OS/360 on Hercules: Overview

1. Overview

1.1 What is Hercules?

Hercules is an emulator for the IBM System/370, ESA/390, and z/Architecture mainframes. The goal of the project is to build an emulator that runs on Linux, Windows, and any other Unix-style OS, capable of running any IBM operating system and program. Check out the Hercules home page for all the details, and join the Hercules mailing list at Yahoo! Groups to get in on the discussion of the system. There's also a group, hercules-os360, dedicated to discussion of OS/360 on Hercules. Updates to this document will be announced there.

1.2 What is OS/360?

OS/360 is the operating system IBM intended all along to be what people ran on their System/360 mainframe line. It's a famous case of software taking much, much longer to deliver than anyone had planned or hoped, and was quite buggy when it was first released. Fred Brooks was a manager at IBM on the OS/360 project, and his classic work on software engineering, The Mythical Man-Month, is based on his experiences with that project.

OS/360 came in three flavors: PCP, MFT, and MVT. PCP (Primary Control Program) was the simplest, capable of only running one task at a time. MFT (Multiprogramming with a Fixed number of Tasks) could, as the name implies, run several things simultaneously, but the number was fixed by a limit set when the system was built. MVT (Multiprogramming with a Variable number of Tasks) removed the build-time restriction on how much could be run at once; instead, the limit was how much could be squeezed into the host computer's memory and CPU resources. MVT is the most capable of the three flavors, and the one we'll be getting to run.

OS/360 lives on even today, greatly enhanced and improved, as z/OS. Over the years, it has been transformed to the point that very little to none of the actual OS/360 code still is used, but the basic design is still very much intact, and many of the features of MVT will be readily recognizable to an MVS, OS/390, or z/OS systems programmer. This is especially true of the HASP job control and spooling system, which is the forerunner of today's JES2.

1.3 What are we trying to accomplish?

The goal here is to have a running OS/360 MVT system, with as many of the features as can be made to work. In particular, TSO (Time Sharing Option, the interactive terminal subsystem), and the FORTRAN, PL/1, COBOL, ALGOL, and RPG compilers are included. Since MVT doesn't have virtual storage, and since Hercules is a software-based emulator, it won't handle a really huge workload, but it's definitely adequate for systems hacking and demonstration.

1.4 Overview of the process

Getting from the raw source materials to a running system is straightforward, but somewhat involved. The Hercules emulator is installed. A small MFT system, referred to as the "driving system", is constructed, and used to build the actual MVT "target system". This is done in two steps: stage 1, which takes a machine-readable description of the target system's hardware and software and builds the jobs necessary to construct it, and stage 2, which uses the output of stage 1 to do the actual work. After stage 2 is complete, a small amount of final preparation is done, and the target system is IPLed and run. HASP and TSO/TCAM are then loaded and generated. At this point, the system is ready to do work.

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Jay Maynard, jmaynard@conmicro.cx

Last updated 21 February 2003