sd2iec - a controller/interface adapting storage devices to the CBM serial bus
Copyright (C) 2007-2012  Ingo Korb <ingo@akana.de>
Parts based on code from others, see comments in main.c for details.
JiffyDos send based on code by M.Kiesel
Fat LFN support and lots of other ideas+code by Jim Brain
Final Cartridge III fastloader support by Thomas Giesel

Free software under GPL version 2 ONLY, see comments in main.c and
COPYING for details.

FIXME:This file still needs to be expanded. A lot.

Deprecation notices
===================
The following feature(s) will be removed in the next release:
- M2I support
   M2I support has been redundant since the introduction of transparent
   P00 support. To continue to use your M2I-format software, convert
   your files to P00 format (e.g. with m2itopc64.c) and set your device
   to extension mode 2 (XE2).

Introduction:
=============
sd2iec is firmware, used in hardware designs like MMC2IEC, SD2IEC, or uIEC,
that allows the Commodore serial bus to access removable storage devices
(MMC, SD, CF) - think of it as a 1541 with a modern storage medium instead
of disks. The project was inspired by (and uses a few bits of code from)
MMC2IEC[1] by Lars Pontoppidan and once ran on the same hardware before it
grew too big for the ATmega32 used there.

Currently, the firmware provide good DOS and file-level compatibility with CBM
drives, but much work remains.  Unless specifically noted, anything that tries
to execute code on the 1541 will not work, this includes every software
fastloader.

[1] Homepage: http://pontoppidan.info/lars/index.php?proj=mmc2iec

Please note: Whenever this file talks about "D64 images" the text applies to
all Dxx image types, i.e. D64/D71/D81/DNP unless specifically noted.

If you are the author of a program that needs to detect sd2iec for
some reason, DO NOT use M-R for this purpose. Use the UI command
instead and check the message you get for "sd2iec" and "uiec" instead.


Supported commands:
===================
- General notes:
  Any command not listed below is currently not supported.

- Directory filters:
  To show only directories, both =B (CMD-compatible) and =D can be used.
  On a real Commodore drive D matches everything.
  To include hidden files in the directory, use *=H - on a 1541 this doesn't
  do anything. sd2iec marks hidden files with an H after the lock mark,
  i.e. "PRG<H" or "PRG H".

  CMD-style "short" and "long" directory listings with timestamps are supported
  ("$=T"), including timestamp filters. Please read a CMD manual for the syntax
  until this file is updated.

- Partition directory:
  The CMD-style partition directory ($=P) is supported, including filters
  ($=P:S*). All partitions are listed with type "FAT", although this could
  change to "NAT" later for compatibility.

- CD/MD/RD:
  Subdirectory access is compatible to the syntax used by the CMD drives,
  although drive/partition numbers are completely ignored.

  Quick syntax overview:
    CD:_         changes into the parent dir (_ is the left arrow on the C64)
    CD_          dito
    CD:foo       changes into foo
    CD/foo       dito
    CD//foo      changes into \foo
    CD/foo/:bar  changes into foo\bar
    CD/foo/bar   dito

  You can use wildcards anywhere in the path. To change into an M2I or D64
  image the image file must be named after the :, it will not be recognized
  otherwise.

  MD uses a syntax similiar to CD and will create the directory listed
  after the colon (:) relative to any directory listed before it.

    MD/foo/:bar  creates bar in foo
    MD//foo/:bar creates bar in \foo

  RD can only remove subdirectories of the current directory.

    RD:foo       deletes foo

  CD is also used to mount/unmount image files. Just change into them
  as if they were a directory and use CD:_ (left arrow on the C64) to leave.
  Please note that image files are detected by file extension and file size
  and there is no reliable way to see if a file is a valid image file.

- CP, C<Shift-P>
  This changes the current partition, see "Partitions" below for details.

- C:
  File copy command, should be CMD compatible. The syntax is
   C[partition][path]:targetname=[[partition][path]:]sourcename[,[[p][p]:]sourcename...]
  You can use this command to copy multiple files into a single target
  file in which case all source files will be appended into the target
  file. Parsing restarts for every source file name which means that
  every source name is assumed to be relative to the current directory.
  You can use wildcards in the source names, but only the first
  file matching will be copied.

  Copying REL files should work, but isn't tested well. Mixing REL and
  non-REL files in an append operation isn't supported.

- D
  Direct sector access, this is a command group introduced by sd2iec.
  Some Commodore drives use D for disk duplication between two drives
  in the same unit, an attempt to use that command with sd2iec should
  result in an error message.

