iSeries Architecture: Fundamental Strength of the iSeries 13iSeries Architecture: Fundamental Strength of the iSeriesiSeries ArchitectureTechnology Independent Machine InterfaceThe iSeries servers are atypical in that they are defined by software, not by hardware. Inother words, when a program presents instructions to the machine interface for execution, it“thinks” that the interface is the system hardware. But it is not! The instructions presented tothat interface pass through a layer of microcode before they are “understood” by the hardwareitself.This comprehensive design insulates application programs and their users from changinghardware characteristics. When a different hardware technology is deployed, IBM rewritessections of the microcode to absorb the fluctuations in hardware characteristics. As a result,the interface presentedto the customer remains the same.This interface is known as the iSeries Layer, or Technology Independent Machine Interface(TIMI). The microcode layer is known as the System Licensed Internal Code (SLIC).Many of the frequently-executed routines run inSLIC. Supervisory resource management functionsin SLIC include validity and authorization checks. Ona customary system, these routines reside in theoperating system. Because SLIC is closer to thesilicon, routines performed there are faster thanroutines placed “higher” in the machine.The brilliance of this design was dramaticallyillustrated when the AS/400 system changed itsprocessor technology from Complex Instruction SetComputing (CISC) processors to 64-bit ReducedInstruction Set Computing (RISC) processors in1995. With any other system, the move from CISC toRISC would involve recompiling (and possibly some rewriting) of programs. Even then, theprograms would run in 32-bit mode on the newer 64-bit hardware.This is not so with the iSeries server, because of TIMI. Customers were able tosaveprograms off their CISC AS/400 systems, andrestore them on their RISC AS/400e models.The programs run as 64-bit programs. As soon as they made this transition, customers had64-bit application programs that ran on a64-bit operating system, containing a64-bitrelational database that fully exploited the64-bit RISC hardware.TIMI and SLIC take 64-bit RISC processor technology in stride. These same architecturalfeatures will be exploited to fully accommodate post-RISC technologies, which mayincorporate 96-bit or 128-bit processors.ProgramsTIMISLIC64-bit RISC Hardware
