Linux 2.6.28 crashes when mem=1024M is passed as boot parameter from u-boot

I am working on building an linux for powerpc 8641d board. My U-boot has been modified to recognize 1 GB of memory(RAM) and it does. But Linux 2.6.28 crashes when mem=1024M is passed as boot parameter from u-boot. I have specified the memory size as 0x40000000 (1GB) in DTS file. Can any one help me to findout the reason?

But the same Linux image works when I specify bootargs as 'mem=256M'. But OS idetifies the total memory of 256MB only but I want it to identify complete 1GB memory.

Here is the dump of linux boot: ** It hangs at this point:

CPU:   8641D, Version: 3.0, (0x80900130)
Core:  E600 Core 0, Version: 2.2, (0x80040202)
Clock Configuration:
   CPU:1000 MHz, MPX:400  MHz
   DDR:200  MHz (400 MT/s data rate), LBC:25   MHz
L1:    D-cache 32 KB enabled
       I-cache 32 KB enabled
L2:    512 KB enabled
Board: Wind River SBC8641D
I2C:   ready
DRAM:      DDR: 1 GiB
FLASH: 16 MiB
PCI-EXPRESS 1: Configured as Host 
IGAPS: PCIE link is Up
Scanning PCI bus 01
        01  00  10cf  2034  0380  ff
    PCI-EXPRESS 1 on bus 00 - 01
PCI-EXPRESS 2: Disabled
In:    serial
Out:   serial
Err:   serial
    IGAPS: Interrupts enabled...!!!!
    Net:   eTSEC1, eTSEC2, eTSEC3, eTSEC4
Hit any key to stop autoboot:  0 

=> bootm 1000000 - 2000000
WARNING: adjusting available memory to 30000000
## Booting kernel from Legacy Image at 01000000 ...
   Image Name:   Linux-2.6.28
   Image Type:   PowerPC Linux Kernel Image (gzip compressed)
   Data Size:    2417424 Bytes = 2.3 MiB
   Load Address: 00000000
   Entry Point:  00000000
   Verifying Checksum ... OK
## Flattened Device Tree blob at 02000000
   Booting using the fdt blob at 0x2000000
   Uncompressing Kernel Image ... OK
   Loading Device Tree to 007fb000, end 007ffb2d ... OK
WARNING: your dts/dtb is old. You must update before the next kernel release
Using MPC86xx HPCN machine description
Total memory = 1024MB; using 2048kB for hash table (at cfe00000)
Linux version 2.6.28 (TCS@localhost) (gcc version 4.5.4 (Buildroot 2012.11.1) ) #42 SMP Tue Aug     20     22:32:49 IST 2013
CPU maps initialized for 1 thread per core
console [udbg0] enabled
setup_arch: bootmem
mpc86xx_hpcn_setup_arch()
Found FSL PCI host bridge at 0x00000000f8008000. Firmware bus number: 0->1
Zone PFN ranges:
  DMA      0x00000000 -> 0x00030000
  Normal   0x00030000 -> 0x00030000
  HighMem  0x00030000 -> 0x00040000
Movable zone start PFN for each node
early_node_map[1] active PFN ranges
    0: 0x00000000 -> 0x00040000
Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 260096
Kernel command line: console=ttyS0,115200 mem=1024M     ip=172.19.180.10:172.19.180.30:172.19.180.1:255.255.255.0:sbc8641d:eth0:off rw root=/dev/nfs     nfsroot=172.19.180.30:/opt/nfsRootfs
PID hash table entries: 4096 (order: 12, 16384 bytes)
clocksource: timebase mult[2800000] shift[22] registered
Console: colour dummy device 80x25
Dentry cache hash table entries: 131072 (order: 7, 524288 bytes)
Inode-cache hash table entries: 65536 (order: 6, 262144 bytes)
Memory: 1031376k/1048576k available (4632k kernel code, 16396k reserved, 144k data, 175k bss, 208k init)

Any reason why it hangs here?