  D has three subcommands: DI (Info), DR (Read) and DW (Write).
  Each of those commands requires a buffer to be opened (similiar
  to U1/U2), but due to the larger sector size of the storage devices
  used by sd2iec it needs to be a large buffer of size 2 (512 bytes)
  or larger. The exception is the DI command with page set to 0,
  its result will always fir into a standard 256 byte buffer.
  If you try to use one of the commands with a buffer that is too
  small a new error message is returned, "78,BUFFER TOO SMALL,00,00".

  In the following paragraphs the secondary address that was used
  to open the buffer is called "bufchan".

  - DI
    In BASIC notation the command format is
      "DI"+chr$(bufchan)+chr$(device)+chr$(page)

    "device" is the number of the physical device to be queried,
    "page" the information page to be retrieved. Currently the
    only page implemented is page 0 which will return the
    following data structure:
     1 byte : Number of valid bytes in this structure
              This includes this byte and is meant to provide
              backwards compatibility if this structure is extended
              at a later time. New fields will always be added to
              the end so old programs can still read the fields
              they know about.
     1 byte : Highest diskinfo page supported
              Always 0 for now, will increase if more information
              pages are added (planned: Complete ATA IDENTIFY
              output for IDE and CSD for SD)
     1 byte : Disk type
              This field identifies the device type, currently
              implemented values are:
                0  IDE
                2  SD
                3  Dataflash
     1 byte : Sector size divided by 256
              This field holds the sector size of the storage device
              divided by 256.
     4 bytes: Number of sectors on the device 
              A little-endian (same byte order as the 6502) value
              of the number of sectors on the storage device.
              If there is ever a need to increase the reported
              capacity beyond 2TB (for 512 byte sectors) this
              field will return 0 and a 64-bit value will be added
              to this diskinfo page.

    If you want to determine if there is a device that responds
    to a given number, read info page 0 for it. If there is no
    device present that corresponds to the number you will see
    a DRIVE NOT READY error on the error channel and the
    "number of valid bytes" entry in the structure will be 0.

    Do not assume that device numbers are stable between releases
    and do not assume that they are continuous either. To scan
    for all present devices you should query at least 0-7 for now,
    but this limit may increase in later releases.

  - DR/DW
    In BASIC notation the command format would be
      "DR"+chr$(bufchan)+chr$(device)
          +chr$(sector AND 255)
          +chr$((sector/256) AND 255)
          +chr$((sector/65536) AND 255)
          +chr$((sector/16777216) AND 255)
    (or "DW" instead of "DR)
    but this won't work on the C64 because AND does not accept
    parameters larger than 32767. The principle should be clear
    though, the last four bytes are a 32 bit sector number in
    little-endian byte order.

    DR reads the sector to the buffer, DW writes the contents
    of the buffer to the sector. Both commands will update the
    error channel if an error occurs, for DR the 20,READ ERROR
    was chosen to represent read errors; for write problems
    during DW it sets 25,WRITE ERROR for errors and 26,WRITE
    PROTECT ON if the device is read-only.

- G-P
  Get partition info, see CMD FD/HD manual for details. The reported
  information is partially faked, feedback is welcome.

- P
  Positioning doesn't just work for REL files but also for regular
  files on a FAT partition. When used for regular files the format
  is "P"+chr$(channel)+chr$(lo)+chr$(midlo)+chr$(midhi)+chr$(hi)
  which will seek to the 0-based offset hi*2^24+midhi*65536+256*midlo+lo
  in the file. If you send less than four bytes for the offset, the
  missing bytes are assumed to be zero.

- N:
  Format works only if a D64 image is already mounted. This command is
  limited to 1541 disk images, usually known as .D64.

- R
  Renaming files should work the same as it does on CMD drives, although
  the errors flagged for invalid characters in the name may differ.

- S:
  Name matching is fully supported, directories are ignored.
  Scratching of multiple files separated by , is also supported with no
  limit to the number of files except for the maximum command line length
  (usually 100 to 120 characters).

- T-R and T-W
  If your hardware features RTC support the commands T-R (time read) and T-W
  (time write) are available. If the RTC isn't present, both commands return
  30,SYNTAX ERROR,00,00; if the RTC is present but not set correctly T-R will
  return 31,SYNTAX ERROR,00,00.

  Both commands expect a fourth character that specifies the time format
  to be used, T-W expects that the new time follows that character in EXACTLY
  the format returned by T-R with the same format char. Do NOT add a space
  between the "T-W" and the time string.

