www.mediapalatset.com gave me a mystery cart which, after some investigation, turned out to be a floppyspeeder kernal cartridge. He also had a 1540 drive with matching speeder-ROM to accompany the the cart. Thanks ! 🙂
The internal Kernal-ROM can be replaced with an external Kernal-ROM cartridge. This cartridge design is a different one than the standard cartridges mentioned at Making a C64 Cartridge and should not be mixed up. The only common thing is area $E000-$FFFF, which is used by Kernal-ROM and Ultimax-mode (RAM at $E000-$FFFF needs also to be taken into consideration).
You can not use plain Ultimax-mode to create an Kernal-ROM cartridge since Ultimax will bank out all memory between $1000-$7FFF and $A000-$CFFF making it rather unusable. A way to enter Ultimax-mode only when accessing (Kernal-ROM) area $E000-FFFF is needed.
To determine if adressing $E000-$FFFF should be ROM or RAM is done by setting HIRAM bit in $01 CPU I/O-register. Having an external ROM at $E000-$FFFF in the cartridgeport causes problems since the cartridgeport HIRAM-signal is used for accessing the BASIC-ROM at $A000-$BFFF. There is no equivivalent HIRAM-signal for $E000-$FFFF in the cartridgeport.
The simple solution is having some extra logic and attach a wire from the cartridge to the HIRAM-signal inside the computer, therefore kernal replacement carts most often come with a cable and a clip for this purpose. An example of this is the V-DOS / EXOS V3 (REX9805) cartridge. You can leave the wire disconnected but access to RAM at $E000-FFFF is not available which limits the functionality a lot (unless running plain basic).
EasyFlash3 and 1541Ultimate use a more elegant solution without a need of an extra wire, Skoe has written an excellent document in the subject: How to build a real Kernal Cartridge.
Turbo DOS V3
The cartridge had a sticker on it saying “Turbo DOS V3” (Drive returns “Turbo dos v3”, kernal startup-message says “Turbo System v3”). The cart also had a reset-button and a switch to be able to change eprom banks for (inter)national kernal. The wire was connected to the solder dot left of the 74LS30 IC. Delikatess-data was a shop located in Gothenburg, also selling Dolphin DOS at the time. I desoldered all components and scanned the PCB for documentation purposes.
Turbo DOS V3 ROM Download
HIRAM-Signal and the wire
The cartridge needs a wire connected to the HIRAM-signal inside the computer which is located at pin 28 on the CPU, you can study the schematics for more convenient locations of the HIRAM-signal.
Clip connected to pin 28 on the CPU (C64E / Assy 250469).
OpenC64KernalCart by Sukkopera
I was in contact with Sukkopera asking if he interested in doing a PCB for the kernal cartridge. He did a great job and refined the design. You can now have 8 different kernals on the external cartridge selectable by jumpers.
OpenC64KernalCart holding 8 different kernals. The HIRAM-signal should be connected to the yellow pin-header.
You can find his work at https://github.com/SukkoPera/OpenC64KernalCart
