With available (and affordable) memory quickly passing the 64kbyte mark many people employed extra RAM in their system during the 1980s in order to create quick access disk storage. Within OS-9 it is very simple to define extra drives with bespoke drivers to integrate these into the operating system.

Often the access would be memory mapped access through a window of varying size with 1-4kbytes as a common option. Using bank switching this window would cover the full extent of the available RAM disk size.

I have designed the RAM disk for the CS09 system in a different way because I would like to keep the number of memory locations for I/O to a minimum. This method is much like normal disk access via a dedicated controller. There are only four bytes mapped into the CPU's address space through which all data transfer happens. Besides a data register there are 2 registers (16-bits) that set the Logical Sector Number of the sector to be accessed and a further command/status register. That's it.

Internally, the RAM disk logic has an 8-bit counter which auto increments whenever a read or write to the data register is performed. Together with the 16-bit LSN there is room for 2^24 bytes (16Mbyte) on the RAM disk. There is an option for using a 9-bit internal counter for use with UniFLEX which uses sectors of 512-bytes.

Access to the disk would require uploading of the LSN number of the sector followed by 256 reads/writes from/to the data register. The internal 8-bit counter get cleared automatically after any change of the LSN registers. Through the command register the disk can be write-protected.

Using one of the two DMA engines on the CPU board all data transfers are really fast.

RAM disk memory is low-power CMOS and the memory chips have backup batteries so that the RAM disk contents is non-volatile.