  The possible formats are:
   - "A"SCII: "SUN. 01/20/08 01:23:45 PM"+CHR$(13)
      The day-of-week string can be any of "SUN.", "MON.", "TUES", "WED.",
      "THUR", "FRI.", "SAT.". The year field is modulo 100.
   - "B"CD or "D"ecimal:
     Both these formats use 9 bytes to specify the time. For BCD everything
     is BCD-encoded, for Decimal the numbers are sent/parsed as-is.
      Byte 0: Day of the week (0 for sunday)
           1: Year (modulo 100 for BCD; -1900 for Decimal, i.e. 108 for 2008)
           2: Month (1-based)
           3: Day (1-based)
           4: Hour   (1-12)
           5: Minute (0-59)
           6: Second (0-59)
           7: AM/PM-Flag (0 is AM, everything else is PM)
           8: CHR$(13)

      When the time is set a year less than 80 is interpreted as 20xx.

- U0
  Device address changing with "U0>"+chr$(new address) is supported,
  other U0 commands are currently not implemented.

- U1/U2/B-R/B-W
  Block reading and writing is fully supported while a D64 image is mounted.

- B-P
  Supported, not checked against the original rom at all.

- UI+/UI-
  Switching the slightly faster bus protocol for the VC20 on and off works,
  it hasn't been tested much though.

- UI/UJ
  Soft/Hard reset - UI just sets the "73,..." message on the error channel,
  UJ closes all active buffers but doesn't reset the current directory,
  mounted image, swap list or anything else.

- U<Shift-J>
  Real hard reset - this command causes a restart of the AVR processor
  (skipping the bootloader if installed). <Shift-J> is character code 202.

- X: Extended commands. If you use JiffyDOS, you can send them by using
  @"X..." - without quotes you'll just receive an error.

  - XEnum    Sets the "file extension mode". This setting controls if
             files on FAT are written with an x00 header and extension or not.
             Possible values for num are:
               0: Never write x00 format files.
               1: Write x00 format files for SEQ/USR/REL, but not for PRG
               2: Always write x00 format files.
               3: Use SEQ/USR/REL file extensions, no x00 header
               4: Same as 3, but also for PRG
             If you set mode 3 or 4, extension hiding is automatically enabled.
             This setting can be saved in the EEPROM using XW, the default
             value is 1.

             For compatibility with existing programs that write D64 files,
             PRG files that have D64, D41, D71, D81, DNP or M2I as an extension
             will always be written without an x00 header and without
             any additional PRG file extension.

  - XE+/XE-  Enable/disable extension hiding. If enabled, files in FAT with
             a PRG/SEQ/USR/REL extension will have their extension removed
             and the file type changed to the type specified by the file
             extension - e.g. APPLICATION.PRG will become a PRG file named
             "APPLICATION", "README.SEQ" will become a SEQ file named "README".
             This flag can be saved in the EEPROM using XW, the default
             value is disabled (-).

  - XInum    Switches the display mode for mountables files (i.e. disk images
             and M2I). num can be 0, in which case the file will be shown
             with its normal type in the directory or 1 which will show all
             mountable files as DIRectory entries (but they can still be
             accessed as files too) or 2 in which case they will show up
             twice - once with its normal type and once as directory.
             The default value is 0 and this setting can be stored
             permanently using XW.

             It may be useful to set it to 1 or 2 when using software that
             was originally written for CMD devices and which wouldn't
             recognize disk images/M2I files as mountable on its own.
             However, due to limitations of the current implementation of
             the CD command such software may still fail to mount a disk
             image with this option enabled.

  - XB+/XB-  Enable/disable free block calculation for FAT32 drives.
             As the free block calculation for FAT32 can take a lot of time
             it can be disabled using XB-. If it is disabled, sd2iec will
             always report "1 BLOCKS FREE" for FAT32 drives. The free block
             calculation on FAT12/FAT16 isn't affected because it takes just
             two seconds at most. This flag can be saved in the EEPROM using
             XW, the default value is enabled (+).

  - X*+/X*-  Enable/disable 1581-style * matching. If enabled, characters
             after a * will be matched against the end of the file name.
             If disabled, any characters after a * will be ignored.
             This flag can be saved in the EEPROM using XW, the default value
             is enabled (+).

  - XDdrv=val Configure drives.  On ATA-based units or units with multiple
             drive types, this command can be used to enable or reorder
             the drives.  drv is the drive slot (0-7), while val is one
             of:
                0: Master ATA device
                1: Slave ATA device
                4: Primary SD/MMC device
                5: Secondary SD/MMC device
                6: Dataflash device
               15: no device

             Note that only devices supported by the specific hardware
             can be selected.  Unsupported device types will return an
             error if requested.  Also, note that you cannot select a device
             in multiple drive slots.  Finally, while it is possible to
             re-order ATA devices using this functionality, it is strongly
             discouraged.  Use the master/slave jumpers on the ATA devices
             instead.  To reset the drive configuration, set all drive slots
             to "no device".  This value can be permanently saved in the
             EEPROM using XW.

