armlinux oops once access more than 32Mega SDRAM

I use U-boot-1.1.1, and ARM-linux-2.4.27, at91rm9200DK, 2 chip SDRAM 16-bi width.

In uboot context, I can access all the 64Mega of SDRAM well.

In linux context, I can see 64Mega SDRAM by "free", and 46Mega available

I test memory, if the used memory bytes increase to 32Mega, the kerne oops.

Can anybody help me? Any advice would be appreciated!