The iSeries is a medium mainframe from IBM. It is aimed at the business market. Since it was launched in 1988, it has been regularly updated and, unfortunately, regularly renamed. The term AS400 now stands colloquially for this family for computers.
The AS400 has a character based interface. In other words, it does not use icons like an Apple or Windows user interface, but instead uses menus and commands. Command keys are also used extensively. The AS400 is a multi-user computer.
Folders, or directories, are called libraries on the AS400. Libraries are not nested. There is a base or home library, called QSYS. All libraries exist in QSYS. No other library can contain a library. All AS400 objects begin with the letter Q.
The AS400 file system is ideally designed for SQL processing. Data files can contain text records - flat - or structured. A structured file can be indexed. Files have a fixed record length. A data file can have more than one index. An index can join two or more data files. Data files are called physical files, and index files are called logical files.
The AS400 has menus, commands, scripts and compiled programs. The operating system is called OS/400. The scripting language is called CL - Control Language. Scripts can be compiled. Scripts and source programs are stored in source files. The AS400 comes with its own source editor and compilers.
The most common programming language is RPG, now called RPG-ILE. This language was designed by IBM for its medium mainframe family. Other compilers provided include COBOL and C.
The strengths of the AS400 are its ease of use, connectivity and the database system. It is fully coompatible with both IPv4 and IPv6.
A command name is typically made up of verb-adjective-object, for example, WRKACTJOB which stands for work(with)-active-jobs. A large number of the commands are "work with" commands. Other commands start with DSP for display. The command CALL is used to call or execute a program or script.
Each user on the AS400 has a profile, which contains the user(s name and a password with other information about the user. The profile is equivalent to account in some platforms. The AS400 comes with a number of pre-defined profiles, such as security officer, programmer, user. These are called QSECOFR, QPGMR and QUSER respectively. Each installation will create profiles to suit their own users and method of working.
Security on the AS400 uses a hierarchical plan. There is a Security Officer who has full authority to all objects. The Security Officer assigns security to all users. There are different levels of security, i.e. different users can do more or less depending on what has been allocated to them. As well as individual profiles, there are also group profiles. A group profile does not sign on as a user, but users assume the characteristics of the group profile they are assigned to.
Typically, the AS400 does not have its own screens or keyboards. In the majority of cases, the AS400 uses a standard PC as its screen and keyboard. There is a piece of software which runs on the PC to provide the interface. The user uses a standard icon to connect to the AS400.
Work on the AS400 is organised into jobs. Each job is known by a job number, a job name and the user who is running the job.
Jobs on the AS400 runs in subsystems. Each subsystem can have one or more queues. There are two basic types of subsystem, the interactive and the batch. The interactive subsystem is where users do their work. The standard one provided by IBM is QINTER. The standard batch subsystem is QBATCH. Many AS400s in use today have multiple batch subsystems and usually only one interactive subsystem. There are also many background subsystems such as QSYSWRK. Each subsystem can have one or more jobs (programs executing in it).
Each job has a job description. The default job description is QDFTJOBD. This, among other properties, sets the library list for the job. A library list is the list of libraries that the system will search for the objects required by the job. It is used for any object which is not defined as being in a particular library.
All printed output goes to a print queue. Again, a number of queues are provided. Each printer is attached to a queue. There are queues which do not have a printer attached. These are used to storing printouts for future printing or retained as a matter of record, such as job logs.
Printers and screens (display units) are devices and are external to the AS400. The AS400 has a set of standard device descriptions or types and anyone of these can be applied to a particular physical printer or screen. The devices are expect to have their own intelligence and memory to use the pint / display data and control data. Each device can be named separately and its description can be configured separately.
The System Values contains pre-defined values such as date and time, time zone etc. in QSYSVAL.