資源描述：
《總線地址、物理地址、虛擬地址相關概念澄清》由會員上傳分享，免費在線閱讀，更多相關內(nèi)容在教育資源-天天文庫。
1、總線地址、物理地址、虛擬地址相關概念澄清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
