diff --git a/src/greaseweazle/codec/codec.py b/src/greaseweazle/codec/codec.py
index b2cbfdb7..e822a618 100644
--- a/src/greaseweazle/codec/codec.py
+++ b/src/greaseweazle/codec/codec.py
@@ -147,6 +147,7 @@ def read_diskdef_file_lines(filename: Optional[str]) -> Tuple[List[str], str]:
 from greaseweazle.codec.macintosh import mac_gcr
 from greaseweazle.codec.commodore import c64_gcr
 from greaseweazle.codec.apple2 import apple2_gcr
+from greaseweazle.codec.hp import hp_mmfm
 
 def mk_trackdef(format_name: str) -> TrackDef:
     if format_name in ['amiga.amigados']:
@@ -159,6 +160,8 @@ def mk_trackdef(format_name: str) -> TrackDef:
         return mac_gcr.MacGCRDef(format_name)
     if format_name in ['c64.gcr']:
         return c64_gcr.C64GCRDef(format_name)
+    if format_name in ['hp.mmfm']:
+        return hp_mmfm.HPMMFMDef(format_name)
     if format_name in ['apple2.gcr']:
         return apple2_gcr.Apple2GCRDef(format_name)
     if format_name in ['bitcell']:
diff --git a/src/greaseweazle/codec/hp/__init__.py b/src/greaseweazle/codec/hp/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/src/greaseweazle/codec/hp/hp_mmfm.py b/src/greaseweazle/codec/hp/hp_mmfm.py
new file mode 100644
index 00000000..c875dde7
--- /dev/null
+++ b/src/greaseweazle/codec/hp/hp_mmfm.py
@@ -0,0 +1,258 @@
+# greaseweazle/codec/hp/hp_mmfm.py
+#
+# Written & released by Keir Fraser <keir.xen@gmail.com>
+#
+# This is free and unencumbered software released into the public domain.
+
+# From github:brouhaha/fluxtoimd/modulation.py:
+#
+# See the file COPYING for more details, or visit <http://unlicense.org>.
+# An HP-proprietary M2FM floppy format, used by the HP 7902, 9885,
+# and 9895 Flexible Disc Drives.
+# Documentation:
+#   9885 Flexible Disk Drive Service Manual
+#      Hewlett-Packard, September 1976, part number 09885-90030
+#   7902A Disc Drive Preliminary Service Manual
+#      Hwelett Packard, May 1979, part number 07902-90060
+#   7902A & C/9895K Flexible Disc Drive Service Documentation
+#      Hewlett-Packard, January 1981, part number 07902-90030
+#   9895A Flexible Disc Memory Service Manual,
+#      Hewlett-Packard, February 1981, part number 09895-90030
+# 9885: single-sided, 67 track, M2FM format only
+# 7902: double-sided, 77 track, M2FM or IBM 3740 FM formats
+# 9895: double-sided, 77 track, M2FM or IBM 3740 FM formats
+
+from typing import List, Optional
+
+import struct
+from bitarray import bitarray
+import crcmod.predefined
+
+from greaseweazle import error
+from greaseweazle.codec import codec
+from greaseweazle.codec.ibm.ibm import decode, encode, sec_map
+from greaseweazle.track import MasterTrack, PLL, PLLTrack
+from greaseweazle.flux import HasFlux
+
+default_revs = 1.1
+
+bad_sector = b'-=[BAD SECTOR]=-' * 16
+
+# Syncs have an extra clock bit:
+# ff 0e -> 55 55 22 54 -> 55 55 2a 54
+sector_sync_bytes = b'\x55\x55\x2a\x54'
+# ff 0a -> 55 55 22 44 -> 55 55 2a 44
+data_sync_bytes = b'\x55\x55\x2a\x44'
+
+sector_sync = bitarray(endian='big')
+sector_sync.frombytes(sector_sync_bytes)
+
+data_sync = bitarray(endian='big')
+data_sync.frombytes(data_sync_bytes)
+
+crc16 = crcmod.predefined.Crc('crc-ccitt-false')
+
+rev_list = bytearray()
+for x in range(256):
+    y = 0
+    for i in range(8):
+        y <<= 1
+        y |= x & 1
+        x >>= 1
+    rev_list.append(y)
+
+def bitrev(x):
+    return rev_list[x]
+
+# MMFM:
+# Write clock bit iff
+#  1. No preceding clock or data
+#  2. No following data
+mmfm_list = bytearray()
+for x in range(512):
+    for i in [7,5,3,1]:
+        if x & (0xf << (i-1)) == 0:
+            x |= 1<<i
+    mmfm_list.append(x & 255)
+
+def mmfm_encode(dat):
+    y, out = 0, bytearray()
+    for x in dat:
+        if (x & 0xaa) == 0:
+            x = mmfm_list[(y<<8) | x]
+        out.append(x)
+        y = 0 if (x & 3) == 0 else 1
+    return bytes(out)
+
+class HPMMFM(codec.Codec):
+
+    time_per_rev = 60 / 360
+    clock = 1e-6
+
+    verify_revs = default_revs
+
+    def __init__(self, cyl: int, head: int, config):
+        self.cyl, self.head = cyl, head
+        self.config = config
+        self.sector: List[Optional[bytes]]
+        self.sector = [None] * self.nsec
+
+    @property
+    def nsec(self) -> int:
+        return self.config.secs
+
+    def summary_string(self) -> str:
+        nsec, nbad = self.nsec, self.nr_missing()
+        s = "HP MMFM (%d/%d sectors)" % (nsec - nbad, nsec)
+        return s
+
+    # private
+    def add(self, sec_id, data) -> None:
+        assert not self.has_sec(sec_id)
+        self.sector[sec_id] = data
+
+    def has_sec(self, sec_id: int) -> bool:
+        return self.sector[sec_id] is not None
+
+    def nr_missing(self) -> int:
+        return len([sec for sec in self.sector if sec is None])
+
+    def get_img_track(self) -> bytearray:
+        tdat = bytearray()
+        for sec in self.sector:
+            tdat += sec if sec is not None else bad_sector
