As you may know I’m busy with pimping a Commodore SX64 and trying to bring it into the 21th century. Since it is fairly old technology loading a program is painstakingly slow. Traditionally modded kernals or cartridges are used to install fastloaders. Modern alternatives like SD2IEC and pi1541 are compatible with them but still are slow. A MagicDesk Cartridge has improved loading times because its memory is directly connected to the CPU instead of a slow serial bus. The MagicDesk Cartridge can store upto 512KB (approximately 2000 blocks) versus a regular disk that holds 170KB (664 blocks).
The MagicDesk Cartridge is a cartridge that can bankswitch upto 1024KB (my version is just 512K) into the memoryspace at $8000 of the Commodore. Swapping is done by writing a byte to IO1 area ($DE00-$DEFF). The first 7 bits selects which bank is switched in. Setting the 8th bit disables the cart. In the first bank a autostarting menu is stored which takes care of the bankswitching, moving the selected program into memory and run it. With some magic also regular 8KB cartridges can be stored and used. Select the right bank with ‘POKE 56832,<bank>‘ and reset the Commodore with ‘SYS 64738‘. With patching other cartridge switchers would be possible.
The original expansion board was removed from the SX64 and replaced with a slighty larger expansion board with the MagicDesk Cartridge added to it. To enable the use of regular carts, it automagicly disables MagicDesk cart when a regular cart is inserted. A switch also can disable the internal cart and a button can reset the SX64 since it is lacking a reset button.
Only one of the 2 5V supply (pin X an Y on the expansion port) is connected to the cart, the other pin is the signal CRTSENSE which is pulled down with a resistor to ground. When no cart is inserted this signal is thus low and the cartridge signals are routed to the internal MagicDesk cartridge. When a cartridge is inserted both pins are tied together and the signal will be high and thus disabling the internal cartridge.
I’ll have a couple of populated leftovers on Tindie for sale.