[U-Boot] how to understand "uboot is 32bit program even it is in the 64bit mips"

[hacklu]

  I have a 64bit cnMips borad.In the mail list archives,it says  "uboot is 32bit,even in the 64bit cpu."
but how to understand that?
I have a guess,is the 64bit cpu support a 32bit-mode,then when does it switch to 64bit-mode to load 64-bit linux?

btw,I have an other question,according to  <see mips run linux>,linux run in the _xkseg_ segment. so the first two bit virtual address in kernel space must be 11,
like 11xx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx (64bit).
but when I printf a variable' address in a foo driver,is say 0xa80000041c2ea180. not start of _11_.
the memory mapping mode has feazed me for _long_ _long_time.

Dear Wolfgang(or someone else who can anwser me),thanks 
 				
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hacklu
2011-06-22