Overview

The ARM floating point functionality is implemented in software because the majority of ARM processors don't have built-in floating point hardware.

The VFP functionality support is enabled when the startup program detects the presence of VFP hardware and sets the system page CPU_FLAG_FPU flag.

The QNX Neutrino procnto and procnto-v6 provides the basic support for managing the VFP context on a per-thread basis. This support uses a lazy mechanism to minimize the cost of saving and restoring the VFP register context as follows:

• The VFP is disabled initially.
• Any VFP access causes an undefined instruction exception, allowing the kernel to initialize the VFP context for a thread.
• First access to the VFP by a thread causes an exception. The kernel exception handler:
  • allocates storage for the VFP context
  • initializes the VFP state
  • records this thread as owning the VFP
  • restarts the VFP instruction
• VFP is disabled on a context switch to allow the kernel to trap subsequent VFP accesses for managing the per-thread VFP context:
  • If the access is performed by the thread owning the VFP, the VFP already contains the correct VFP state. In this case, the VFP is simply re-enabled and the VFP instruction is restarted.
  • If the access is performed by a different thread, a VFP context switch is required:
    • The VFP state is saved in the owning thread's VFP storage.
    • If this is the first access performed by the thread, storage is allocated for the VFP context, and its VFP state is initialized.
    • The VFP state is restored from the new thread's VFP storage.
    • The new thread is recorded as owning the VFP.
    • The VFP instruction is restarted.