What is the "two-drive trick" that can read Amiga disks on a PC?

Question

Standard belief is that the floppy disk controllers used on PCs are incapable of reading Amiga floppy disks, due to the PC disk controller expecting a very specific low-level format of disks, whilst the Amiga disk controller is much more flexible and choosing a low-level format which is incompatible but can fit more data onto a disk. This has been discussed in a few questions on this site (put amiga floppy into the search box to see them).

However there's references to a "two-drive trick" which allows a PC with two floppy drives to successfully read an Amiga floppy disk and generate an ADF image file.

I found adfread-1.1 which implements the two-drive trick. Its instructions are pretty simple:

- Install fdrawcmd.sys from http://simonowen.com/fdrawcmd/
- Insert your Amiga disk in A:
- Insert the PC-formatted disk in B:
- From a command prompt, run:  adfread.exe newimage.adf

There's no information about how the "two-drive trick" works. Source code is included but largely uncommented so I'm mystified as to how this is supposed to work.

How does the two-drive trick work? Why does it require two drives, and how does having two drives get over the fundamental incompatibility of Amiga disks and PC floppy disk controllers?

Answer 1

This only works with 2 drives on the same controller and cable. The floppy controller doesn't know when the disk (hardware) has been switched on the port (software), so a transfer command can be issued on the IBM formatted disk, but run on the Amiga disk.

This answer comes from the documentation for the utility Disk2FDI. Here's an excerpt from docs\tech\rawflopy.doc in the Disk2FDI zip file:

I.2. The algorithm

This algorithm requires two disk drives installed (read the 1DISKDRV.TXT file to get an explanation why reading raw data generally with only 1 disk drive is physically impossible). Insert the disk you want to fully read in one drive (let’s say A: in this example). Insert another disk in the other drive (B: in this example). This disk must be IBM-formatted.

Select drive A: using port 3F2h. Also turn on motors for both A: and B:. Go to the desired track and side using standard FDC 765 commands. Swap to B: using port 3F2h. Select density appropriate for IBM-formatted disk in B: using port 3F7h. Issue a “Read a Track (Diagnostic)” command using the FDC 765. The parameters should match a sector that is physically present on the IBM-formatted disk, for example sector #1. For this command, set a sector size of at least 8KB (even if the physical sector is 512 bytes long). 16KB and 32KB sector sizes can be set to read more raw data. DMA registers should have been set accordingly. Watch continuously the DMA address until it is different from the starting address. When it is so, it means that the 765 has begun transferring sector data, so it has previously found the sector header on B:. As soon as the DMA address is increased, swap to A: using port 3F2h. This is the main idea behind this technique. The 765 has no way to know disk selection has changed because port 3F2h is not linked to it. This 1st step was discovered on the 11th of December, 1999. Change density (bit rate) using port 3F7h. For a full track read, including MFM synchronization bits, you must set a bit rate twice the standard value. For example, when reading a 250 000 bits/sec track (double-density track), set the bit rate to 500 000 bits/sec. This 2nd step was discovered on the 18th of December, 1999.

The FDC 765 will now read the disk in drive A: from now on, thinking it is a big sector on disk in drive B:. Wait for FDC interrupt. Of course, most status bits at the end of this operation should be just ignored, such as the data error (CRC) flag (which will be obviously set). The main indicator of a successful operation is the DMA counter or address. For a 32KB “sector” read, the DMA address will equal the starting address plus 32768 if the operation was successful.

Since some bytes were read at first from drive B:, and the swap of drives and bit rates will require a little time to settle, it is wise not to consider the first 50 bytes read.

If the track to be read contains an IBM sector, drive swapping may not be necessary. Yet, bit rate swapping can be useful, especially for protected or non-standard tracks.

This is credited in the documentation to Bernd Schmidt

Answer 2

You can actually read about 98% of an Amiga disk with a PC controller and just one drive, but at risk of destroying 2% of the data. The two-drive solution overcomes these limitations.

Credit for the two-drive trick should probably go to Vincent Joguin, who first implemented this in 1999. He applied it to NEC's floppy controller, but others are compatible with this chip.

Vincent indeed cited Bernd Schmidt, but only to show that the problem was thought to be impossible to solve. Toni Wilen and Simon Owen wrote adfread (able to run on Windows NT/2000 and later systems), which was inspired by Vincent's Disk2FDI (which ran only on Windows 9x/Me) and paper. You can read more about the various tools at https://www.amigaforever.com/kb/13-118.

To read Vincent Joguin's high-quality presentation from 2000 get DISK2FDI.ZIP from http://web.archive.org/web/20010723045656/http://fast.emuunlim.com/disk2fdi/, then look for DOCS/TECH/RAWFLOPPY.DOC inside the ZIP.