+        return tdat
+
+    def set_img_track(self, tdat: bytes) -> int:
+        totsize = self.nsec * 256
+        if len(tdat) < totsize:
+            tdat += bytes(totsize - len(tdat))
+        for sec in range(self.nsec):
+            self.sector[sec] = tdat[sec*256:(sec+1)*256]
+        return totsize
+
+    def decode_flux(self, track: HasFlux, pll: Optional[PLL]=None) -> None:
+        raw = PLLTrack(time_per_rev = self.time_per_rev,
+                       clock = self.clock, data = track, pll = pll)
+        bits, _ = raw.get_all_data()
+
+        for offs in bits.itersearch(sector_sync):
+
+            if self.nr_missing() == 0:
+                break
+
+            offs += 2*16
+            idam = decode(bits[offs:offs+4*16].tobytes())
+            if len(idam) != 4:
+                continue
+            if crc16.new(idam).crcValue != 0:
+                continue
+            cyl = bitrev(idam[0])
+            sec_id = bitrev(idam[1])
+            head = (sec_id & 128) == 128
+            sec_id &= 127
+            if cyl != self.cyl or head != self.head or sec_id > self.nsec:
+                print('T%d.%d: Ignoring unexpected sector C:%d H:%d R:%d'
+                      % (self.cyl, self.head, cyl, head, sec_id))
+                continue
+            if self.has_sec(sec_id):
+                continue
+
+            # Find data
+            offs += 8*16
+            dat_offs = bits[offs:offs+50*16].search(data_sync)
+            if len(dat_offs) != 1:
+                continue
+            offs += dat_offs[0] + 2*16
+
+            sec = decode(bits[offs:offs+258*16].tobytes())
+            if len(sec) != 258:
+                continue
+            if crc16.new(sec).crcValue != 0:
+                continue
+
+            # bit swap, and byte swap
+            sec = bytes(map(lambda x: bitrev(x), sec[:256]))
+            sec = struct.pack('<128H', *struct.unpack('>128H', sec))
+
+            self.add(sec_id, sec)
+
+
+    def master_track(self) -> MasterTrack:
+
+        # Post-index track gap.
+        t = encode(bytes(100))
+
+        for sec_id in sec_map(self.nsec, self.config.interleave,
+                              self.config.cskew, self.config.hskew,
+                              self.cyl, self.head):
+            sector = self.sector[sec_id]
+            data = bad_sector if sector is None else sector
+            # Header
+            t += encode(bytes([0xff]*3)) + sector_sync_bytes
+            idam = bytes(map(lambda x: bitrev(x),
+                             [self.cyl, sec_id | (self.head << 7)]))
+            idam += struct.pack('>H', crc16.new(idam).crcValue)
+            t += encode(idam)
+            t += encode(bytes(1 + 16 + 4))
+            # Data
+            t += encode(bytes([0xff]*3)) + data_sync_bytes
+            data = struct.pack('<128H', *struct.unpack('>128H', data))
+            data = bytes(map(lambda x: bitrev(x), data))
+            data += struct.pack('>H', crc16.new(data).crcValue)
+            t += encode(data)
+            t += encode(bytes(1 + 34 + 4))
+
+        # Add the pre-index gap.
+        tlen = int((self.time_per_rev / self.clock)) & ~31
+        t += encode(bytes((tlen//8-len(t))//2))
+
+        t = mmfm_encode(t)
+
+        track = MasterTrack(bits = t, time_per_rev = self.time_per_rev)
+        track.verify = self
+        return track
+
+
+    def verify_track(self, flux):
+        readback_track = self.__class__(self.cyl, self.head, self.config)
+        readback_track.decode_flux(flux)
+        return (readback_track.nr_missing() == 0
+                and self.sector == readback_track.sector)
+
+
+class HPMMFMDef(codec.TrackDef):
+
+    default_revs = default_revs
+
+    def __init__(self, format_name: str):
+        self.interleave = 1
+        self.cskew, self.hskew = 0, 0
+        self.secs: Optional[int] = None
+        self.finalised = False
+
+    def add_param(self, key: str, val) -> None:
+        if key == 'secs':
+            val = int(val)
+            self.secs = val
+        elif key in ['interleave', 'cskew', 'hskew']:
+            n = int(val)
+            error.check(0 <= n <= 255, '%s out of range' % key)
+            setattr(self, key, n)
+        else:
+            raise error.Fatal('unrecognised track option %s' % key)
+
+    def finalise(self) -> None:
+        if self.finalised:
+            return
+        error.check(self.secs is not None,
+                    'number of sectors not specified')
+        self.finalised = True
+
+    def mk_track(self, cyl: int, head: int) -> HPMMFM:
+        return HPMMFM(cyl, head, self)
+
+
+# Local variables:
+# python-indent: 4
+# End:
diff --git a/src/greaseweazle/codec/ibm/ibm.py b/src/greaseweazle/codec/ibm/ibm.py
index 92cf26b8..425a70b8 100644
--- a/src/greaseweazle/codec/ibm/ibm.py
+++ b/src/greaseweazle/codec/ibm/ibm.py
@@ -97,6 +97,19 @@ def decode(dat):
 
