NOSS

NAS Operational Support System

The IBM 9020 ran the National Airspace System (NAS) program operationally. NAS consisted of the NAS Monitor (derived from OS/360 and written in BAL) and the ATC Application programs (written in JOVIAL). NAS is still in use today, though the S/360s are long gone!

This page is about NOSS – the NAS Operational Support System – which was used to maintain the NAS program. NOSS was developed in the 1960s and consists of a Monitor and various utility subprograms, the two main ones being a JOVIAL complier and a BAL assembler, called BALASM. This is regular BAL with the addition of the special 9020 instructions.

The original NAS program ran on a tape-only S/360 Model 50 complex. The Model 50s were selected by the FAA before the new S/360 family was announced to the public in 1964 (IBM had shared, in confidence, basic details of S/360 the previous year). Development of NOSS ran in parallel to the development of IBM’s mainstream Operating Systems. As the first 9020s were tape-only, NOSS is probably very similar to TOS/360 (Tape Operating System/360).

Disks were later added to all 9020s and many Model 50 complexes were upgraded to use Model 65s as the main processor. With the advent of disks, the support OS evolved into OS/9020, almost identical to OS/360, but NOSS was retained as it could run in a tape-only subsystem. This was useful because an offline sub-system without disks could be configured out of an operational triplex system.

NAS software development took place at the FAA's NAFEC facility, where there were three separate 9020 complexes (A, D, and E versions). As NAFEC was non-operational a whole system could be scheduled to run the OS/9020 support system without any regard to redundancy, etc. However, certain NAS support functions had to be run at the operational centres. An example is the Prefiled Flight Plan Assembly utility, which created a tape of repetitive flight plans (unique at each centre) for subsequent input to the operational system. The tape-only NOSS could be used for this as it didn't require disks.

There were three tape sub-systems, any one of which could be configured into the offline system without significantly impacting the operational system’s redundancy. [This is a slight over-simplification as an offline sub-system only had access to two tape channels - see below. Exactly which two channels depended on which one of the three I/O Control Element was configured into the offline system. Note that only one of the two accessible tape controllers would be actually configured into the offline system via the special 9020 hardware Configuration Control Register that every major element contained.]