IBM AS/400e User Handbook Manual page 45

AS/400 System Architecture: Underlying Strength of the AS/400e
from changing hardware characteristics. When a different hardware technology is deployed,
IBM rewrites sections of the microcode to absorb the fluctuations in hardware characteristics.
As a result, the interface presented to the customer remains the same .
Program
SLIC
64-bit RISC
Hardware
This interface is known as the Technology-Independent Machine Interface (TIMI). The
microcode layer is known as the System Licensed Internal Code (SLIC). The brilliance of this
design was dramatically illustrated when the AS/400 system changed its processor
technology from Complex Instruction Set Computing (CISC) processors to 64-bit Reduced
Instruction Set Computing (RISC) processors in 1995. With any other system, the move from
CISC to RISC would involve recompiling (and possibly some rewriting) of programs. Even
then, the programs would run in 32-bit mode on the newer 64-bit hardware. This is not so with
the AS/400 system because of TIMI. Customers were able to save programs off their CISC
AS/400 systems, restore them on their new RISC AS/400e servers, and the programs would
run . Not only did they run, but they were fully 64-bit programs.
As soon as they made this transition, customers had 64-bit application programs that ran on a
64-bit operating system containing a 64-bit relational database that fully exploited the 64-bit
RISC hardware . TIMI and SLIC take 64-bit RISC processor technology in stride. These same
architectural features will be exploited to fully accommodate post-RISC technologies, which
may have 96-bit or 128-bit processors.
Many of the frequently-executed routines have been moved to the SLIC. On a customary
system, these routines reside in the operating system. Because the SLIC is closer to the
silicon, routines placed there run faster than routines placed "higher" in the machine. There is
a valuable performance gain. Supervisory resource management functions in SLIC include
validity and authorization checks.
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AS/400 System Architecture: Underlying Strength of the AS/400e