Sending commands to devices behind port multiplier

• Its support is indicated to system software via CAP.SPM . (ICH6 does not support port multiplier)
• If CAP.SCLO → “1”，then enable PxCMD.CLD ，else disable PxCMD.CLD .
• 
  If PxCMD.CLD → “1”，then clean PxTFD.STS.BSY =0 and PxTFD.STS.DRQ =0 .