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CNTFRQ register on Cortex-A8

John Conover
  • Note: This was originally posted on 22nd October 2012 at http://forums.arm.com

    Thanks,

      After going through the Architecture Reference Manual again, I see the optional sections listed in the beginning:
    Floating-point (VFP) Extension
    Advanced SIMD Extension
    Security Extensions
    Virtualization Extensions
     
      The Generic Timer is listed in chapter B8 and it mentions that it is optional. I guess that I expect the document to flow from a high level where the optional elements are listed, into a detailed view of how they work. It would have been nice if all the optional elements were listed in a single place. So when I came into the manual with assembly code from Linux on how to access the CNTFRQ register, I never made the connection that it is part of the Generic Timer (the assembly of how to access the CNTFRQ is listed in section B4.1, VMSA System control registers descriptions.)

    Thanks again for the clarification.
  • Arm Employee Badge
    Note: This was originally posted on 21st October 2012 at http://forums.arm.com

    The forums (and the Cortex-A8 docs) are correct, this register is not present.
    The ARM architecture includes a number of optional features. A given implementation (I.e. processor) chooses which of the optional features to implement. The CNTFRQ register is part of the optional Generic Timer spec.  As far as I know, only the CA15 and CA7 implement this.