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IWM chip Apple IIGS

A QFP version of an IWM chip in an Apple IIGS.

The Integrated Woz Machine (IWM), codenamed "Liron", is a single-chip version of the floppy disk controller board used by Apple II series computers.[1] It was also employed in early Macintosh computers.[2]


Disk II Interface Card 1978

The first Disk II interface card, released in 1978 for the Apple II.

When first developing a floppy drive for the Apple II, Apple Computer co-founder Steve Wozniak felt that the models available on the market were too complicated, expensive, and inefficient. Rather than use the existing floppy drives from Shugart Associates, Wozniak decided to use only the drive mechanism, but develop his own electronics and controller for the drive.[3]

Disk II IWM Controller Card 1983

A 1983 controller card that was simplified by the IWM chip.

Wozniak's design became the highly successful Disk II floppy drive and controller card, which contained a greatly reduced number of 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.[3] The Disk II floppy drive controller was originally built with 8 integrated circuits (ICs), or microchips, two of which were the programmable ROMs (PROMs), containing tables for the encoder and decoder, the state machine, and some code.[1]

Consolidation into the IWM[]

IWM chip Apple IIGS schematic

Schematic of the IWM chip's placement in the Apple IIGS.

To make it easier to fit the drive controller into newer motherboard designs, Apple staff scientist Wendell Sander collaborated with Bob Bailey of Synertek to consolidate Wozniak's entire controller board onto a single chip that came to be known as the Integrated Woz Machine (IWM). Wendell Sander's involvement led to it sometimes being referred to as the "Integrated Wendell Machine".[2][4] The IWM was employed in the Apple IIc, Apple IIGS, and all early Macintosh models up to the Macintosh II. Later revisions of Apple II controller cards were also simplified by replacing pre-existing ICs with a single IWM chip.[1]

Replacement by the SWIM[]

An updated version, known as the SWIM (Sander-Wozniak Integrated Machine), was introduced with the Macintosh IIx on September 19, 1988. This new version added the capability of reading and writing FM and MFM-formatted (PC) floppy disks when paired with a FDHD mechanism, also marketed as "SuperDrive".[5]


The SWIM controller was used through most of the "beige" era of 68K and PowerPC Macs until it was phased out with the introduction of the iMac in 1998.[6] The very first iMac G3 revision still had a SWIM and floppy drive connector on the motherboard, allowing a floppy drive to be retrofitted by knowledgeable enthusiasts.[7]


  1. 1.0 1.1 1.2 Disk Drives: Controllers, Wouter's Classic Computer Collection. 2011-12-13.
  2. 2.0 2.1 Five Different Macintoshes by Andy Hertzfeld, Folklore. Accessed 2006-04-24.
  3. 3.0 3.1 "The Apple Story / Part 2: More History and the Apple III", BYTE, UBM Technology Group, January 1985, p. 167. Retrieved on 2013-10-26. 
  4. Milestones:Apple Macintosh Computer, Engineering and Technology History Wiki. Accessed 2021-07-31.
  5. SuperDrives Stumble, TidBITS no.2. 1990-04-23.
  6. #1 Temporal Loop - Birth of the iMac by Thomas Hormby, The Mac Observer. 2007-05-25.
  7. iMac Floppy Kit. Corporate Systems Center (2001-04-14). Archived from the original on 14 April 2001. Retrieved on 24 January 2020.

External links[]

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