英语翻译
ThesimplestlayoutisaccomplishedbylocatingtheapplicationinROMataddress0in
thememorymap.Theapplicationcanthenbranchtotherealentrypointwhenitexecutes
itsfirstinstructionattheresetvectorataddress0x0.But,therearedisadvantages
withthislayout.ROMistypicallynarrow(8or16bits)andslowcomparedtoRAM,
requiringmorewaitstatestoaccessit.Thisslowsdownthehandlingofprocessor
exceptions,especiallyinterrupts,throughthevectortable.Moreover,ifthevectortable
isinROM,itcannotbemodifiedbythecode.
SinceRAMisnormallyfasterandwiderthanROM,itisbetterforthevectortableand
interrupthandlersifthememoryat0x0isRAM.AlthoughItisnecessarythatRAMbe
locatedat0x0duringnormalexecution,ifRAMislocatedataddress0x0onpower-up,
thereisnotavalidinstructionintheresetvectorentry.Therefore,ROMmustbelocated
at0x0atpower-uptoassurethatthereisavalidresetvector.Thechangeoverfrom
resettothenormalmemorymapisnormallyaccomplishedbyperformingtheremap
command.