Commodore 64 reloaded JiffyDOS mod using a commodity version of JiffyDOS

C64 Reloaded JiffyDOS

On the C64 Reloaded boards, all ROM images have been burned into a single EPROM chip. As described by Individual Computers, this ‘…gives some extra space for a second Kernal ROM and a second character set’. As I will be using the C64 Reloaded with my JiffyDOS loaded 1541 Ultimate II, I ordered a JiffyDOS kernal IC from Retro Innovations.

The JiffyDOS mod is really easy as no hole has to be drilled into the case. To switch between Kernals, the RESTORE key must be pressed for 3+ seconds. The JiffyDOS IC comes with a jumper which selects which Kernal is the default. When the jumper is removed, JiffyDOS is the default Kernal and the CBM Kernal can be selected using the RESTORE key. When the jumper is installed the standard CBM Kernal is default and JiffyDOS can be reached using the RESTORE key.

First I tested it with the the jumper removed from the C64 Reloaded JiffyDOS ROM so the machine booted directly into JiffyDOS at startup. In this mode, the standard CBM Kernal is 2nd. I then attached my 1541 Diskette Drive which had already been modified with a 1541 JiffyDOS ROM from Retro Innovations as described in the 1541 JiffyDOS Mod.  The ‘@’ command showed the message of ’73,JIFFYDOS 5.0 1541, 00,00′ indicating that everything worked as expected. I got the exact same information when I did it with the 1541 Ultimate II attached. So that worked as well.

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With the jumper attached, the default Kernal is the standard CBM Kernal. This is the way I like my JiffyDOS modded machines to boot. I guess it’s a nostalgic thing, but I really like being greeted by the original Commodore 64 boot screen and then I have to do something actively with the machine, like pressing a button/flipping a switch, in order to swap to JiffyDOS! The C64 Reloaded Wiki states that just pressing the RESTORE button for 3+ seconds should do the trick. However, something odd happened when I wanted to switch into the JiffyDOS Kernal. In order to do this, I had to hold the RESTORE key for 3+ seconds, release the RESTORE key and then flip the power button up (this is the reset function of the button) in order to boot the system into JiffyDOS mode. The C64 Reloaded Wiki states that just pressing the RESTORE button for 3+ seconds should do the trick. The combination of the RESTORE key + Reset switch should actually change the character set to whatever 2nd character set that is installed on the ROM chip. I have not tested this with an alternative character set as I don’t think I will ever need another character set anyways on this machine. The bottom line is that the provided JiffyDOS ROM worked as the 2nd Kernal and the dual Kernal system now works the way I like it.

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As a side note, despite which Kernal is active (CBM or JiffyDOS), using the reset function of the power switch will reset the system back to whatever Kernal that was active before resetting. This is a neat feature as it is not neccesary to repeat the Kernal swap drill over and over again every time a new game is loaded.

UPDATE: Want to burn your own EPROM with JiffyDOS? Find the tutorial on how to do it here.

© breadbox64.com 2015

7 comments on “C64 Reloaded JiffyDOS

  1. Great article. Was wondering if you could describe the process you went through to put all of the ROMS that reside on the 64reloaded EPROM onto the ROM-el you got from Retro-Innovations.

    As I understand it, 2 character roms, 1 basic rom and 2 kernals reside on the one chip. In order to get this JiffyDOS install to work, you’d have to transfer everything that’s on the factory installed chip to the ROM-el with the exception of replacing one of the provided kernals with the JiffyDOS Kernal.

    Could you describe how you went about doing that?

    Thanks,
    COREi64

  2. Thanks for the comment. The JiffyDOS Kernal ROM Overlay IC I got from Retro Innovations (RI) had already been programmed with the CBM Kernal and JiffyDOS at the right memory locations. So no programming was needed. I just replaced the original C64Reloaded EPROM with the RI one. It’s a 30 seconds job. I therefore have no direct experience on programming the ROM-el chip from RI.

    However, the C64Reloaded Wiki is a little unclear on how to switch between the 1st and 2nd Kernal ROMs on the EPROM. I therefore fiddled around with my own EPROM chip for testing purposes (before realizing that the RI ROMs were actually working after all and switching between Kernals had to be done as described in the post above). I used a cheap EPROM burner (TOP853) and a 27C256 EPROM IC that I got off Ebay. I then dumped the content of the original C64Reloaded ROM using the software that came with the burner. I then scrolled down to memory location $6000 (which is the start location for the 2nd Kernal ROM) and copied/inserted JiffyDOS (version 6.01). The JiffyDOS file is in binary format (.bin) and should take up memory $6000-$7fff (the ROM file can be bought from RI). Finally, I burned the updated data file onto the 27C256 EPROM IC and inserted it into the C64Reloaded board. And that’s it. The homebrew EPROM had the exact same functionality as the one provided by RI.

  3. Fantastic! Thanks very much for the detailed explanation. Going to take some time and do this for myself. Great site BTW!

    COREi64

  4. Thanks! I just added a jumper to my C64R using your instructions. I too prefer the default CBM Kernal.

  5. I just did the mod as you did with a homebrew EPROM and it behaves exactly like yours. I did a search on the net to see if that is normal and it is. I was hoping it was just holding RESTORE for 3 seconds, but you also have to press the reset switch up for it to work.
    As a side note I inserted a custom JiffyDOS ROM with a nice rainbow logo as seen here! (link)

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