Intro - http://wiki.osdev.org/ATA
How to use PIO mode - http://wiki.osdev.org/ATA_PIO_Mode

Deep docs - http://www.t13.org/Documents/UploadedDocuments/project/d1410r3b-ATA-ATAPI-6.pdf
http://suif.stanford.edu/~csapuntz/ide.html
http://www.t13.org/
http://www.ata-atapi.com/
http://www.repairfaq.org/filipg/LINK/F_IDE-tech.html
http://www.seagate.com/support/disc/manuals/ata/1621pma.pdf
http://www.scsita.org/aboutscsi/sas/tutorials/SAS_ATA_upper_layers_public.pdf