Integrated Woz Machine

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The Integrated Woz Machine (or IWM for short) is a single-chip version of the floppy disk controller for the Apple II. It was also employed in Macintosh computers.


When developing a floppy drive for the Apple II, Apple Inc. co-founder Steve Wozniak felt that the existing ones available on the market were too complicated, expensive and inefficient. Rather than adopting the existing floppy drives from Alan Shugart, Wozniak decided to keep only the drive mechanics and to develop his own electronics for both the drive as well as the controller.[1]

Wozniak successfully came up with a working floppy drive with many fewer electronic components. Instead of storing 8–10 sectors (each holding 256 bytes of data) per track on a 5.25-inch floppy disk — something standard at that time, Wozniak utilized group-coded recording (GCR), and with 5-and-3 encoding he managed to squeeze as many as 13 sectors on each track using the same mechanics and the same storage medium. In a later revision, this number was bumped up to 16 sectors per track with 6-and-2 encoding.[1]

At first, the floppy drive controller was built with many ICs and a PROM. To cope with increasing orders, Wendell Sander at Apple later put all these components into one single chip and developed a single-chip version — the IWM.[2]

Application and updates[edit]

The IWM is essentially a disk controller on one IC. It was employed in the Apple IIgs and all Mac models up to the Macintosh II. Later, an extended version, known as SWIM (Sander-Wozniak Integrated Machine), was introduced. This new version added the capability of reading and writing FM- and MFM-formatted (PC-formatted) floppy disks. In later Mac models, more and more peripheral components were added to the SWIM, until Apple finally phased out floppy drives from the Macs. The floppy controller function still stayed in the chipset for a while, even though the provision of floppy drives for the Macs had already ceased. For instance, the first iMacs still had a floppy drive connector on the motherboard, allowing a floppy drive to be retrofitted by knowledgeable enthusiasts.[3]


  1. ^ a b Williams, Gregg; Moore, Rob (January 1985). "The Apple Story / Part 2: More History and the Apple III". BYTE (interview): 166. Archived from the original on 2012-02-12. Retrieved 2013-10-26.  [1] (NB. Interview with Steve Wozniak, where he describes creating the Apple version of GCR.)
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Further reading[edit]