BOS/360

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BOS/360 (Basic Operating System/360) was an early IBM operating system.

BOS was the predecessor to TOS/360, the Tape Operating System, on the System/360 and it was IPL'd from a card reader. It was intended for very small 360s with no disks and limited tape drives.

BOS died out very early as disks such as the IBM 2311 and IBM 2314 became common with the IBM System/360, whereas disks had been a real luxury on the IBM 7090.

Actually, BOS was a very important tool for the very earliest 360s. It came in 2 versions, 4K BOS and 8K BOS. The latter evolved into DOS which some suggest was really version 6 of BOS. BOS in turn was preceded by BPS, Basic Programming Support. The first 360s to be shipped were the models 30 and 40. BOS was the first disk based operating system and was released in early 1965. 4K BOS was for machines with 8 or 16K of core storage and as its name implies, assembled a Supervisor of about 4K. With very few exceptions, all of the early 360s (but not the model 20) shipped with 2, 3 or more 2311 disk drives. Thus BOS was the only disk based operating system for a machine that was marketed as disk based. The Supervisor was IPL'd and the date and time were entered by the operator on the console typewriter.

The operating system disk contained a source language library, a macro library and a core image library. The macro library included all the options for the supervisor, which was assembled by the 360 assembly language compiler, the only language available at the time, although RPG came a little later. The other crucial component was the Job Controller, which was fed by punched cards using JCL (Job Control Language). Most BOS users upgraded to DOS and added more core, typically to 32K or even a gargantuan (for the time) 64K. DOS was the first System/360 operating system to introduce multi-programming. It could be configured with 2 foreground partitions, typically used for spooling while the main job stream ran in the background.

TOS was a failure. It was intended as an OS for 360s replacing tape only 1401s. Each time the Supervisor needed to load a sub routine the tape drive would wind or rewind frantically while the machine remained idle. IBM's experience with disk drives grew through the 1440 which came with 1311 disk drives, the forunner of the 2311. This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.