 crc16 = crcmod.predefined.Crc('crc-ccitt-false')
 
+# Create logical sector map in rotational order
+def sec_map(nsec: int, interleave: int, cskew: int, hskew: int,
+            cyl: int, head: int) -> List[int]:
+    sec_map, pos = [-1] * nsec, 0
+    if nsec != 0:
+        pos = (cyl*cskew + head*hskew) % nsec
+    for i in range(nsec):
+        while sec_map[pos] != -1:
+            pos = (pos + 1) % nsec
+        sec_map[pos] = i
+        pos = (pos + interleave) % nsec
+    return sec_map
+
 def sec_sz(n):
     return 128 << n if n <= 7 else 128 << 8
 
@@ -762,16 +775,6 @@ def sec_n(i):
         t.time_per_rev = 60 / rpm
         t.clock = t.time_per_rev / tracklen_bc
 
-        # Create logical sector map in rotational order
-        sec_map, pos = [-1] * nsec, 0
-        if nsec != 0:
-            pos = (cyl*config.cskew + head*config.hskew) % nsec
-        for i in range(nsec):
-            while sec_map[pos] != -1:
-                pos = (pos + 1) % nsec
-            sec_map[pos] = i
-            pos = (pos + config.interleave) % nsec
-
         pos = gap4a
         if gap1 is not None:
             pos += t.gap_presync
@@ -780,11 +783,11 @@ def sec_n(i):
 
         id0 = config.id
         h = head if config.h is None else config.h
-        for i in range(nsec):
-            sec = sec_map[i]
+        for sec in sec_map(nsec, config.interleave,
+                           config.cskew, config.hskew, cyl, head):
             pos += t.gap_presync
             idam = IDAM(pos*16, (pos+synclen+4+2)*16, 0xffff,
-                        c = cyl, h = h, r= id0+sec, n = sec_n(sec))
+                        c = cyl, h = h, r = id0+sec, n = sec_n(sec))
             pos += synclen + 4 + 2 + gap2 + t.gap_presync
             size = 128 << idam.n
             datsz = size*2 if mark_dam == Mark.DAM_DEC_MMFM else size
diff --git a/src/greaseweazle/data/diskdefs.cfg b/src/greaseweazle/data/diskdefs.cfg
index 32a16192..d34fceeb 100644
--- a/src/greaseweazle/data/diskdefs.cfg
+++ b/src/greaseweazle/data/diskdefs.cfg
@@ -599,6 +599,30 @@ disk gem.1600
     end
 end
 
+# HP 9885 Flexible Disk Drive
+disk hp.mmfm.9885
+    cyls = 67
+    heads = 1
+    tracks * hp.mmfm
+        secs = 30
+        interleave = 4
+        cskew = 4
+        hskew = 1
+    end
+end
+
+# HP 9895 (and 7902) Flexible Disk Drive
+disk hp.mmfm.9895
+    cyls = 77
+    heads = 2
+    tracks * hp.mmfm
+        secs = 30
+        interleave = 4
+        cskew = 4
+        hskew = 1
+    end
+end
+
 disk ibm.160
     cyls = 40
     heads = 1