[U-Boot] Question about mpc85xx startup using Nor Flash

I'm porting u-boot to P2040 based board these days.
As u-boot/arch/powerpc/mpc85xx/start.s commented: *The processor starts at 0xffff_fffc and the code is first executed in the last 4K page in flash/rom.*
The E500MCRM Chapter 6.6 mentioned:
*This default TLB entry translates the first instruction fetch out of reset(at effective address 0xffff_fffc).* *This instruction should be a branch to the beginning of this page.*
So, if I configure the HCW to let powerpc boot from Nor Flash, why should I suppose that the Nor Flash's last 4K is mapped to 0xffff_f000~0xffff_ffff. Since there're no LAW setup yet and the default BR0/OR0 of Local Bus does not match that range. And what is the instruction at 0xffff_fffc and where is it from?
Best Regards, Hook Guo
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郭佳