Editing Group coded recording (section)

=== {{anchor|GCR 4/5|Commodore}}Commodore ===
Independently, [[Commodore Business Machines]] (CBM) created a group coded recording scheme for their [[Commodore 2040]] floppy disk drive (launched in the spring of 1979). The relevant constraints on the 2040 drive were that no more than two zero bits could occur in a row; the drive imposed no special constraint on the first bit in a byte. This allowed the use of a scheme similar to that used in {{val|6250|u=bpi}} tape drives. Every four bits of data are translated into five bits on disk, using the same 5-bit codes as IBM to ensure there are never more than two zero bits in a row, but in a different order:
{|
|
{| class="wikitable"
! colspan="2"|4-bit value !! colspan="2"|GCR code<ref name="Hildon_1985_Anthology" />
|-
! hex !! bin !! bin !! hex
|-
| 0x0 || 0000 || 0.1010 || 0x0A
|-
| 0x1 || 0001 || 0.1011 || 0x0B
|-
| 0x2 || 0010 || 1.0010 || 0x12
|-
| 0x3 || 0011 || 1.0011 || 0x13
|-
| 0x4 || 0100 || 0.1110 || 0x0E
|-
| 0x5 || 0101 || 0.1111 || 0x0F
|-
| 0x6 || 0110 || 1.0110 || 0x16
|-
| 0x7 || 0111 || 1.0111 || 0x17
|}
|
{| class="wikitable"
! colspan="2"|4-bit value !! colspan="2"|GCR code<ref name="Hildon_1985_Anthology" />
|-
! hex !! bin !! bin !! hex
|-
| 0x8 || 1000 || 0.1001 || 0x09
|-
| 0x9 || 1001 || 1.1001 || 0x19
|-
| 0xA || 1010 || 1.1010 || 0x1A
|-
| 0xB || 1011 || 1.1011 || 0x1B
|-
| 0xC || 1100 || 0.1101 || 0x0D
|-
| 0xD || 1101 || 1.1101 || 0x1D
|-
| 0xE || 1110 || 1.1110 || 0x1E
|-
| 0xF || 1111 || 1.0101 || 0x15
|}
|}
Like the IBM code, at most eight one bits in a row are possible, so Commodore used sequences of ten or more one bits in a row as a [[synchronization sequence]].

This more efficient GCR scheme, combined with an approach at [[constant bit-density]] recording by gradually increasing the clock rate ([[zone constant angular velocity]], ZCAV) and storing more physical sectors on the outer tracks than on the inner ones ([[zone bit recording]], ZBR), enabled Commodore to fit {{val|170|u=KiB}} on a standard single-sided single-density 5.25-inch floppy, where Apple fit {{val|140|u=KiB}} (with 6-and-2 encoding) or {{val|114|u=KiB}} (with 5-and-3 encoding) and an FM-encoded floppy held only {{val|88|u=KiB}}.