1972: Introduction to VM/370

Thanks, early computer systems imposed a difficult problem on their users due to the relatively small size of their primary storage. Virtual storage techniques have substantially reduced this problem with the implementation of a single large address space of up to 16 megabytes of virtual storage size. The address space appears real to the user, although in reality it is possible through tables and translation of hardware and software that the user is unaware of. The single virtual storage approach has many advantages, such as more efficient use of real storage. Easier job scheduling. Simpler program design and improved multiple functions. -Programming operations, the virtual storage technique is even more powerful than this, however, because with some additional functions, a system control program can be extended to support multiple virtual storages, all implemented with a single main storage.

With this approach, each user of the system is either a batch multiple scheduler. work, an online data communication system or a programmer using interactive computing facilities has access to an address space, storage devices and a part of the CPU, all controlled by the virtual storage control program, in a sense , in a multiple virtual storage environment, each user has access to their own machine and if that is the case, why not allow one user to load and run an operating system on this machine, such as the disk operating system? Because there is a unique but important problem in this environment: the user's operating system would not have access to the system 370 privileged state that is necessary to exercise control over the computer's basic resources, for example, as soon as the user's operating system tried to issue the machine's start i o instruction, it would fail.

The use of the start i o statement is restricted to the actual control program running in the privileged state of the system 370 now this is a shame because there are many functions that could be performed in an environment where an individual user has access to their own system operating system, for example, could test complex applications, generate an operating system or develop programs with complete flexibility in programming and without interfering with anyone else's work and it is precisely this type of capability that is provided by the operating system called vm 370, the installation of the virtual machine 370. The control program vm 370 allows users to execute privileged instructions in a pseudo-privileged state within their address space that is, the control program intercepts and parses these instructions and executes them for the user.

Therefore, the vm 370 control program can allow any system 370 or system 360 operating system to reside in a virtual address space and appear to run as it would in a dedicated space. The machine, with the exception of elapsed time, vm 370 creates an environment capable of supporting full system operations. We call this a virtual machine. Access to a virtual machine is provided through a terminal, for example, a video display 3270 that is connected to a system 370 running under the vm. 370 control program each terminal connected to system 370 can then provide a separate virtual machine to its user the terminal becomes the console of the virtual machine console operations such as displaying storage are performed on the terminal using vm commands 370 However, to support full system operations a virtual machine must have the same functional components as a real machine, so let's examine how vm 370 implements these virtual machine components.

The storage of a virtual machine is implemented as a virtual address space in the same way it is handled in other virtual storage operating systems. Through a paging device and translation hardware and software, a virtual CPU is provided by giving each virtual machine a share of the real CPU time. Storage devices, such as tape recording drives and direct access storage devices, are implemented in various ways when a virtual machine requires a tape drive, the vm operator 370 assigns an actual tape device to the virtual machine, and Then, during program execution, the VM 370 control program translates this real device address to the address of the virtual tape drive if actual physical intervention is required, as would be the case in this case mounting the user's tape reel. performed by the vm370 operator.

Tape drives are not typically used by a virtual machine, but most virtual machine users require single-track devices, such as card readers and printers, and so it would be impractical to dedicate actual devices to each one. Instead of the virtual machine, unit log operations for virtual machines are supported on VM 370 using print spooling techniques, for example, output for printing is written to a shortcut device and made to the user of the virtual machine as soon as a real printer is available under the virtual shortcut of vm 370. Storage devices are implemented to support all virtual machine requirements. Most user needs are met by assigning one or more minidisks to their virtual machine.

A minidisk in vm370 serves as a virtual disk drive. The minidisk appears as a real disk on the user's virtual computer. machine, it is called an MD because it is generally smaller than a pack of physical disks, a fixed number of cylinders are assigned to each MD depending on the user's needs, most MDs are smaller than a package of physical disks, but If a virtual machine requires it, a minidisk disk can be mapped to an entire disk package, as we said before, the console of each virtual machine is simulated in a terminal with vm 370 commands typed that replace physical operations, such as pressing the IPL button, normal communications from the operator to the system are done in the custom terminal as it would be in the console of a dedicated system, so now that we have a virtual machine, let's see what we can do with it.

Well, we could start by loading the operating system that we have available in our store. This could be a system like the system we uploaded. It can then be used for batch production just as it would be on a dedicated machine, but why do this? We could load the OS directly onto our real machine and avoid the overhead of a somewhat slower operation caused by the addition of the vm370 control program, so this doesn't seem to make much sense, but let's take a closer look at this. The point is that once the vm 370 control program is running, any number of virtual machines can be loaded and used for different purposes, such as batch production, system testing, or program development for each user.

