The central part of EXDOS is the Filing System Handler. This provides all the faclities necessary to access and manipulate files on disks. The user's applications program does not normally talk to the filing system handler directly but rather goes through two possible routes, the Disk Device or the EXDOS CLI.
The Disk Device is an EXOS device driver which EXDOS automatically links in when it starts up. This can be used like any other EXOS device, allowing the user to open and close channels to disk files, and read or write data to them
The EXDOS CLI is an EXOS system extension which interprets certain command strings (coloncommands from ISBASIC) to provide functions such as renaming and deleting files and giving directory listings.
When EXDOS needs to access a disk, it always does so through a unit handler. There is a built in unit handler (called UNITH) contained in the EXDOS ROM which provides an interface to up to four disk drives connected to the disk controller card. Additional unit handlers can be linked in to allow EXDOS to access any other disk hardware which may be created. In addition to UNITH, the EXDOS ROM also contains another unit handler (called RAMUNIT) which can be linked in by an EXDOS CLI command to provide a RAMdisk facility.
UNITH does not access the disk controller hardware directly. Instead, it always goes through DISKIO which provides a very low level interface to the disk controller. This low level interface is also available directly to the user for accessing nonstandard disks, and also for functions like disk formatting.
The interrelation of all these various sections of code is shown in the diagram below. ISDOS is included in this diagram to show how it fits in, although it is not contained in the EXDOS ROM. The user in this diagram refers to EXOS applications programs such as ISBASIC, and also to the person using these programs.