IBM's Disk Operating System for the 360 series of mainframes was the first IBM operating system intended to be booted/IPLd from a disk. All of IBM's prior computers IPL'd cards or a tape. (More details here.)
Many of IBM's predecessor systems used disks for storage of programs and data, and several included or later saw the development of monitor programs that provided for streamlined transition between jobs in a multi-job deck. But monitors were relatively small permanently-resident programs that didn't do much more than fetch the appropriate program for the next activity identified by control cards in the deck.
For example, the IBM 1130 Disk Monitor System V2 used a 480-byte resident monitor, at 12% of the 4KB minimum configuration storage. The 1620 Monitor II required 1400 characters of the 20,000 characters (7%) in the minimum configuration needed for Monitor II.
A 1410/7010 Operating System resident monitor for a disk-oriented system required 11660 characters of the 60,000 character minimum machine requirement (19%). All generated functions were permanently resident in both the 1410/7010 System Operation File and the Disk and Tape IOCS programs for the 1401/1460. A 7040 IBSYS resident nucleus would require about 4KB of a 32KB machine (25%). IBSYS used permanently-resident code for infrequently-used functions such as file label processing, and IBSYS for included I/O support.
By contrast, the smallest version of the resident portion of DOS/360 was 6KB, or 38% of the minimum configuration System/360 memory size. This 6KB nucleus handled all I/O operations and task management for a single program partition. When configured with additional options, it supported execution of up to three concurrent programs--something DOS/360's predecessors did not do.
And unlike its predecessor systems, DOS/360 used disk overlays for comparatively infrequently-used operating system functions like error recovery ($$A transients) and file and volume open and close ($$B transients). In prior systems, code for these functions was compiled or linked as part of the executable program or was permanently resident, consuming memory that might have been used for the executable program. DOS/360 thereby was able to provide:
A standard set of tape and disk file label processing capabilities without requiring system generation options (1401 IOCS) or permanent dedication of processor memory (70xxx, 14xx systems)
A comprehensive error recovery suite, not limited to what could be tolerated as a permanent memory commitment.
A disk volume file directory managed by the operating system, eliminating the need to create application program code to locate data files on disk. (The 1130 and 1620 monitors did create a disk file structure but did not provide facilities for application interpretation of that structure. Either application coding or application runtime subroutines were needed.)
A relatively simple system generation process, compared to its predecessors, that needed to be performed relatively infrequently.