Disk access uses the 8kb address space above the top of RAM. The 8kb window is mapped to the disk space with a write to I/O. Left over map space is used to select up to four disks:
Although the method is fast it costs 8kb of RAM (i.e. 56kb rather than 64kb of RAM). After boot up it is not necessary to have ROM in the address space. I should look at this again later.