Group Code Recording (GCR) is a floppy disk data encoding format used by the Apple II and Commodore Business Machines in the 5ΒΌ" disk drives for their 8-bit computers (the best-known drives being the Disk II for the Apple II family and the Commodore 1541, used with the Commodore 64 computer). It was also used on the Apple Macintosh. A floppy disk is a data storage device that is composed of a circular piece of thin, flexible (i. ... For other senses of the word code, see code (disambiguation). ... The Apple II was one of the most popular personal computers of the 1980s. ... Commodore is the commonly used name for Commodore International, an electronics company who was a major player in the 1980s home computer field. ... The Commodore 1541 (originally called VIC-1541), made by Commodore Intl, was the best-known floppy disk drive for the Commodore 64 home computer. ... The Commodore 64 (C64, CBM 64) was a popular home computer of the 1980s. ... The iMac G5, Apples flagship consumer desktop. ...
The purpose of GCR is to avoid too many consecutive zeroes (i.e. absence of transitions), because the ones (transitions) synchronize the read clock. GCR permits consecutive ones.
GCR is more efficient than FM coding, but less efficient than MFM. Because MFM does not permit consecutive transitions, it runs at twice the clock rate of GCR. Modified Frequency Modulation, commonly MFM, is the magnetic data recording scheme used by most floppy disk formats, notably by most CP/M machines, as well as PCs running DOS. MFM is a modification to the original FM (frequency modulation) scheme for encoding data on single-density floppy disks. ...
GroupCodeRecording (GCR) is a floppy disk data encoding format invented by Commodore Business Machines and used in the 5¼" disk drives for their 8-bit home/-personal computers (the best-known drive probably being the single-sided Commodore 1541, used with the C64 computer).
While other formats such as MFM used a doubling of each 2-bit group to avoid this, GCR was tailored to give a much higher data density on the disc and avoid unnecessary redundancy, under the assumption that a few zeroes in a row was OK, but not too many.
A disk data encoding format similar to GCR was adopted by the original Apple Macintosh 3½" floppy disk drive, though no attempt was made to be compatible with Commodore's system – in fact since the physical form of the disk itself was new, Apple was free to implement it any way they chose.
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