Unable to mount root fs on unknown-block

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nios2-terminal: connected to hardware target using JTAG UART on cable 

nios2-terminal: "USB-Blaster [USB-0]", device 1, instance 0 nios2-terminal: (Use the IDE stop button or Ctrl-C to terminate)  Linux version 2.6.11-uc0 (new@MICROSOF-7693F8) (gcc version 3.4.1 (Altera Nios I I 5.1 b73))# 3 Mon Aug 21 10:49:42 2006     uClinux/Nios II  Altera Nios II support © 2004 Microtronix Datacom Ltd.   setup_arch: No persistant network settings signature at 003F0000  Built 1 zonelists  Kernel command line: root=/dev/mtdblock0 ro  PID hash table entries: 64 (order: 6, 1024 bytes)  Dentry cache hash table entries: 2048 (order: 1, 8192 bytes)  Inode-cache hash table entries: 1024 (order: 0, 4096 bytes)  Memory available: 6340k/8192k RAM, 0k/0k ROM (1439k kernel code, 295k data)  Mount-cache hash table entries: 512 (order: 0, 4096 bytes)  NET: Registered protocol family 16  Serial: JTAG UART driver $Revision: 1.4 $  ttyJ0 at MMIO 0x80681158 (irq = 2) is a jtag_uart  io scheduler noop registered  io scheduler anticipatory registered  io scheduler deadline registered  io scheduler cfq registered  RAMDISK driver initialized: 16 RAM disks of 4096K size 1024 blocksize  dmfe_probe:eth0, 50000 Khz Nios  <DM9000> eth0 I/O: 80681000, VID: 90000a46    ADDR: 00:07:ed:00:00:00   dmfe_probe:eth1, 50000 Khz Nios  microtronix[mtd]: RAM probe address=0x200000 size=0x0  Creating 1 MTD partitions on "RAM":  0x00000000-0x00000000 : "ROMfs"  mtd: partition "ROMfs" is out of reach -- disabled  microtronix[mtd]: set ROMfs to be root filesystem  NET: Registered protocol family 2  IP: routing cache hash table of 512 buckets, 4Kbytes  TCP established hash table entries: 512 (order: 0, 4096 bytes)  TCP bind hash table entries: 512 (order: -1, 2048 bytes)  TCP: Hash tables configured (established 512 bind 512)  NET: Registered protocol family 1  NET: Registered protocol family 17  end_request: I/O error, dev mtdblock0, sector 2  EXT2-fs: unable to read superblock  end_request: I/O error, dev mtdblock0, sector 0  FAT: unable to read boot sector  VFS: Cannot open root device "mtdblock0" or unknown-block(31,0)  Please append a correct "root=" boot option  Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(31,0)     why?Can you help me ?