    XD?      View the current drive configuration.  Example result:
             "03,D:00=04:01=00:02=01,10,01".  The track indicates the
             current device address, while the sector indicates extended
             drive configuration status information.

  - X        X without any following characters reports the current state
             of all extended parameters via the error channel, similiar
             to DolphinDOS. Example result: "03,J-:C152:E01+:B+:*+,08,00"
             The track indicates the current device address.

  - XS:name  Set up a swap list - see "Changing Disk Images" below.
    XS       Disable swap list

  - XR:name  Set the file used for file-based M-R emulation.
    XR       Disable file-based M-R emulation.
             See "M-R, M-W, M-E" below. This setting can be
             permanently saved in the EEPROM using XW.

  - XW       Store configuration to EEPROM
             This commands stores the current configuration in the EEPROM.
             It will automatically be read when the AVR is reset, so
             any changes you made will persist even after turning off
             the hardware.

             The stored configuration include the extension mode,
             drive configuration and the current device address.
             If you have changed the device address by software,
             sd2iec will power up with that address unless
             you have changed the device address jumpers (if available) to
             a different setting than the one active at the time the
             configuration was saved. You can think of this feature as
             changing the meaning of one specific setting of the jumpers
             to a different address if this sounds logical enough to you.

             The "hardware overrides software overrides hardware" priority
             was chosen to allow accessing sd2iec even when it is soft-
             configured for a device number that is already taken by
             another device on the bus without having to remove that
             device to reconfigure sd2iec (e.g. when using a C128D).

  - X?       Extended version query
             This commands returns the extended version string which
             consists of the version, the processor type set at build time
             and the suffix of the configuration file (usually corresponds
             to the short name of the hardware sd2iec was compiled for).

- M-R, M-W, M-E
  When no file is set up using XR, M-R will check a small internal
  table of common drive-detection addresses and return data that
  forces most of the supported fast loaders into a compatible mode
  (e.g. 1541 mode for Dreamload and ULoad Model 3, disabled fastloader
  for Action Replay 6). If the address is not recognized, more-or-less
  random data will be returned.

  Unfortunately GEOS reads rather large parts of the drive rom using
  M-R to detect the drive, which cannot be reasonably added into the
  internal table. To enable the GEOS drive detection to work properly
  with sd2iec and to allow switching between 1541/71/81 modes,
  file-based M-R emulation has been implemented. If a file has been
  set up as M-R data source using the XR command, its contents will be
  returned for M-R commands that try to read an address in the range
  of $8000-$ffff. The rom file should be a copy of the rom contents of
  a 1541/71/81 drive (any headers will be skipped automatically), its
  name must be 16 characters or less. When an M-R command is received,
  the file will be searched in three locations on the storage medium:
    1) in the current directory of the current partition
    2) in the root directory of the current partition
    3) in the root directory of the first partition

  The internal emulation table will be used if the file wasn't found
  in any of those locations or an error occured while reading
  it. Please be aware that the rom file is ONLY used for M-R
  commands. Except for some very specific situations where drive
  detection fails (e.g. GEOS) it will probably decrease compatibility
  of sd2iec because most of the implemented fast loaders will only
  recognize the 1541 variation of the loader.

  Memory writing knows about the address used for changing the device
  address on a 1541 and will change the address of sd2iec to the
  requested value. It will also check if the transmitted data
  corresponds to any of the known software fastloaders so the correct
  emulation code can be used when M-E is called.

- E-R, E-W
  NOTICE: These commands will be removed in the next release.

  Both commands work like M-R and M-W, but instead of reading/writing RAM
  they allow access to a user-area of the EEPROM. This area currently holds
  512 bytes and accesses beyond its end will result in a 32 SYNTAX ERROR.
  It is strongly recommended to work on a protocol for sharing this area
  between multiple applications that want to store their configuration in
  there, but that is beyond the scope of this project.

  As the contents of the EEPROM have to be copied to RAM before they can
  be sent to the computer it is not possible to read more data with a
  single command than the error message buffer (default size: 36 bytes)
  can hold. Similiarly, writing is restricted by the size of the command
  buffer (at least 42 bytes for compatibility, expected to be at least
  100 bytes in release versions).

