FS_Entry reason codes:

             0 - NoOp  - Just return.
             1 - Get device name

                r0 - 1 (Reason code)
                r1 - FS number.
                r2 -> Buffer.
                r3 -> device name / id.
                 
                Exit:

                r0 - length of name 
                r1 - preserved.
                r2 - preserved.
                r3 - preserved.

            
             2 - Get free space for device.

                r0 - 2 (Reason code)
                r1 - FS number.
                r2 -> Buffer
                r3 -> Device name / id .

                Exit:

                Registers preserved.

                [r2]    - Total size of device.
                [r2+4]  - Free space on device.
                [r2+8]  - Used space on device.

                OR

                r2 = 0 if size unchanged from last time read.

             
             3 - Compare device.

                r0 - 3 (Reason code)
                r1 - FS number.
                r2 -> filename.
                r3 -> Device id.
                r6 -> Special field.

                Exit:   

                Registers preserved.

                Z set if r2 & r6 result in a file on the device pointed to by r3.

                This call can simply return with Z set if the filing system is a FAST
                filing system (E.G. RAMFS).