|MadSci Network: Computer Science|
EIDE controllers actually do very little themselves - the electronics in the drive itself do most of the hard work. The drive electronics respond to requests from the PC to read or write data on the disk, work out exactly where on the disk the required data is located, position the head, and control the flow of data between the disk and a small internal RAM cache, and then on to the host PC. The drive also deals with keeping track of of any physical manufacturing defects on the disk surface, so data is not stored in bad areas. The PC's EIDE interface simply handles the communication of commands and data to the drive, and the transfer of the data to the PC's main memory.
Try the links in the MadSci Library for more information on Computer Science.