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).