Depends on what you want to do :) If you just want to read/write sectors of 512 bytes at a time, it's pretty easy to implement even on a simple MCU.

There's a couple of registers which need to be set: https://wiki.osdev.org/ATA_PIO_Mode and you can then just read/write the data from the parallel lines.

Here's a similar project using an ATmega32: https://github.com/zwostein/idetrol (i used this code, ported to ARM to check my hardware before I wrote the kerneldriver)