Section 4.0  Host Computation
The host side of the SkyWriter Host Integrated Compute Image Generator is supported by an
advanced symmetfic multi-processing (SMP) based computational subsystem derived from the
Silicon Graphics POWER Series(tm) project supercomputers.  This subsystem combines parallel
CPUS, memory, secondary storage, and I/O to provide SkyWriter with the ability to calculate
real-time vehicle dynamics, communicate with A-to-D and D-to-A boards and devices, and
access databases in addition to being able to render realistic out-the-window (OTW) scenes.
4.1 CPUs
 
The parallel CPUs are packaged two to a board, and thus must be installed in lots of two.
SkyWriter can support either 2 or 4 parallel CPUs and their corresponding floating point
processors.  All Silicon Graphics POWER Series and SkyWriter systems are based upon the
Mips R3000/R3010 RISC microprocessor and floating point processor.  This means that any
software developed for one machine will be inherently binary compatible with any other
machine in the same product line.  This is true for most all of the SGI product line, including
the Personal IRIS and the new desktop IRIS Indigo.

While the entire product line shares the same processor, the clock rate varies from machine to
machine.  The product line includes Mips R3000 processors running anywhere from 20MHz to
40MHz.  SkyWriter supports both the 33MHz and the 40MHz processors with either 2 or 4
processors.  If desired, these clock rates can even be mixed within a single system, as the main
MPlink bus connecting the CPUs to memory and graphics runs asynchronously to the
processors.
 

4.2 Cache
 
To enhance the performance of the computational subsystem, each MIPS R3000 RISC
processor has its own first and second level data and instruction caches.  There is a significant
difference between the 33MHz and the 40MHz system performance due at least partly to the
fact that while both the 33MHz and.the 40MHz R3000 systems offer 64K instruction and 64K
first level data caches, the 33MHz systems, the RS-4D320IG2 and the RS-4D340IG2, offer a
256K second level data cache while the 4OMHz systems, RS-4D420IG2 and the RS-
4D440IG2, offer a 1MB second level data cache.

4.3 Main Memory
 

SkyWriter systems have one slot for a memory board.  This board may contain either low-
density (1MB SIMMS) or high-density (4MB SIMMS) memory, but not a mixture of the two.
Standard systems are configured with 8MB of low-density memory, but this may be upgraded
at any time to 32MB of high density memory which swaps out the existing low density
memory.

If the system is configured exclusively with low-density memory, the maximum system
memory that can be configured is 64MB.  When using the high-density memory modules, this
maximum is extended to a total of 256MB.