Apple II
Technical Notes
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                                                  Developer Technical Support


UniDisk 3.5
#2:    UniDisk 3.5 ID Bytes

Revised by:    Matt Deatherage                                  November 1988
Written by:    Mike Askins                                           May 1985

This Technical Note describes the signature bytes of the UniDisk 3.5.
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The signature bytes for the UniDisk 3.5 are the same as those for any 
SmartPort device:

    $Cn01 = $20
    $Cn03 = $00        ProDOS Block Device
    $Cn05 = $03

    $Cn07 = $00        SmartPort Interface

where n is the slot number of the device.

When searching the slots for a UniDisk 3.5 it is very important to check all 
the signature bytes, since there are other peripherals with similar ID bytes.  
Once you find a SmartPort card (or port), you should do a SmartPort STATUS 
call to determine which devices are connected to it.  Any number of different 
devices could match the SmartPort ID bytes, so trying to identify a device 
without making a SmartPort STATUS call is very likely to produce inaccurate 
results.


Why the UniDisk 3.5 Does Not Auto-Boot on Older Machines

If you look carefully, you will notice that the older (][, ][+ and unenhanced 
IIe) Autostart Monitor will not boot any SmartPort device because the ID byte 
at $Cn07 = $00 instead of $3C (like the old Disk II).  If Apple had left the 
ID bytes the same as the Disk II, then older versions of Apple II Pascal (1.2 
and earlier) would assume that the drive was a Disk II.


Where This Leaves You

The enhanced IIe ROMs, as well as the UniDisk 3.5 IIc ROMs and later (which 
you have if you are using a UniDisk 3.5 on a IIc) check only the first three 
ID bytes.  This check means that they will not only auto-boot the UniDisk 3.5, 
but any SmartPort or ProDOS block device.  On an older machine, you can boot 
one of these devices by typing PR#n from AppleSoft or Cn00G from the Monitor.


Further Reference
o    Apple IIGS Firmware Reference