This code is intended to be used to read MFM disks using a beaglebone
board with a disk interface daughter card. A makefile is provided to
build mfm_read and mfm_util on the beaglebone.

The site for this code is http://www.pdp8online.com/mfm/mfm.shtml

The Makefile assumes am335x_pru_package-master is in the parent directory
of the source. This provides libprussdrv.a for accessing the PRUs, the
PRU assembler, and necessary include files.  The version I used to build it
is at https://github.com/beagleboard/am335x_pru_package. The Debian
distribution includes a version of this package with bugs so for now you
still need to download the current version.

The mfm_util program can also be built on a little endian Linux system.
See the pdp8online site above for more information on building.

For reading disks the code running on the PRU is controlled by commands
from the ARM which are documented in cmd.h and prucode0.p. The commands
do disk control such as seeking, checking status, and reading a track.
The track is read as the timing of rising edges of the MFM data signal. The
time is represented as delta time between the edges. These delta transitions
are put in a shared DDR memory buffer as 16 bit words with a count stored in
the PRU local memory. The ARM reads the count and deltas for processing.
The raw transitions can be stored for later processing and/or decoded back
into the disk contents. The mfm_util program is used to decode the
saved transitions into the decoded disk contents.

See mfm_read_util_doc.* for how to run the programs.

All the code I created is released under GPLv3 or later.

Source file organization
mfm_read.c	Top file for reading MFM disks
mfm_util.c	Top file for processing existing MFM delta transitions file

For mfm_read
analyze.c	Try to determine the track format of a disk drive
deltas_read.c	Routines for transferring deltas from PRU
drive.c		Routines for controlling the disk drive
pru_setup.c	Routines for initializing the PRU
prucode0.p	The assembly program running in the PRU
cmd.h		The PRU code commands, return status, and various memory
		location definitions

Used by both
crc_ecc.c	Routines for performing CRC and ECC operations
mfm_decoder.c	Routines for performing the MFM decoding
msg.c		Routines for printing messages
parse_cmdline.c	Routines for parsing and printing the command line options
wd_mfm_decoder.c	Routines for processing "Western Digital" format
xebec_mfm_decoder.c	Routines for processing Xebec format tracks
emu_tran_file.c Routines for reading and writing emulation and transition files
mfm_decoder.h	Defines for data structures used by the code
Makefile	Makefile for building the two executables and PRU code
<other>.h	Various header files which define function prototypes

Other files
crc_reverse.c	Routines to be manually used for determining CRC data for a disk
setup_mfm_read  Script to configure the beaglebone pins
mfm_read-00A0.dts Device tree file to configure pins

The DRIVE_PARAMS structure in mfm_decoder.h defines most of the information
for decoding the track. It can either be set from the command line or by
analyze_disk routine. Some decoding information which is hardcoded in the
routines might be better moved into the structure.

See emu_tran_file.c header for data file formats. 

The decoded disk contents is the concatenated sector data. It is in the
order specified in the sector headers, not the physical order on disk.
This will be different if the drive was formatted with interleave.
If the sector header was bad the sector data will be all zeros. If the
data area had a CRC error the data read will be written to the file.

David Gesswein
djg@pdp8online.com