英语翻译
The simplest layout is accomplished by locating the application in ROM at address 0 in
the memory map.The application can then branch to the real entry point when it executes
its first instruction at the reset vector at address 0x0.But,there are disadvantages
with this layout.ROM is typically narrow (8 or 16 bits) and slow compared to RAM,
requiring more wait states to access it.This slows down the handling of processor
exceptions,especially interrupts,through the vector table.Moreover,if the vector table
is in ROM,it cannot be modified by the code.
Since RAM is normally faster and wider than ROM,it is better for the vector table and
interrupt handlers if the memory at 0x0 is RAM.Although It is necessary that RAM be
located at 0x0 during normal execution,if RAM is located at address 0x0 on power-up,
there is not a valid instruction in the reset vector entry.Therefore,ROM must be located
at 0x0 at power-up to assure that there is a valid reset vector.The changeover from
reset to the normal memory map is normally accomplished by performing the remap
command.