Problem mounting ramdisk image

["Häpe, Sebastian, HRD/AB"]

Hi everybody,

I have a litte problem with an initrd-image on a custom board:

I created a gzip-compressed image of an initrd which is loaded to RAM by the
bootloader (U-Boot) before it starts the kernel. The kernel is compiled with
support for initial ramdisks and knows the right addresses of the image in
RAM.
Now the following effects occur:

 -  if i do not tell the kernel which filesystem I have in my image (only
entering: 'root=/dev/ram rw init=/bin/busybox')
    he tells me the following:
    
    RAMDISK driver initialized: 16 RAM disks of 4096K size 1024 blocksize
    RAMDISK: Compressed image found at block 0
    Freeing initrd memory: 709k freed
    Kernel panic: VFS: Unable to mount root fs on 01:00

 -  if I tell the kernel to use TMPFS or RAMFS (rootfs=tmpfs or
rootfs=ramfs), the error is

    RAMDISK: Compressed image found at block 0
    Freeing initrd memory: 709k freed
    VFS: Mounted root filesystem (tmpfs/ramfs)
    Kernel panic: No init found. Try to pass init=... To the kernel command
line     

    When I examine the return value of the execve() function, I get
error-code 2, which means: File or directory not found

My distribution (ELinOS) creates two files, 'Image.gz' and 'Image.initrd'.
If I extract Image.gz, the full filesystem is extracted. 
How can I check wheather the 'Image.initrd' file is correct?
(U-Boot tells me it is correct if I enter 'imi [address of image]')

Or does anyone did have the same problem? Have I forgotten any kernel
options?

Greetings,
Sebastian Häpe