Jelena Mamčenko Operating Systems Lecture Notes on Operating Systems 52 To allow the use of more partitions in a compatible way a new partition type was introduced (in MS-DOS 3.2, January 1986), the extended partition , which was actually just a container for additional partitions called logical drives . Originally only 1 logical drive was possible, allowing the use of hard-disks up to 64 MB. In MS-DOS 3.3 (August 1987) this limit was increased to 24 drives; it probably came from the compulsory letter-based C: - Z: disk naming. The logical drives were described by on-disk structures which closely resemble MBRs, probably to simplify coding, and they were chained/nested in a way analogous to Russian matryoshka dolls. Only one extended partition was allowed. Prior to the introduction of extended partitions, some hard disk controllers (which at that time were separate option boards, since the IDE standard did not yet exist) could make large hard disks appear as two separate disks. Alternatively, special software drivers, like Ontrack's Disk
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