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Building tebl's switchless kernal switcher for Commodore 64
May 17, 2026
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From Wikipedia:
KERNAL is Commodore's name for the ROM-resident operating system core in its 8-bit home computers: from the original PET of 1977, followed by the extended but related versions used in its successors: the VIC-20; Commodore 64; Plus/4; Commodore 16; and Commodore 128.
Given the myriad of custom kernals that are available for the Commodore 64, it may be useful to switch between multiple on the same machine. Historically, this could be done by replacing the kernal ROM with a larger EPROM and adding an externally mounted toggle switch to select a kernal via one of the EPROM's higher address lines.
About 10 years ago bwack made a little board that uses a microcontroller to sense when the RESTORE key is pressed and then twiddle the address lines of a 27C256 accordingly. No more drilling holes in a C64 case! This design was forked by tebl, I used their version since the same PCB can also be used as an adapter to replace the DOS ROM in the Commodore 1541 disk drive. I paid $1.15 for five of these boards to be made by JLCPCB.
It was a straightforward build for me. I used a PIC K150 to program the PIC12F629 microcontroller.
The only problem I had was that the machined pin headers I'd gotten must've been shorter than the ones tebl used, the PIC in its socket was blocking the assembly from fully seating onto the kernal ROM socket. I don't think this would've happened if I soldered the PIC directly to the board. I got a pair of single row machined pin sockets to raise the assembly height and then it was able to fit.
Here are photos of the kernal switcher installed in my C64. As you can see it taps the RESTORE key signal from the mainboard as well as RESET. The power LED plugs into the switcher, when you hold down RESTORE it blinks to let you pick 1 of 6 operations. What isn't pictured here are the Male to Female jumpers I had to add moments later to extend the length of the power LED cable for correct fitment.
It works!! The BASIC prompt now says "JiffyDOS".
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