Editing IBM AS/400 (section)

=== 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.