For those that don't know, JiffyDOS is a ROM replacement for a Commodore 64 (and 128, I believe) and its 1541 disk drive. This ROM replacement provides OS shortcuts, faster load times, etc.

I've noticed that many Commodore 64 (and perhaps 128) users install an external switch to disable the JiffyDOS ROM and switch back to the original Commodore ROM.

What was the main reason for this? Did some games or other software programs not support JiffyDOS?

I realize a switch is a convenient way of not having to open the hardware so my question isn't on the switch...but why many users opted for a switch.


  • Guess: It wasn't compatible with all software, so sometimes you needed to switch it off to make software work.
    – dirkt
    May 17, 2017 at 18:21
  • @dirkt, specifically, it's probably incompatible with copy-protection measures.
    – Mark
    May 17, 2017 at 18:52
  • 2
    JiffyDOS is available for the VIC-20, C-16, PLUS/4, C-64, and C-128, so basically everything except the PET/CBM.
    – Tim Locke
    May 18, 2017 at 11:09

1 Answer 1


As an owner of a JiffyDOS-equipped SX-64, the reasons that @dirkt and @Mark gave are exactly correct: there are some fast loaders that just don't work with JiffyDOS.

The reason for the switch? Most JiffyDOS vendors include the switch and socket hardware. It's especially relevant now, as 2732 or 2764 EPROMS are not pin-compatible with Commodore's layout, so some kind of adapter must be supplied to make the ROMs work.

  • 1
    JiffyDOS uses the space occupied by the tape routines in the Kernal so you can't use a tape and JiffyDOS together, therefore having the means to revert back to the original Commodore Kernal is necessary.
    – bodgit
    Sep 19, 2018 at 12:42

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