UniDisk #5
Architectural Differences Between 3.5" Drives

Revised by Matt Deatherage (November 1988)
Written by Cameron Birse & Mike Askins (October 1986)

This Technical Note provides information of interest to those developers writing low-level software for the UniDisk 3.5 and Apple 3.5 disk drives.

Definition of Drives

It is important to understand the differences between Apple's 3.5" drives if you are considering writing low-level software for use on the Apple II family drives.

UniDisk 3.5
is an intelligent drive, meaning that it has a microprocessor-based controller inside the drive enclosure that communicates with the host computer in an intelligent fashion through the IWM port. The host sends commands to the intelligent controller in the drive and the controller manipulates the drive hardware to read or write, and sends the data back to the host in a "packet" format.
Apple 3.5 Drive
is an unintelligent drive that depends on the host computer to manipulate the drive hardware to read and write data to and from the drive. Apple IIGS low-level routines for this drive will be essentially the same as those downloaded to the UniDisk 3.5 controller RAM, except they will reside in the host computer's memory. New device-specific control calls must be used for the Apple 3.5 Drive.

Tips for Low-Level Drive Access

The following calls are not guaranteed to be compatible in the future; for the highest level of compatibility, avoid disk access at this level.

As always, in order to promote compatibility between your software and future Apple II systems and to avoid writing utilities which will only work on one kind of drive, you should avoid low-level calls that are specific to a particular device or CPU.

Further Reference

This and all of the other Apple II Technical Notes have been converted to HTML by Aaron Heiss as a public service to the Apple II community, with permission by Apple Computer, Inc. Any and all trademarks, registered and otherwise, are properties of their owners.