Add a reverse-engineered checksum routine, and some documentation.

README.md

@@ -81,6 +81,7 @@ physical disks with these formats and so know they work.
 | [Commodore 64 1541](doc/disk-c64.md)     |  🦖   |        | and probably the other GCR formats  |
 | [Brother 120kB](doc/disk-brother.md)     |  🦄   |        |                                     |
 | [Brother 240kB](doc/disk-brother.md)     |  🦄   |   🦄   |                                     |
+| [Brother FB-100](doc/disk-fb100.md)      |  🦖   |        | Tandy Model 100, Husky Hunter, knitting machines |
 | [Macintosh 800kB](doc/disk-macintosh.md) |  🦖   |        | and probably the 400kB too          |
 | [TRS-80](doc/disk-trs80.md)              |  🦖   |        | a minor variation of the IBM scheme |
 {: .datatable }
@@ -100,6 +101,7 @@ at least, check the CRC so what data's there is probably good.
 | [Victor 9000](doc/disk-victor9k.md)      |  🦖   |        | 8-inch        |
 | [Zilog MCZ](doc/disk-zilogmcz.md)        |  🦖   |        | 8-inch _and_ hard sectors |
 {: .datatable }
+
 ### Notes
 
   - IBM PC disks are the lowest-common-denominator standard. A number of other

doc/disk-fb100.md

@@ -0,0 +1,32 @@
+Disk: Brother FB-100
+====================
+
+The Brother FB-100 is a serial-attached smart floppy drive used by a several
+different machines for mass storage, including the Tandy Model 100 and
+clones, the Husky Hunter 2, and (bizarrely) several knitting machines. It was
+usually rebadged, sometimes with a cheap paper label stuck over the Brother
+logo: the most common variant appears to be the Tandy Portable Disk Drive or
+TPDD:
+
+<a href="http://www.old-computers.com/museum/computer.asp?c=233&st=1"> <img src="tdpp.jpg" alt="A Tandy Portable Disk Drive"/></a>
+
+It's a bit of an oddball: the disk encoding is FM with a very custom record
+scheme: 40-track single-sided 3.5" disks storing 100kB or so each. Each track
+had only _two_ sectors, each 1280 bytes, but with an additional 17 bytes of
+ID data used for filesystem management.
+
+There was also apparently a TPDD-2 which could store twice as much data, but
+I don't have access to one of those disks.
+
+Reading discs
+-------------
+
+Just do:
+
+```
+.obj/fe-readfb100
+```
+
+You should end up with an `fb11.img` of the appropriate size. It's a simple
+array of 80 1297-byte sectors (17 bytes for the ID record plus 1280 bytes for
+the data).

lib/fb100/decoder.cc

@@ -30,6 +30,89 @@ static bool search(const RawBits& rawbits, size_t& cursor)
     return false;
 }
 
+/* 
+ * Reverse engineered from a dump of the floppy drive's ROM. I have no idea how
+ * it works.
+ * 
+ * LF8BA:
+ *         clra
+ *         staa    X00B0
+ *         staa    X00B1
+ *         ldx     #$8000
+ * LF8C2:  ldaa    $00,x
+ *         inx
+ *         bsr     LF8CF
+ *         cpx     #$8011
+ *         bne     LF8C2
+ *         ldd     X00B0
+ *         rts
+ * LF8CF:
+ *         eora    X00B0
+ *         staa    X00CF
+ *         asla
+ *         asla
+ *         asla
+ *         asla
+ *         eora    X00CF
+ *         staa    X00CF
+ *         rola
+ *         rola
+ *         rola
+ *         tab
+ *         anda    #$F8
+ *         eora    X00B1
+ *         staa    X00B0
+ *         rolb
+ *         rolb
+ *         andb    #$0F
+ *         eorb    X00B0
+ *         stab    X00B0
+ *         rolb
+ *         eorb    X00CF
+ *         stab    X00B1
+ *         rts
+ */
+
+static void rol(uint8_t& b, bool& c)
+{
+    bool newc = b & 0x80;
+    b <<= 1;
+    b |= c;
+    c = newc;
+}
+
+static uint16_t checksum(const Bytes& bytes)
+{
+    uint8_t crclo = 0;
+    uint8_t crchi = 0;
+    for (uint8_t a : bytes)
+    {
+        a ^= crchi;
+        uint8_t t1 = a;
+        a <<= 4;
+        bool c = a & 0x10;
+        a ^= t1;
+        t1 = a;
+        rol(a, c);
+        rol(a, c);
+        rol(a, c);
+        uint8_t b = a;
+        a &= 0xf8;
+        a ^= crclo;
+        crchi = a;
+        rol(b, c);
+        rol(b, c);
+        b &= 0x0f;
+        b ^= crchi;
+        crchi = b;
+        rol(b, c);
+        b ^= t1;
+        crclo = b;
+    }
+
+    return (crchi << 8) | crclo;
+}
+
 void Fb100Decoder::decodeToSectors(const RawBits& rawbits, unsigned,
     RawRecordVector& rawrecords, SectorVector& sectors)
 {
@@ -58,14 +141,19 @@ void Fb100Decoder::decodeToSectors(const RawBits& rawbits, unsigned,
         br.seek(1);
         const Bytes id = br.read(FB100_ID_SIZE);
         uint16_t wantIdCrc = br.read_be16();
+        uint16_t gotIdCrc = checksum(id);
         const Bytes payload = br.read(FB100_PAYLOAD_SIZE);
         uint16_t wantPayloadCrc = br.read_be16();
+        uint16_t gotPayloadCrc = checksum(payload);
+
+        if (wantIdCrc != gotIdCrc)
+            continue;
 
         uint8_t abssector = id[2];
         uint8_t track = abssector >> 1;
         uint8_t sectorid = abssector & 1;
 
-        int status = Sector::BAD_CHECKSUM;
+        int status = (wantPayloadCrc == gotPayloadCrc) ? Sector::OK : Sector::BAD_CHECKSUM;
         auto sector = std::unique_ptr<Sector>(
             new Sector(status, track, 0, sectorid, payload));
         sectors.push_back(std::move(sector));