Improved Apple //c (ROM 4X) and Apple IIc Plus (ROM 5X) firmware.
ROM 4X and 5X are enhancements to the Apple //c version 4 and Apple IIc Plus firmware ROMs.
It adds the following features to the Apple //c and IIc Plus firmware:
The first feature listed above is the raison d'etre for this project. The larger story is down below but in short: The Apple //c memory card driver keeps certain information in the "screen holes" in main memory, which are required to use the memory card as a RAM disk. Should these screen hole values disappear, the card is re-initialized to empty when ProDOS boots. This happens even when the card is battery-backed and already has a RAM disk. The card data is not damaged until ProDOS boots, but if you attempt to manually boot the RAM disk it will say "UNABLE TO START FROM MEMORY CARD" because the screen hole values are not initialized.
This firmware enhancement identifies a ProDOS boot block on the RAM disk and, if found, restores the appropriate screen holes to make the RAM disk bootable and prevent firmware or ProDOS from re-initializing it.
See the ROM 4X README.md.
See the ROM 5X README.md.
Why did I do this? See story.md.