Ramdisk images for test

[HappyPhot]

Hello,
  I am trying to use ramdisk for my board. (not success yet)
  I followed setion 7.6 of DULG and found got two Ramdisk images
for test for ppc_82xx. One is pRamdisk, another is ramdisk_image.gz.
  Who can tell what the difference is between them ?

thank you,
/HappyPhot