A series of micromagnetic models including simulations of the 3D thin film write head field, the GMR read head, the thin film media and channel codes are utilized to study the recording performance in longitudinal hard disk drives (HDD) at extremely high densities. The (0, 4/4) encoder is utilized to translate the user data into (0, 4/4) constrained codes, before the write process is performed. The write current is achieved from the constrained code in the NRZ format. The read back voltage is reshaped to the PR-IV signal and the Viterbi detector is utilized to recover the data. In a medium of 10 nm grains, the recording linear density limits with the PRML method are about 1000 kfci, which is 1.5 times of those with the PD channel.