INtime® 3.1 Software - tenAsys

Chapter 2: Understanding INtime software architecture
About the Windows HAL
INtime software uses the Windows HAL, but intercepts certain functions to perform
the following actions:
• Traps attempts to modify the system clock rate so that the RT kernel can control
the system time base.
• Traps attempts to assign interrupt handlers to interrupts reserved for RT kernel use.
• Ensures that interrupts reserved for RT kernel use are never masked by
Windows software.
INtime software is compatible with all HAL files shipped with the standard Windows
products.
About thread scheduling
Priority-based scheduling
The RT kernel switches between threads and makes sure the processor always executes
the appropriate thread. The kernel’s scheduling policy is that the highest priority
thread that is ready to run is/becomes the running thread. The kernel maintains an
execution state and a priority for each thread and enforces its scheduling policy on
every interrupt or system call.
A priority is an integer value from 0 (the highest priority) through 255.
Range
Usage
0–127 Used by the OS for servicing external interrupts. Creating a thread that
handles internal events here masks numerically higher interrupt levels.
128–130 Used for some system threads.
131–252 Used for application threads.
Execution state
Interrupt threads mask lower-priority (numerically higher) interrupt levels. When you
assign interrupt levels, give a higher-priority (numerically lower) level to interrupts
that can’t wait, such as serial input, and a lower priority (numerically higher) to
interrupts that can wait, such as cached input.
The execution state for each thread is, at any given time, either running, ready, asleep,
suspended, or asleep-suspended. The RT kernel enforces the scheduling policy in
which the highest priority ready thread is always the running thread.
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