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C64 Switchless Multi Kernel Switch

  • 27C256/512 rom adapter for C64's and disk drives.


The CAD files are available on GitHub:

All text above must be included in any redistribution

SKS64 Switchless Kernal Switcher

A kernal ROM adapter and switcher for C64, C64C, 1541, 1541C and 1541-II. The software is written in the Arduino IDE.

How it works

Switching Kernals

The current selected Kernal is indicated by a color using the RGB LED as power LED replacement. Holding the RESTORE key for several seconds until the LED flashes. You then release it and tap RESTORE until you get the kernal (color) you want. Wait a couple of seconds and the C64 reset with the new kernal selected.

1541 disk drives: Now you can have the disk drive and the C64 switch at the same time! The reset pulse issued by the SKS64 now has prepended serial data indicating the bank number. When the SKS64 board in the disk drive is setup in 1541 mode, it will switch to the upper bank when the C64 is switched to bank1 (for example, JiffyDOS).


Hold the RESTORE key for two seconds. The LED flashes. Release and wait two seconds for the reset.

EXROM Reset (optional)

Hold the RESTORE key for five seconds. Some machine language programs change the way computer returns after a reset. In that case the only way to reset to basic is to turn the machine off. By controlling both RESET and EXROM it is possible to "cold start" the machine.

Note: EXROM reset is only available when the ATtiny's RESET pin is disabled (fuse bit #RSTDISBL). This can only be done with HV-programmers (like TL866).


There are two different PCBs for the SKS64.

  • "longboard": works with C64 longboards and 1541/1541Late.
  • "shortboard": works with C64C shortboards and 1541C/1541-II.

Note: The internals of C64/C64C and 1541/1541C are often mixed up, and you must look inside their cases to be sure which boards they have.

Gerbers and hex-files (for programming) are found under the release-tab above.


Programming is done with ISP programmers or High-Voltage programmers. You can use an Arduino Uno as ISP. Please see the User Guide for more information.


After programming, the board must enter setup mode. Please refer to the User Guide.

ROM layout

ROM images are placed in 8k banks.

  • C64 Longboard
    • place 4 kernals in a 32k image (27C256).
    • place 8 kernals in a 64k image (W27C512).
  • C64 Shortboard
    • place BASIC + 3 kernals in a 32k image (27C256).
    • place BASIC + 7 kernals in a 64k image (W27C512).
  • 1541
    • Place CBM DOS + JiffyDOS in a 16k image (27C128)

For 1541C/1541-II ROM images are placed in 16k banks.

  • 1541C/1541-II
    • Place KERNAL+CBMDOS + KERNAL+JiffyDOS in a 32k image (27C256)

Via locations

You can solder 1x1mm pin headers into motherboard vias to wire up RESTORE, RESET and EXROM.

3D_front 3D_front 3D_front 3D_front 3D_front


The Switchless Kernal Switcher for shortboards is based on Sven Petersen's C64 Kernal Adaptor Switch shortboard. The old shortboard version is located at DiscoHR's repository. It is possible to use the original LED and cable assembly. This was an idea I found on tebl's repository.

Project links

Change log

longboard version

  • Rev 1.21 (2019-07-23):
    • Support 27C512 and 8 banks
  • Rev 1.20 (2019-07-14):
    • Eagle -> KiCad.
    • PIC12F629 -> ATTINY45/85. Note: Pinout is different! Don't mix up PIC12F with ATTINY!
    • Programming with Arduino IDE.
    • Programmers: ISP: USBasp or Arduino UNO as ISP. High-Voltage: TL866.
    • Use the original LED or a common cathode RGB LED.
    • User Guide rewritten for this revision.
  • Rev 1.13 (2016-09-28):
    • Moved IC2 down to clear space for the socket in the c64.
    • Changes to the silk screen.
  • Rev 1.12 (2016-09-18):
    • Increased pad and hole sizes.
    • Removed ground fill from top layer.
    • Added CERN OHL v1.2 License.
  • Rev 1.11 (2016-09-10):
    • RGB Led header following RGB led pinout (Blue-Green-GND-Red).
    • Edited silkscreen.
  • Rev 1.1 (2016-09-08):
    • sch+brd.
    • Swapped signals #RESTORE and RED_LED. GP3 is input only.

shortboard version

  • Rev 1.0 (2019-07-18)
    • Initial design
  • Rev 1.1 (2021-08-10)
    • jumpers for 1541C/1541-II support

Note: The schematic revision follows the board revision number.


Kernal switcher for the C64/C64C/C128




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