A card with 4MB NVRAM for Apple ][ compatible computers which is a ProDOS compliant R/W device. This is a project based on my Prodos 512kB NVRAM drive which is intended to work with 8 NVRAM chips 39SF040 and compatible. The NVRAM is organized in 2048 banks of 2kB each, numbered #000 to #7FF. Banks are selected by writing #00 - #FF into $C0N0 and 0-7 into #C0N1, where N = 8 + [slot number]. When writing to $C0Nx at the same time the NVRAM chips are also enabled. When a bank is selected, its content is seen in the address space $C800-$CFFF. To de-enable the NVRAM drive a write must be performed to address $CFFF or reset executed. If a user program is accessing the card, it is important that at the end a write is performed to $CFFF so that the card is deactivated and does not conflict with other hardware, which is using the same address space. The bootloader is programmed in the highest 256 bytes of the last chip. The boot loader is always available at addresses $CM00-$CMFF, where M = [slot number]. The bootloader can also be seen at addresses $CF00-$CFFF of bank #7FF. The image contains a ProDOS volume which is loaded by the boot loader. Additional firmware resides in ProDOS Block $01. The card requires the presence of 64kB AUX memory, otherwise will be read only. Recommended installation is in Slot 7. The boot loader has functionality that it captures the boot sequence of the computer and loads ProDOS.
As of version 5.0 a design change was implemented - a write RAM buffer was added onboard to avoid using the system memory for buffering writes. This required a firmware update, which was also optimized to be now only 512 bytes large and fits completely in block 0001. The minimum firmware version is 42. A sencond PLD is used to combine the functions of the 'LS133 and 'LS138.
Version 5.2 is the same as 5.0, except that all small chips are hidden under the big DIP memory chips.
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