About The AS400 is a business computer from IBM. It is classed as a medium mainframe. The AS400 is also known as iSeries, System i, i5 and Power Systems.
Batch Jobs Batch jobs run in the background, i.e. there is not a screen interface with batch jobs. They typically perform long-running data processing tasks. They can also be waiting for messages, such as polling a queue. There are system batch jobs as well as user batch jobs. Batch jobs can be scheduled to run at particular times. Batch jobs are similar to cron jobs in a unix system.
Commands There are many commands to carry out operations on an AS400. Commands can be system commands or can be created by users. Most commands take parameters. Parameters are known as keyed parameters in that each parameter is identified by a key word, such as file(…). Apart from some simple commands, such as CALL, the majority commands are composites of verb-adjective-noun or verb-noun. An example is WRKACTJOB, for WORK with ACTIVE JOBS or CRTPF for CREATE PHYSICAL FILE.
Communication One of the strengths of the AS400 is how it communicates with networks and other computers. It is fully TCP/IP compatible.
Devices Any peripheral is regarded as a device on the AS400. This includes screens, printers and tape drives. Any number of peripherals can be added to an AS400, but each has to be configured. Each AS400 comes with default configurations for many types of device. Screens do not use the AS400 main memory. Instead, they use up memory on the PC or laptop. Communication between a screen and the AS400 is by way of a character string consisting of control characters and display characters. AS400 does not print directly to a printer. All printed output is stored in print queues on the AS400. Not all print queues are attached to a printer. Every device or peripheral has a label or name on the AS400.
Features The AS400 comes with its own operating system, OS/400, and its own database, DB2. DB2 is fully compatible with SQL. The programming languages which come with the AS400 are RPG, Cobol and C. Compilers for other languages are available.
File System The file system uses physical files for storing data. Physical files can be indexed. A physical file can have one or more logical files related to it. The main use of a logical file is to provide a different sequence by way of different indexes. An index consists of one or more fields on the physical file. Files exist in libraries, which are similar to a directories of folders in Windows and Unix. However, libraries cannot be nested as in Windows or Unix. The base library in AS400, or root directory is always QSYS. (All system objects start with the letter q on AS400s). All other libraries exist in QSYS. Libraries can be system libraries or user libraries. Libraries are frequently grouped into library lists for calling programs and running jobs.
Interactive Jobs An interactive job is where the user is working with the AS400.
Job Description A job description provides run values for a job. All jobs have a default job description. Specific job descriptions can be created for various groups of job. A job description provides information such as a library list to be used or a printer queue.
Jobs All work on an AS400 is performed as jobs. A job has a name, a number and an owner. The owner is the user profile initiating the job. When you sign on to an AS400, a job is created for your session. This is an interactive job. Jobs can be batch as well as interactive.
History The AS400 has its roots in the old System 3 family of computers from IBM. After System 3 there was the System 34 and System 38, and then the S36.
Memory Memory on the AS400 is divided into pools. Base memory is where most of the work takes place. There are then pools for interactive jobs and for various batch jobs and subsystems.
Messages The AS400 communicates with users by means of messages. There are many kinds of messages. All are routed through message queues.
Objects All programs, files, queues etc are regarded as objects on an AS400. A library is a collection of objects. An object can exist in only one library at a time, unlike in Unix.
Operating System The operating system uses commands and menus. Much of the operating system can be managed from sets of menus. The commands offer more flexibility. A few of the commands are commonplace, e.g. CALL to call a program or NETSTAT to look at network statistics. But the majority of the commands are made up of verb-object-action combinations and use 3-letter mnemonics. For example, many of commands to display information about an object start with DSP. To display an object description, the command is DSPOBJD. Another common verb is WRK, to work with, e.g. WRKACTJOB to work with active jobs.
Queues Queues are used in many areas of the AS400. There are message queues, printer (aka spool) queues, job queues, data queues to name but a few. This is not to be confused with MQSeries however.
Security Each person who can sign onto an AS400 typically has a user name and a password. The user name is also the name of their profile. The profile defines to some extent what they can do on the computer. It gives their level of security, from security officer - the highest - to user - the lowest. A number of profiles can be placed under a group profile.
Subsystems The AS400 divides all work into subsystems. The two main subsystems are QINTER and QBATCH where much of the user jobs execute. There are other system subsystems such as QSYSWRK and QCTL. All work is directed into the subsystems via job queues.
System Values Values such as date format and password rules are stored as system values.
Unix The AS400 comes with a Unix partition or shell, the Q shell.
User Interface As already noted, the operating system utilises commands and menus to control and manage the AS400. All of these are text based. There are no icons on the AS400. Indeed, the AS400 does not have its own screens, it piggy-backs onto a standard Windows or Unix personal computer or laptop. (It is possible to get dedicated terminals for AS400, but these are rare nowadays). The AS400 screen is colloquially known as a "green screen" because the default colours are a light green font on a very dark green, almost black background. The size of the standard screen is 80 columns by 24 rows. Some people like to use a 132 column by 27 rows. These are the only two sizes available. This means that all the characters on the screen are the same size. There are eight colours available are red, green, black, turquoise, yellow, blue, white and pink. The other display font characteristics are non-display, highlight, underline and blink.
User Profile Every person who can sign onto an AS400 has a user profile. The user profile has a name and typically a password. Every job is run by a user profile.