In ancient times one had to specify the parameters of one's MFM or RLL disk to the BIOS, or add them as user-defined type (often type 47 or 48).
Often, the table with these parameters is found at 0xe400 (0162000). Below the results found for five different BIOSes and two programs to investigate other machines.
The first program, dump_rom, will
write the BIOS area to a file. One can do the same from DOS
using debug:
A:> debug > dump
d f000:0,ffff
q
A:>
(and then turn the text output of debug into a binary file using a small script).
The second program, hdtypes, will search some file,
or /dev/mem for the hard disk types table, and print it.
Note that it uses memmem - avoid it on pre-glibc2 systems.
The program cmoshdtype reads or writes the hard disk type
for hd0 or hd1 in CMOS.
Links: dump_rom.c, hdtypes.c and cmoshdtype.c.