4.4 Real-Time Support
 
Silicon Graphics operating system, IRIX, is based upon a POSIX-compliant version of UNIX
System V.3. It has, however, required special modifications to support multiple processors, a
concept not covered by standard UNIX.  These special functions at the heart of IRIX have been
combined under the banner of REACT, the REal time ACcess Technology kernel.

REACT offers a set of system calls enabling real-time operation of systems running IRIX.  As
real-time functionality is key to the operation of simulators, REACT acts as the core of the host
capability of the SkyWriter system.  REACT provides for deterministic response to interrupts
and deterministic process dispatch latency.  Latencies for a multiprocessor system running
IRIX 4.0 are shown in the diagram below:

IRIX with REACT is true UNIX, so time-critical processes are identical to all other processes.
This means that all UNIX software development tools are available to real-time developers.
REACT is an integral part of IRIX that is included with every copy of the operating system.

Some of the key features of REACT include:

o   Standard UNIX with deterministic response.

o   Fast total interrupt response.

o   Shared memory, symmetric multiprocessing w/full user control over system resources

o   Highly productive software development environment

o   High performance networking

IRIX Release 4.0 conforms to numerous interface standards intended to provide application
portability, including most features specified in Draft 11 of the POSIX 1003.4 (real-time)
standard.  Interrupt latency, context switch time and process dispatch latency are clearly
specified.

A multiprocessor system can be configured to ensure that the appropriate user-level process
will begin execution within 200 usec after a VME interrupt occurs.