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1、总线地址、物理地址、虚拟地址相关概念澄清Now,onnormalPCsthebusaddressisexactlythesameasthephysicaladdress,andthingsareverysimpleindeed.However,theyarethatsimplebecausethememoryandthedevicessharethesameaddressspace,andthatisnotgenerallynecessarilytrueonotherPCI/ISAsetups.
2、 Now,justasanexample,onthePReP(PowerPCReferencePlatform),theCPUseesamemorymapsomethinglikethis(thisisfrommemory): 0-2GB "realmemory" 2GB-3GB "systemIO"(inb/outandsimilaraccessesonx86) 3GB-4GB "IOmemory"(sharedmemoryovertheIObus)N
3、ow,thatlookssimpleenough.However,whenyoulookatthesamethingfromtheviewpointofthedevices,youhavethereverse,andthephysicalmemoryaddress0actuallyshowsupasaddress2GBforanyIOmaster.SowhentheCPUwantsanybusmastertowritetophysicalmemory0,ithastogivethemasterad
4、dress0x80000000asthememoryaddress.So,forexample,dependingonhowthekernelisactuallymappedonthePPC,youcanendupwithasetuplikethis:physicaladdress: 0virtualaddress: 0xC0000000busaddress: 0x80000000wherealltheaddressesactuallypointtothesamething.
5、It'sjustseenthroughdifferenttranslations..Similarly,ontheAlpha,thenormaltranslationisphysicaladdress: 0virtualaddress: 0xfffffc0000000000busaddress: 0x40000000(buttherearealsoAlphaswherethephysicaladdressandthebusaddressarethesame).Anyway,the
6、waytolookupallthesetranslations,youdo #include phys_addr=virt_to_phys(virt_addr); virt_addr=phys_to_virt(phys_addr); bus_addr=virt_to_bus(virt_addr); virt_addr=bus_to_virt(bus_addr);Now,whendoyouneedthese?thereareactually_th
7、ree_differentwaysoflookingatmemoryaddresses,andinthiscaseweactuallywantthethird,theso-called"busaddress".Essentially,thethreewaysofaddressingmemoryare(thisis"realmemory",thatis,normalRAM--seelateraboutotherdetails):-CPUuntranslated. Thisisthe"physica
8、l"address. Physicaladdress0iswhattheCPUseeswhenitdriveszeroesonthememorybus.-CPUtranslatedaddress.Thisisthe"virtual"address,andiscompletelyinternaltotheCPUitselfwiththeCPUdoingtheappropriatetranslationsinto"CPUuntranslated".-busaddress.Thisis
