By providing complete,control over process priority as well as the assignment of processes and
interrupts to processors, IRIX enables simplified system software implementations, minimal
latency, and maximal CPU utilization.

4.4.1 Deterministic Response
 

Unix was originally designed to distribute CPU time and other system resources equally
among users,of a time-sharing system.  As a result, predicting precisely when any event will
occur in a generic Unix system is-impossible.  IRIX with REACT includes the following
extensions to generic Unix which, together, result in a fully deterministic system that is
suitable for real-time applications.
 
4.4.2 Interruptible Kernel
 
When running in kernel mode, IRIX periodically allows user-level interrupts to be processed.
 
4.4.3 Precise Process Priority Control
 
Users can assign a process to one of three priority bands: above, equal to, or below normal
Unix processes.  Within each band, processes can be assigned one of at least 32 vertical priority
levels.
 
4.4.4 Non-Degrading Priority
 
Users can also prevent the Unix scheduler's aging mechanism from decreasing the priority of a
process as its run time accumulates.  This freezes the process' priority at the assigned level.
 
4.4.5 Locking Memory Pages
 
A process can be locked into mehiory, ensuring that it will never be swapped out onto disk.
 
4.4.6 Processor Isolation
 
On a multiprocessor system, a subset of the processors can be isolated from the non-
determinism introduced by disk and network I/O processing.

POWER Lock(tm) Processor Isolation, a feature of IRIX Release 4.0, goes beyond isolating a
processor from non-determinism to allow a user to isolate a processor from certain memory
management activities that extend interrupt latency.

Communication among processors via shared memory is not affected by processor isolation.
Processes running on isolated processors have full access to all Unix services.

4.5 Input/Output
 

No system is an island, so SkyWriter supports a variety of options for I/O to secondary storage
devices, networks, and third party devices.