linux 2.4.26 on 440GX 512MB of RAM

I am booting linux from, Uboot on a custom 440GX board.

U-boot is able to access the 512MB of RAM I have on this board.
Unless I add the option mem=3D256M to the linux boot line,=20
linux is not able to fully boot.

I know it calls MMU_init (head_44x.S) but does not seem to jump to =
start_kernel after that.

Is there something I am missing here?=20