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Detail:
  Previously, UnthreadV was only called when the IRQsema chain was empty, the
  link for the just-completed interrupt having just been removed. However,
  the information in the link is necessary to allow OS_Heap to be called from
  UnthreadV context, and patching up IRQsema within the UnthreadV handler
  prevents the implementation of a prioritised threading scheme. As a result,
  we must call UnthreadV every time the interrupt dispatch unthreads, and
  leave it up to the UnthreadV handler to distinguish between return to
  thread context and return from a nested interrupt handler.
Admin:
  Will require some sort of patch to enable heap-safe prioritised threading
  on RISC OS-STB 5.0.0 or RISC OS 5.07, the only two released OSes with the
  previous UnthreadV behaviour.

Version 5.35, 4.79.2.77. Tagged as 'Kernel-5_35-4_79_2_77'