Editing IBM AS/400 (section)

== System architecture ==
According to [[Frank Soltis]], one of the architects of the AS/400 platform, the AS/400's architecture is defined by five architectural principles. Most of these principles are inherited from System/38.{{sfnp|Soltis|2001|p=1–5, 97–177}}

=== Technology Independence ===
[[File:IBM AS-400e Model 150 (cropped).jpg |upright=0.6|thumb|right|IBM AS/400e Model 150.]]

The high-level [[instruction set]] (called TIMI for "Technology Independent Machine Interface" by IBM) allows [[application program]]s to take advantage of advances in hardware and software without recompilation. TIMI is a [[p-code machine|virtual instruction set]] independent of the underlying machine instruction set of the CPU. User-mode programs contain both TIMI instructions and the machine instructions of the CPU, thus ensuring hardware independence. This is conceptually somewhat similar to the [[virtual machine]] architecture of programming environments such as [[Java (programming language)|Java]] and [[.NET Framework|.NET]].

Unlike some other virtual-machine architectures in which the virtual instructions are interpreted at [[Run time (program lifecycle phase)|run time]], TIMI instructions are never interpreted. They constitute an intermediate [[compile time]] step and are [[binary translation|translated into the processor's instruction set]] as the final compilation step. The TIMI instructions are stored within the final program object, in addition to the executable machine instructions. This is how application objects compiled on one processor family (e.g., the original [[Complex instruction set computer|CISC]] AS/400 48-bit processors) could be moved to a new processor (e.g., [[PowerPC]] 64-bit) without re-compilation. An application saved from the older 48-bit platform can simply be restored onto the new 64-bit platform where the operating system discards the old machine instructions and re-translates the TIMI instructions into 64-bit instructions for the new processor.

The system's instruction set defines all pointers as 128-bit. This was the original design feature of the [[IBM System/38|System/38]] (S/38) in the mid-1970s planning for future use of faster processors, memory, and an expanded address space. The original AS/400 CISC models used the same 48-bit address space as the S/38. The address space was expanded in 1995 when the [[RISC]] [[PowerPC]] [[RS64]] 64-bit CPU processor replaced the 48-bit CISC processor.

=== Software integration ===
{{Main|IBM i}}

OS/400 (now known as IBM i) is the native operating system of the AS/400 platform and was the sole operating system supported on the original AS/400 hardware. Many of the advanced features associated with the AS/400 are implemented in the operating system as opposed to the underlying hardware, which changed significantly throughout the life of the AS/400 platform. Features include a [[Relational database management system|RDBMS]] ([[Db2 for i]]), a menu-driven interface, support for multiple users, [[block-oriented terminal]] support ([[IBM 5250]]), and printers.

=== Object-based design ===
Unlike the "[[everything is a file]]" principle of [[Unix]] and [[Unix-like|its derivatives]], on IBM i everything is an object (with built-in persistence and garbage collection).{{citation needed|date=January 2024}}

=== Single-level store ===
IBM uses a [[single-level store]] virtual memory architecture in the AS/400 platform. For 64-bit PowerPC processors, the virtual address resides in the rightmost 64 bits of a pointer while it was 48 bits in the S/38 and CISC AS/400. The 64-bit address space references main memory and disk as a single address set, which is the single-level store concept.

=== Hardware integration ===
Later generations of hardware are also capable of supporting various guest operating systems, including [[System Support Program|SSP]], [[IBM AIX|AIX]], [[Linux]], [[Microsoft]] [[Windows 2000]], and [[Windows Server 2003]]. While OS/400, AIX, and Linux are supported on the POWER processors on [[Logical partition (virtual computing platform)|LPARs (logical partitions)]], Windows is supported with either single-processor internal blade servers (IXS) or externally linked multiple-processor servers (IXA and iSCSI). SSP guests were supported using emulation from OS/400 V3R6 through V4R4 using the Advanced 36 Machine facility of the operating system, a feature distinct from the System/36 Environment compatibility layer, which requires System/36 software to be recompiled.