Issue 150 January 2003
Construct an ATA Hard Drive Controller

GETTING FAT

All of the reading and writing up to this point was completed with simple C routines that were teamed together to perform a much larger and more complex task. Believe it or not, designing the ATA hard drive controller hardware and finishing the C coding for the controller I/O functions was the easy part. The Ethernet code was just as easy, because I copied AVR code that was already written for the Packet Whacker microcontroller NIC. The next step in the process of assembling a microcontroller-based networkable mass storage system was a bit more demanding.

I completed a tremendous amount of research in preparation for writing AVR code to implement Microsoft’s FAT32 file system. Thanks to a series of Circuit Cellar articles written by our own Jeff Bachiochi (Circuit Cellar 143–146), I had a good idea about the roads I will travel and battles I will fight.

The journey starts with the bytes in Photo 4, the master boot record. Because I’m not executing instructions on a legacy x86 machine and using a PC BIOS or MSDOS, I’ll have to interpret the data and adapt it to the AVR. For instance, there’s executable code in the MBR that I don’t care about. The problem is that I have to navigate through it to find markers that either give me information about where FAT-related constants and parameters reside or point me to places where I can read and write my data.

In the case of the MBR, I’m interested only in the last 66 bytes, because that’s where the partition table resides. The fun starts at offset 0x1BE in the MBR, which is the first partition entry. Because the drives I formatted were secondary drives, the FDISK program couldn’t make their partitions active. So, the first byte at offset 0x1BE was 0x00 (inactive) on my drives.

Other interesting information resides at offset 0x1C6 in the MBR. This is the number of sectors between the MBR and the first sector of the first partition. As you can see in Photo 4, WinHex shows that number as 0x3F. I used the WinHex program to dial in 0x3F sectors beyond the MBR and, lo and behold, there was the FAT32 boot sector with additional fields of information to parse. The plan is to collect documentation and use WinHex to obtain the actual visuals concerning how data and control areas on a FAT32 hard disk are defined and laid out.

So, you see that I have my work cut out for me. The good news is that you’ll be able to share the fruits of my labor, because I will make the ATA hard drive controller hardware available to those of you who want one. The code discussed in this article is available for you to download from the Circuit Cellar ftp site. All of the basic functions necessary to read and write to an ATA hard drive can be found in the code I’m providing.

After you have all of the ImageCraft C source code, a copy of WinHex, and an ATA hard drive controller in front of you, you’ll see that everything you’ve read about in this article isn’t complicated, it’s simply embedded.