RTEMS CPU Architecture Supplement
During the execution of directive calls, critical sections of code may be executed. When these sections are encountered, RTEMS disables interrupts to level seven (15) before the execution of this section and restores them to the previous level upon completion of the section. RTEMS has been optimized to insure that interrupts are disabled for less than RTEMS_MAXIMUM_DISABLE_PERIOD microseconds on a RTEMS_MAXIMUM_DISABLE_PERIOD_MHZ Mhz ERC32 with zero wait states. These numbers will vary based the number of wait states and processor speed present on the target board. [NOTE: The maximum period with interrupts disabled is hand calculated. This calculation was last performed for Release RTEMS_RELEASE_FOR_MAXIMUM_DISABLE_PERIOD.]
[NOTE: It is thought that the length of time at which the processor interrupt level is elevated to fifteen by RTEMS is not anywhere near as long as the length of time ALL traps are disabled as part of the "flush all register windows" operation.]
Non-maskable interrupts (NMI) cannot be disabled, and ISRs which execute at this level MUST NEVER issue RTEMS system calls. If a directive is invoked, unpredictable results may occur due to the inability of RTEMS to protect its critical sections. However, ISRs that make no system calls may safely execute as non-maskable interrupts.
RTEMS CPU Architecture Supplement
Copyright © 1988-2008 OAR Corporation