After some revisions of this project I decided to split the project in an EPROM and SRAM version. The EPROM version should be good for the general audience and the SRAM version for developers. The most obvious thing to program the EPROM with is EPYX fastloader as this is supported with the SD2IEC firmware. For copyright issues I will not offer any downloads of ROMs.
The EPYX fastloader uses some clever tricks to switch itself out of the way and frees up precious BASIC RAM after initialisation. When the C64 is powered up, EXROM is pulled low because the !RST was pulled low after reset and thus the capacitor is discharged. The PLA chip inside detects this and switches the EPROM in and executes the code stored herein, because of the magic string at $8000. As long as ROML is pulled down, the capacitor can’t be charged and EXROM stays low. After initialisation is done and no code is executed anymore from the ROM area, the capacitor get charged and ROML is released. At this time the PLA will switch out the cart and switches the RAM back in. When the ROM is needed a smal stub in IO2 area ($DE00-$DEFF) is called, which accesses the IO1 area ($DD00-$DDFF) and that enables the ROM and its code. This cart is equiped with the same logic, DIP switches selects the desired bank in the EEPROM (upto 8 when using a 27c512 EPROM). There is a solderjumper to enable this logic or just tie ROML down. An optional (external) switch can be used. More details in this topic on retrohackers (the schematic was posted there)
The board uses the larsp-64k hardware with a uSD slot. Writeprotect is not implemented (cart is always writeable) and the IEC address can be selected with a solderjumper (ID 8 is default). Nothing big is changed here.
Please contact me if you are interested in getting one. With enough interest I’ll produce a small run.