You can perform any testing you want independently of other users, let's look at the program development feature now, while it is possible to use a batch operating system like dos for this purpose, a programmer in a terminal does not need all the facilities provided for said type. a system can be used instead of a control program that specifically supports program development and interactive computing and vm370 provides a system called conversational monitor system or cms instead of loading an operating system like dos or os the user loads cms cms is a control program designed specifically to support interactive conversational programming and data manipulation in a virtual machine, the cms user can invoke any compiler available in the system library, cms provides a conversational compiler interface for development and testing, cms It also provides other facilities appropriate for conversational processing, such as data input manipulation and text processing with CMS, then multiple users can simultaneously perform interactive computing operations on the same system 370.

So far we have seen the operation of multiple virtual machines on a real machine, each virtual machine can be used to satisfy a specific requirement the system programmer is testing a new version of an operating system the application programmer is developing test programs and an operator is running a batch system in production mode The batch operating system we have illustrated so far is dos, however the operating system running on a virtual machine can be one that supports a virtual address space such as dos vs or os vs1, we can even run a system that supports multiple virtual storage, such as os mvs, in one virtual machine these possibilities further enhance the functions that can be performed in a vm 370 environment, for example vm-370, can be used to assist in the conversion of dos or dos vs to os vs1 before conversion.

The new system can be tested in a virtual machine during the conversion. Both systems can run simultaneously on virtual machines as needed. It is even possible to run the vm 370 control program in a virtual machine. This mode of operation is used by IBM development staff when creating a new version of vm 370. A vm 370 user can benefit from this capability by generating and testing a new release of vm 370 on a virtual machine simultaneously with daily operations, As mentioned above, virtual machine operators communicate through terminals, the consoles of their virtual machines that are connected to the VM 370 control program, depending on the requirements of the virtual machine, the terminal may be located in the room of actual machines in the local area or in a completely remote location, for example, the virtual console of a batch production system would probably be located in the machine room of system 370, it is quite possible that the vm 370 operator and the batch machine operator would be On the other hand, the same person, application programmers and word processor users using cms, would probably be using terminals located in their own work area and sometimes this can present some issues.

Consider, for example, the situation where a programmer located a few miles from the actual computer system can examine its output on the display screen of a 3270; However, there are times when you need a printout indicating that the printout should be available at your remote location rather than on the actual system printer 370. and VM 370. has an answer to this problem by providing a network called remote queuing communication subsystem rscs allows input and output operations to be performed for a virtual machine at a remote location a virtual machine can command its virtual drive registration devices to be simulated at a remote location workstation controlled by rscs instead of on the local printer or card reader.

This remote workstation can be a stand-alone drive registration configuration, such as an IBM 3780, or a computer such as an IBM System 3 or other 370 system with connected drive registration devices for rscs use. RSCS can be used to submit jobs from a remote workstation to a batch system running in a virtual machine and then the output of the jobs can be transmitted to a printer on the remote workstation. The batch system can be an operating system such as dos or os or it can be the cms batch system, cms batch makes the scheduling functions of cms available to a workflow.

These jobs can be submitted locally or remotely under the control of rscs. The vm370 rscs subsystem can act as a remote workstation for another operating system. Therefore, a programmer can develop a program under VM 370 and then submit it for execution to a 370 system under the control of an operating system such as OS MVS. This allows you to combine individual systems, each satisfying specific objectives, into a network of systems to meet all your needs. In this

introduction

we have described the functions provided by vm 370 to demonstrate many functions that your users can perform using vm 370. It is possible to back up a production system that is normally running on a real 370 system.

Allows you to use virtual machines to migrate. to a new system you can run cms on virtual machines to support interactive manipulation of computer data and word processing you can test a complex application on a virtual machine without interrupting system operations in case of failure with vm 370 you can perform a generation of the system on a virtual machine machine during main shift operations can perform operating system maintenance on virtual machines can serve local and remote users on a vm 370 network and function as part of a network with other systems 370 operating under programs system control like osmvs which hasseen some of the many possible configurations of the vm 370 system, for example combining a batch multi-programming system like os vs1 with interactive computing activity in cms, you have seen how rscs can be used to support processing on remote workstations and How a vm 370 system itself can be part of an even larger network by supporting the installation of virtual machines, the vm 370 system, especially when combined with another IBM virtual storage operating system, can provide powerful features that benefit many types of computing facilities.