Editing XPL (section)

===XMON===
XPL is run under the control of a monitor, '''XMON''', which is the only operating system-specific part of this system, and which acts as a "loader" for XCOM itself or any programs which were developed using XCOM, and also provides three auxiliary storage devices for XCOM's use, and which are directly accessed by block number. The originally published XMON was optimized for [[History of IBM magnetic disk drives#IBM 2311|IBM 2311]]s.  An XMON parameter FILE= enabled the monitor to efficiently use other disks with larger block sizes.<ref>A Compiler Generator page 251</ref>  The working disk block size was also a compile-time constant in
XCOM.<ref>A Compiler Generator, page 372</ref>

XMON used a very simple strategy for disk direct access.  NOTE provided the address of a disk track.  POINT set the location of the next disk track to be the address previously returned by NOTE. This strategy was adopted to allow easy porting of XMON to other OSes, and to avoid the much more complicated disk direct access options available at that time.<ref>A Compiler Generator Appendix A1,7</ref>

Converting XMON from its primitive use of NOTE, POINT and READ/WRITE disk operations—with precisely 1 block per track—to [[EXCP]] (i.e., write/create new records) and [[Execute Direct Access Program|XDAP]] (i.e., read/update old records)—with n blocks per track, where n was computed at run-time from the target device's physical characteristics and could be significantly greater than 1—achieved significantly improved application performance and decreased operating system overhead.

Although originally developed for [[OS/360]], XMON (either the original NOTE, POINT and READ/WRITE implementation; or the EXCP and XDAP enhancement) will run on subsequently released IBM OSes, including OS/370, XA, [[OS/390]] and [[z/OS]], generally without any changes.