IBM i

IBM i (the "i" standing for "integrated") is an operating system developed by IBM for IBM Power Systems. It was originally released in 1988 as OS/400, alongside the IBM AS/400 line of systems. It was renamed to i5/OS in 2004, before being renamed a second time to IBM i in 2008. It is an evolution of the System/38 CPF operating system, with compatibility layers for System/36 SSP and AIX applications. It inherits a number of distinctive features from the System/38 platform, including the Machine Interface, the implementation of object-based addressing on top of a single-level store, and the tight integration of a relational database into the operating system.
History.
Fort Knox and Silverlake.
In the early 1980s, IBM management became concerned that IBM's large number of incompatible midrange computer systems were hurting the company's competitiveness, particularly against Digital Equipment Corporation's VAX. In 1982, a project named "Fort Knox" commenced, which was intended to consolidate the System/36, the System/38, the IBM 8100, the Series/1 and the IBM 4300 series into a single product line based around the IBM 801 processor, while retaining backwards compatibility with all the systems it was intended to replace. This project proved to be overly ambitious, and ran into multiple delays and changes of scope before being cancelled in 1985.
During this time, a skunkworks project was started at IBM Rochester by engineers who believed that Fort Knox's failure was inevitable. These engineers developed code which allowed System/36 applications to run on top of the System/38, and when Fort Knox was cancelled, their project was given official approval in December 1985. The project became known as "Silverlake" (named for Silver Lake in Rochester, Minnesota). Silverlake's goal was to deliver a replacement for the System/36 and System/38 in as short of a timeframe as possible, as the Fort Knox project had stalled new product development at Rochester, leaving IBM without a competitive midrange system.
The Silverlake system built upon IBM Rochester's attempts to consolidate the System/38 and System/36, and was essentially an evolution of the System/38 which reused some of the hardware and software developed for the Fort Knox project. The operating system for Silverlake was codenamed XPF (Extended CPF), and had originally begun as a port of CPF to the Fort Knox hardware. In addition to adding support for System/36 applications, some of the user interface and ease-of-use features from the System/36 were carried over to the new operating system.
Silverlake was available for field test in June 1988, and was officially announced in August of that year. By that point, it had been renamed to the Application System/400, and the operating system had been named Operating System/400.
The move to PowerPC.
In 1990, IBM Rochester began work to replace the AS/400's original System/38-derived 48-bit CISC processors with a 96-bit architecture known as "C-RISC" (Commercial RISC). Rather than being a clean-slate design, C-RISC would have added RISC-style instructions to the AS/400's processor, while maintaining backwards compatibility with the System/370-style "Internal Microprogrammed Interface" (IMPI) instruction set and the microcode used to implement it. In 1991, at the insistence of IBM president Jack Kuehler, a team at IBM Rochester under the leadership of Frank Soltis delivered a proposal to adapt the 64-bit PowerPC architecture to support the needs of the AS/400 platform. Their derivative of the PowerPC, known "Amazon" and later as IBM RS64, was approved by IBM management instead of the C-RISC design, and formed the basis of the RISC AS/400 hardware.
The port to PowerPC required a rewrite of most of the code below the TIMI. Early versions of OS/400 inherited the Horizontal and Vertical Microcode layers of the System/38, although they were renamed to the "Horizontal Licensed Internal Code" (HLIC) and "Vertical Licensed Internal Code" (VLIC) respectively. The port to the new hardware replaced IMPI and the associated microcode, which required the VLIC to be rewritten to target PowerPC instead of IMPI, and for the operating system functionality previously implemented in the HLIC microcode to be re-implemented elsewhere. This lead to the HLIC and VLIC being replaced with a single layer named the "System Licensed Internal Code" (SLIC). The SLIC was implemented in an object oriented style with over 2 million lines of C++ code, replacing a mixture of PL/MP, Modula-2, C, IMPI assembly code, and processor microcode. Due to the amount of work needed to implement the SLIC, IBM Rochester hired several hundred C++ programmers for the project, who worked on the SLIC in parallel to new revisions of the VLIC for the CISC AS/400 systems. The first release of OS/400 to support PowerPC-based hardware was V3R6.
