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. (more…)