Technical details DOS/360 and successors

1 technical details

1.1 transients
1.2 multiprogramming
1.3 program libraries
1.4 utilities
1.5 telecommunications

technical details

the following description applies dos/360 except otherwise noted. later versions offer additional functionality.

because dos/360 designed run on low-end models of system/360 memory usage concern. possible generate dos supervisor, resident portion of operating system, small 5902 bytes. detailed charts listed memory requirements each sysgen option, little 100 bytes. minimum system leave on 10kb of storage available single batch partition enough run utilities , compilers except cobol, pl/i, , full fortran iv. keep memory usage small possible, dos coded entirely in assembly language.

transients

to further reduce memory usage supervisor employed overlays called transients read 1 of 2 reserved transient areas required. 2 types of transient routines, called physical transients , logical transients handled hardware exceptions , provided program services such open/close respectively. transient module names began $$a or $$b sped loading storing names first in directory. referred a-transients or b-transients.

multiprogramming

like system/360 operating software initial releases of dos run 1 program @ time. later versions of real dos able run 3 programs concurrently, in separate memory partitions, supported same hardware memory protection features of more scalable os/360 operating system. these identified bg (background), f1 (foreground 1) , f2 (foreground 2). multiprogramming optional feature of dos/360, selectable @ system generation. sysgen option allowed batch operation run in either fg partition. otherwise foreground programs had manually started computer operator.

dos-vs allowed 7 concurrent programs, although 5 or 6 more common number due smaller scale of hardware hosting dos systems. both dos , dos-vs allow number of partitions set @ ipl (initial program load), ibm term boot load.

program libraries

executable programs stored in core image library. while running, dos not reclaim space programs deleted or replaced newer versions. when core image library became full, had compressed 1 of utility programs, , halt development work as half day. many shops froze changes day, compressed cil off-line , , ipled new core image library @ beginning of business day. relocatable library linkable object programs , source statement library assembler macros , include text supported. installations define additional private relocatable , source statement libraries on other disk volumes.

utilities

dos/360 had set of utility programs, assembler, , compilers fortran, cobol , pl/i. , supported useful range of file organizations access methods in using them:

sequential data sets read or written, 1 record block @ time beginning end.
in indexed (isam) files specified section of each record defined key used specific records.
in direct access (bdam) files, application program had specify physical location on disk of data wanted access. bdam programming not easy , customers never used themselves; fastest way access data on disks , many software companies used in products, database management systems such adabas, idms , ibm s dl/i.

sequential , isam files store either fixed-length or variable-length records, , types occupy more 1 disk volume.

telecommunications

dos/360 offered btam, data communications facility primitive , hard use later standards. however, btam communicate type of terminal, big advantage @ time when there little standardization of communications protocols. simplicity of api allowed relatively easy interface of external communications processors, facilitated dos/360 machines becoming nodes in multi-tier networks of large organizations.