Rebranding.
The AS/400 product line was rebranded multiple times throughout the 1990s and 2000s. As part of the 2004 rebranding to "eServer i5", OS/400 was renamed to "i5/OS"; the "5" signifying the use of POWER5 processors. The first release of i5/OS, V5R3, was described by IBM as "a different name for the same operating system".
In 2006, IBM rebranded the AS/400 line one last time to "System i". In April 2008, IBM consolidated the System i with the System p platform to create IBM Power Systems. At the same time, i5/OS was renamed to "IBM i", in order to remove the association with POWER5 processors. The two most recent versions of the operating system at that time, which had been released as i5/OS V5R4 and V6R1, were renamed to IBM i 5.4 and 6.1.
Along with the rebranding to IBM i, IBM changed the versioning nomenclature for the operating system. Prior releases used a "Version, Release, Modification" scheme, e.g. V2R1M1. This was replaced with a "Version.Release" scheme, e.g. 6.1. Beginning with IBM i 7.1, IBM replaced the Modification releases with "Technology Refreshes". Technology Refreshes are delivered as optional PTFs for specific releases of the operating system which add new functionality or hardware support to the operating system.
Architecture.
IBM i is split into two layers, the hardware-dependent "System Licensed Internal Code" (SLIC) and the hardware-independent "Extended Control Program Facility" (XPF). These are divided by a hardware abstraction layer called the "Technology Independent Machine Interface" (TIMI). IBM often uses different names for the TIMI, SLIC and XPF in documentation and marketing materials, for example, the IBM i 7.4 documentation refers to them as the "IBM i Machine Interface", "IBM i Licensed Internal Code" and "IBM i Operating System" respectively.
TIMI.
The TIMI isolates users and applications from the underlying hardware. This isolation is more thorough than the hardware abstractions of other operating systems, and includes abstracting the instruction set architecture of the processor, the size of the address space and the specifics of I/O and persistence. This is accomplished through two interrelated mechanisms:
The TIMI is a backwards-compatible extension of the System/38 Machine interface, and allows IBM i applications to be completely independent of the underlying hardware. The hardware isolation provided by the TIMI allowed IBM to replace the AS/400's 48-bit IMPI architecture with the 64-bit RS64 architecture in 1995. Applications compiled on systems using the IMPI instruction set could run on top of the newer RS64 systems without any code changes, recompilation or emulation, while also allowing those applications to avail of 64-bit addressing.
SLIC.
The SLIC consists of the code which implements the TIMI on top of the IBM Power architecture. In addition to containing most of the functionality typically associated with an operating system kernel, it is responsible for translating TIMI instructions into machine code, and it also implements some high level functionality which is exposed through the TIMI, such as IBM i's integrated relational database. The SLIC implements IBM i's object-based storage model on top of a single-level store addressing scheme, which does not distinguish between primary and secondary storage, and instead manages all types of storage in a single virtual address space. The SLIC is written in C++ and assembly code, and replaced the HLIC and VLIC layers used in versions of OS/400 prior to V3R6.
XPF.
The XPF consists of the code which implements the hardware-independent components of the operating system, which are compiled into TIMI instructions. Components of the XPF include the user interface, the Control Language, data management and query utilities, development tools and system management utilities. The XPF also contains the "System/36 Environment" and "System/38 Environment", which provide backwards compatibility APIs and utilities for applications and data migrated from SSP and CPF systems. The XPF is IBM's internal name for this layer, and as the name suggests, began as an evolution of the System/38 Control Program Facility. The XPF is mostly implemented in PL/MI, although other languages are also used.
PASE.
The "Portable Application Solutions Environment" (originally known as the "Private Address Space Environment") allows IBM AIX applications to be ported to IBM i.
