could I set the VTOR register of M3 core In SmartFusion chip to 0x8800 0000 ?

hello masters,

Now ,I am using M2S090-FGG484I chip , it has a cortex M3 core.  I am programing it.

 I want to offest the vector table to 0x8800 0000 ,but I found the VTOR regsiter  bit 31-30 is reserved and TBLOFF is bit 28-7. So I think that it couldn't set the vector table to 0x8800 000.

However, when I set VTOR regsiter is 0x8800 0000,  it aright worked . the vector table is seemingly located at 0x8800 0000 .

So, I want to know that the reason of it. could you help me ?

                                                                 thanks very much

                                                                                jent