpose is to emulate a 24 KB (kilobyte) RAM expansion, and map it into the 6502 address space. There's also a cog to control the on-board address decoder but for this project it's basically configured to do the same as the on-board address decoder. Oh, and the Microsoft BASIC interpreter for the KIM-1 ("KB9") is preloaded in the RAM area when the Propeller starts.

    The result is that I can now power up my MicroKim and QuickStart, and instantly use the BASIC interpreter by jumping into it from the monitor program. I don't have to download it via the terminal emulator first, so that saves me probably 3 minutes every time I want to use BASIC. And if I don't want to use BASIC, I can simply overwrite it; after all it's in (simulated) RAM, not ROM.

    The setup is the same as described in the project description. The software can be downloaded at .

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Hacker404 wrote 04/22/2015 at 19:53 point

Those old EPROMs are erased by UV light. There is a component of UV in sunlight so you have to cover the round glass window with a UV shield or it will eventually be corrupted. 

  Are you sure? yes | no

Jac Goudsmit wrote 04/22/2015 at 21:45 point

Yup. Unfortunately I don't buy 5.25" floppy disks anymore. The write-protect tabs that came with those were the perfect thing to cover EPROM windows :-)

Incidentally, the problem with my MicroKim EPROM was that some bits that were supposed to be 1, were 0. And this happened only for one of the pins of the data bus. When an EPROM gets erased, bits go from 0 to 1, and EPROMs that get too much UV light will show random 0 to 1 changes everywhere, not just in one bit of the data bus. So I think the chip had just gone defective and didn't get accidentally erased.

  Are you sure? yes | no

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