IDE or EIDE is a hard drive via host controller interface generally used for connecting hard drive, cdrom drives and tape or floppy drives to an IBM style PC. Its kind of well documented but not succinctly.
It emerged as a replacement for the ST506 or Shugart Technologies MFM drive and controller combination. Shugart was known for developing the 5 1/4 inch floppy disks of the 1980s among other things.
The idea for the interface was to "offload" the mangement of positioning the heads and control of the reading of the disk as much as possible to a microcontroller onboard a seperate "controller" for the hard drive.
To the PC however this is presented as a device connected to the 8 bit ISA bus.
The 8 bit ISA bus was capable of transmitting 16 bits at a time, but for early equipment 8 bits was the maximum. Address and decoder logic circuits to actuate and pulse control lines would indicate to the connected HDD controller when commands were ready on the bus to be read, and whether the PC were ready or had finished offloading data from the HDD controller memory "buffer". and into the PC memory. Write to the HDD wass accomplished much the same way by "presenting" data on the bus, and then pulsing or actuating control lines to indicate to the microprocessor on the HDD controller card that data was ready to be read.
Initially the PC understood or had to make it requests assuming the HDD were made of Cylinders, Heads and Sectors.. corresponding to the location of "blocks" of data of approximately 512 bytes of data per block. Due to the size of the data types used to access the HDD by the 8 bit (and later 16 bit) BIOS, limits on the Total size of "addressable" storage on HDD were imposed.
Physically however HDD had limits on the actual number of Heads in a drive, but the data type for this number was confiscated and reused as a "general" variable for a virtual drive with many more heads than physically possible. The HDD controller then could translate that into a method for accessing larger and larger HDD storage capacity. But to get away from proprietary "translations" it became common place to begin using all of the CHS "bit space" to represent a linear 24 bit adress call a "Logical Block Address" or LBA. Later LBA technology would get an upgrade to LBA-48 to access even more storage.
To a PC it has been said the HDD "looks like a chip" attached to its mini-ISA or IDE bus. It communicates to this "chip" using memory and address and control lines.
The speed at which the PC communicates when reading or writing data is an interpretation of the result of sending commands, waiting for the HDD to raise a "ready" line and then offloading that data into its memory. And vice versa for writing.
HDD limitations due to "seeking" or "retries" of sectors are hidden or delt with by the HDD controller but can be reported to the PC if queried.
PC limitations due to "calculation" time for locating a file in a virtual file system that exists as a consequence of storing a pattern on the hard drive with an indexing system are hidden or delt with by the PC.. but do add to time required to retrieve or write data.
A simple microcontroller that wishes to access an IDE hard drive merely needs to have enough 5 volt buffered ISA lines corresponding to the subset of the ISA bus lines that define an IDE interface.
Many microprocessors today have only 3.3 volt I/O lines, and these must be protected or buffered by sufficient interface circuits or buffer chips.
The protocol for actuating control lines and presenting data to an IDE connected drive are documented by timing diagrams, but in general PIO - Programmed Input Output access mode is under the control and speed of the host microprocessor.
Microprocessors with custom peripherals or FPGA defined co-controllers to offload the task of communicating over a built-in IDE interface were more common at one time from Freescale.. but have passed mostly into history. Today discrete chips to create a supporting IDE interface are less common but can still be done.. though SPI and software defined FPGA circuts to provide that function are becoming more common.
Its quite common in fact to redefine the IDE bus as a chip function as part of of a more general purpose USB attached peripheral, and that as a SATA device and then attach a SATA to IDE interface to complete the connection.
This minimizes the electrical demands and potential incompatibilites of custom designed circuits.