PASE for i provides an AIX runtime environment on the IBM i, including three shells (ksh, sh, and csh) and standard Unix utilities, allowing many AIX applications to run on the IBM i operating system with little or no change. AIX programs are binary compatible with IBM i when using PASE. Many AIX binaries which do not make direct system calls to the AIX kernel may be executed without modification or recompilation in the PASE environment. PASE programs can be built on IBM i systems using the GNU Compiler Collection (GCC), or alternatively with the IBM XL C/C++ compilers. Support is provided for running both 32-bit and 64-bit AIX executables.
IBM i also provides Qshell, which is a Unix-like layer built on top of IBM i, which does not rely on PASE.
Features.
Database management.
IBM i features an integrated relational database currently known as IBM Db2 for IBM i. The database evolved from the non-relational System/38 database, gaining support for the relational model and SQL. The database originally had no name, instead it was described simply as "data base support". It was given the name "DB2/400" in 1994 to indicate comparable functionality to IBM's other commercial databases. Despite the Db2 branding, Db2 for IBM i is an entirely separate codebase to Db2 on other platforms, and is tightly integrated into the SLIC layer of IBM i as opposed to being an optional product.
IBM i provides two mechanisms for accessing the integrated database - the so-called "native interface", which is based on the database access model of the System/38, and SQL. The native interface consists of the "Data Description Specifications" (DDS) language, which is used to define schemas and the codice_1 command or codice_2 query API. Certain Db2 for i features such as object-relational database management require SQL and cannot be accessed through the native interface. IBM i has two separate query optimizers known as the "Classic Query Engine" (CQE) and "SQL Query Engine" (SQE). These are implemented inside the SLIC alongside a "Query Dispatcher" which selects the appropriate optimizer depending on the type of the query. Remote access through the native interface and SQL is provided by the Distributed Data Management Architecture (DDM) and Distributed Relational Database Architecture respectively.
A storage engine for MySQL and MariaDB named codice_3 allows applications designed for those databases to use Db2 for i as a backing store. Other open source databases have been ported to IBM i, including PostgreSQL, MongoDB and Redis. These databases run on the PASE environment, and are independent of the operating system's integrated database features.
Networking.
IBM i supports TCP/IP networking in addition to the proprietary IBM System Network Architecture.
IBM i systems were historically accessed and managed through IBM 5250 terminals attached to the system with twinax cabling. With the decline of dedicated terminal hardware, modern IBM i systems are typically accessed through 5250 terminal emulators. IBM provides two terminal emulator products for IBM i:
In addition, IBM provides a web-based management console and performance analysis product named IBM Navigator for i.
Open-source.
Some of the open source applications ported to IBM i include:
Open source software for IBM i is typically packaged using the RPM package format, and installed with the YUM package manager. YUM and RPM replaced the 5733-OPS product, which was previously used to install open source software on IBM i.
Programming.
Programming languages available from IBM for IBM i include RPG, Control Language, C, C++, Pascal, Java, EGL, Smalltalk, COBOL, BASIC, PL/I and REXX. The Integrated Language Environment (ILE) allows programs from ILE compatible languages (C, C++, COBOL, RPG, and CL), to be bound into the same executable and call procedures written in any of the other ILE languages.
IBM systems may also come with programming and development software such as the Programming Development Manager. IBM provides an Eclipse-based integrated development environment for IBM i named IBM Rational Developer for i.
IBM i uses EBCDIC as the default character encoding, but also provides support for ASCII, UCS-2 and UTF-16.
Storage.
In IBM i, disk drives may be grouped into an "auxiliary storage pool" (ASP) in order to organize data to limit the impact of storage-device failures and to reduce recovery time. If a disk failure occurs, only the data in the pool containing the failed unit needs to be recovered. ASPs may also be used to improve performance by isolating objects with similar performance characteristics, for example journal receivers, in their own pool.
By default, all disk drives are assigned to pool 1. The concept of IBM i pools is similar to the Unix/Linux concept of volume groups; however, with IBM i it is typical for all disk drives to be assigned to a single ASP.