Hardware design of Magic Desk and Ocean compatible cartridge for the Commodore 64
This is the hardware part that accompanies the project of Magic Desk Cartridge Generator
 
Magic Desk cartridges, in their original form, could hold up to 128Kb of code, or up to 16 banks of 8Kb each. Bank selection is done by writing a byte to $DE00. If bit 8 is set, then EXROM is disabled and RAM in the range of $8000 - $9FFF is usable.
This cartridge expands on the idea by having 128 banks of 8Kb available, thus the maximum size is 1 Mb.
Inputs of an 8-bit latch (74LS273) are connected to the data lines. Correct decoding for the I/O1 signal is done by combining PHI2 and I/O1 trough the NOR gates of 74LS02. The only feature that is deviant from the original Magic Desk specification, is that when you disable the cartridge by writing a value of $80 or larger to $DE00, the banking mechanism gets locked and no more write attempts to $DE00 will change the register's state until a hardware reset is performed. This had to be done because some programs would unintentionally write to $DE00 (or any other location in the I/O1 range) and sometimes, if $01 is wrongly set by the program, it could result in a crash. Since the use of this particular cartrige is primarily to enable making onefile program compilations trough the Magic Desk Cartridge Generator, we have decided to do it this way. If you want (or need) full compatibility with the original Magic Desk cartridge hardware, we have left a jumper to disable this behaviour. Cut the track exposed on the LOCK jumper ("YES" position) on the bottom side of the cartridge PCB, and connect it to the "NO" position. This way the locking mechanism is disabled, but still you can remove the cartridge from the memory map by setting the bit 7 of the latch high.
The PCB was designed to accomodate various types of EPROMs and their FLASH variants. The smallest EPROM you can use is 27C512 (64Kb), and the largest is 27C040 (512 Kb). There is an option to expand the board to a full 1Mb, which will be explained below.