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, as the sole operating system of 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

Origin

OS/400 was developed alongside the AS/400 hardware platform beginning in December 1985. Development began in the aftermath of the failure of the Fort Knox project, which left IBM without a competitive midrange system. During the Fort Knox project, a skunkworks project was started at Rochester by engineers, who succeeded in developing code which allowed System/36 applications to run on top of the System/38, and when Fort Knox was cancelled, this project evolved into an official project to replace both the System/36 and System/38 with a single new hardware and software platform. The project became known as Silverlake (named for Silver Lake in Rochester, Minnesota).

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

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 led 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 all of the HLIC code, and most of the VLIC code. Owing 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

When IBM i was first released as OS/400, it was 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). Later versions of the operating system gained additional layers, including an AIX compatibility layer named Portable Application Solutions Environment (originally known as the Private Address Space Environment), and the Advanced 36 Machine environment which ran System/36 SSP applications in emulation.

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:

 Compilers for IBM i do not generate native machine code directly, instead they generate a high level intermediate representation defined by the TIMI. When a program is run, the operating system carries out ahead-of-time translation of the TIMI instructions into native machine code for the processor, and stores the generated machine code for future execution of the program. If the translation process changes, or a different CPU instruction set is adopted, the operating system can transparently regenerate the machine code from the TIMI instructions without needing to recompile from source code.
 Instead of operating on memory addresses, TIMI instructions operate on objects. All data in IBM i, such as data files, source code, programs and regions of allocated memory, are encapsulated inside objects managed by the operating system (c.f. the "Everything is a file" model in Unix). IBM i objects have a fixed type, which defines the set of applicable operations which may be carried out on them (for example, a Program object can be executed, but cannot be edited). The object model hides whether data is stored in primary, or secondary storage. Instead, the operating system automatically handles the process of retrieving and then storing the changes to permanent storage.

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.

There are two different formats of TIMI instructions, known as the Original Machine Interface (OMI) and New Machine Interface (NMI) formats. OMI instructions are essentially the same as the System/38 Machine interface instructions, whereas NMI instructions are lower-level, resembling the  W-code intermediate representation format used by IBM's compilers. IBM partially documents the OMI instructions, whereas the NMI instructions are not officially documented. OMI instructions are used by the original AS/400 compilers, whereas NMI instructions are used by the Integrated Language Environment compilers. During the PowerPC port, native support for the OMI format was removed, and replaced with a translator which converted OMI instructions into NMI instructions.

The storing of the TIMI instructions alongside the native machine code instructions is known as observability. In 2008, the release of i5/OS V6R1 (later known as IBM i 6.1) introduced a number of changes to the TIMI layer which caused problems for third-party software which removed observability from the application objects shipped to customers.

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 primarily implemented in C++, 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

PASE provides binary compatibility for user mode AIX executables which do not interact directly with the AIX kernel, and supports the 32-bit and 64-bit AIX Application Binary Interfaces. PASE was first included in a limited and undocumented form in the V4R3 release of OS/400 to support a port of Smalltalk. It was first announced to customers at the time of the V4R5 release, by which time it had gained significant additional functionality.

PASE consists of the AIX userspace running on top of a system call interface implemented by the SLIC. The system call interfaces allows interoperability between PASE and native IBM i applications, for example, PASE applications can access the integrated database, or call native IBM i applications, and vice versa. During the creation of PASE, a new type of single level storage object named a Teraspace was added to the operating system, which allows each PASE process to have a private 1TiB space which is addressed with 64-bit pointers. This was necessary since all IBM i jobs (i.e. processes) typically share the same address space. PASE applications do not use the hardware-independent TIMI instructions, and are instead compiled directly to Power machine code.

PASE is distinct from the Qshell environment, which is an implementation of a Unix shell and associated utilities built on top of IBM i's native POSIX-compatible APIs.

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 OPNQRYF command or QQQQRY 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 IBMDB2I 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 Systems 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:
 IBM i Access Client Solutions is a Java-based client that runs on Linux, macOS and Windows to provide 5250 emulation.
 IBM i Access for Web/Mobile provides web-based 5250 emulation.

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:

 Apache HTTP Server
 Java
 Node.js
 OpenSSL
 Git
 gcc
 nginx
 PHP
 Python
 Ruby
 Lua
 R
 MariaDB
 MySQL
 Perl
 Redis
 MongoDB
 PostgreSQL
 Vim

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. Ports of open source software to IBM i typically target PASE instead of the native IBM i APIs in order to simplify porting.

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.

When PASE was introduced, it was necessary to compile code for PASE on an AIX system. This requirement was removed in OS/400 V5R2 when it became possible to compile code using the IBM XL compiler suite inside PASE itself. Since then, other compilers have been ported to PASE, including gcc.

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.

Security

IBM i was one of the first general-purpose operating systems to attain a C2 security rating from the NSA. Support for C2 level security was first added in the V2R3 release of OS/400.

Release timeline

See also
Comparison of operating systems
Library (IBM i)
Object (IBM i)

References

External links
IBM i site
IBM i Documentation
IT Jungle - IBM i news website  
MC Press Online - IBM Midrange Computer news website

1988 software
AS/400
IBM operating systems
Object-oriented operating systems
Power ISA operating systems