  The user-area does not interfere with the stored configuration (XW) in
  any way.

Large buffers:
==============
To support commands which directly access the storage devices support
for larger buffers was added. A large buffer can be allocated by
opening a file named "##<d>" (exactly three characters" with <d> replaced
by a single digit specifying the number of 256-byte buffers to be
chained into one large buffer - e.g. "##2" for a 512 byte buffer,
"##4" for 1024 bytes etc. Unlike a standard buffer where the read/write
pointer is set to byte 1, a large buffer will start with the r/w pointer
pointing to byte 0 because that seems to be more sensible to the author.

If there aren't enough free buffers to support the size you requested
a 70,NO CHANNEL message is set in the error channel and no file is
opened. If the file name isn't exactly three bytes long a standard
buffer ("#") will be allocated instead for compatibility.

The B-P command supports a third parameter that holds the high byte
of the buffer position, For example, "B-P 9 4 1" positions to byte
260 (1*256+4) of the buffer on secondary address 9.

No command is implemented at this time which uses large buffers.

Long File Names:
================
Long file names (i.e names not within the 8.3 limits) are supported on
FAT, but for compatibility reasons the 8.3 name is used if the long
name exceeds 16 characters. If you use anything but ASCII characters
on the PC or their PETSCII equivalents on the Commodore you may
get strange characters on the other system because the LFN use
unicode characters on disk, but sd2iec parses only the low byte
of each character in the name.

Partition:
==========
sd2iec features a multi-partition support similiar to that of the CMD
drives. The partitions (which may be on separate drives for some hardware
configurations) are accessed using the drive number of the commands
sent from the computer and are numbered starting with 1. Partition 0
is a special case: Because most software doesn't support drive numbers
or always sends drive number 0, this partition points to the currently
selected partition. By default, accesses to partition 0 will access
partition 1, this can be changed by sending "CP<num>" over the command
channel with <num> being an ASCII number from 1 to 255. "C<Shift-P"
(0x42 0xd0) works the same, but expects a binary partition number as the
third character of the command.

Software fastloaders:
=====================
Note: Using sd2iec without an external crystal or similiar precise
      clock source is not a supported configuration.
      If you try that anyway, be prepared to suffer from random
      data corruption. You have been warned.
      Some fastloader implementations will actively refuse to work
      if you use an unsuitable clock source.

  Turbodisk
  ---------
  Turbodisk is detected by the CRC of its 493 byte long floppy code and
  the M-E address 0x0303. The same code seems to be used under various names,
  among them "Turbodisk" (both 2.1 and 2.2) and "Fast-Load".
  It is not known if there is an NTSC-compatible version of this fastloader.

  Final Cartridge III
  -------------------
  Both the fast loader and the fast saver of Final Cartridge III are supported.
  The FC3 is both PAL and NTSC compatible.

  The slightly different fastloader used for files freezed with the FC3
  is also supported.

  EXOS V3 and The Beast System
  ----------------------------
  Both supported, the loader used by these kernals is very similiar to
  the FC3 fast loader.

  Action Replay 6
  ---------------
  The AR6 reads a byte from the drive rom to check which fastloader it should
  use. When file-based M-R emulation is disabled sd2iec returns a value that
  should force the cartridge to use the standard kernal loader instead of its
  many fastloaders/-savers. This means that accessind sd2iec with
  file-based rom emulation enabled will fail because the cartridge
  will enable fastloader that will probably not be recognized.

  Currently the only recognized AR6 fastloader and fastsaver are the
  ones for the 1581.

  Dreamload
  ---------
  Dreamload uses direct track/sector access, so it is only supported
  on D64 or similiar disk image formats. As sd2iec has to wait for commands
  from the C64 constantly the disk change buttons may become unresponsive,
  try multiple times if you need to. Dreamload is a "captive" fastloader,
  sd2iec stay in Dreamload mode until it receives a "quit loader" command
  from the C64. To force sd2iec to resume normal operation, hold the disk
  change button until the red LED turns on (just like sleep mode).

  Please note that Dreamload does not work with more than one device on the
  serial bus due to the way it uses the ATN line.

  ULoad Model 3
  -------------
  ULoad Model 3 uses direct track/sector access, so it is only supported
  on D64 or similiar disk image formats. Currently there is exactly one
  supported variant of ULoad Model 3, which is the one used by
  Ultima 3 Gold. There are no other known variants at this time, but
  this may change.

  If you are a coder and want to use ULoad Model 3 in your own program,
  either configure it to produce the same drive code as U3Gold or
  contact me so we can work out a way to trigger ULoad M3 support
  without uploading any drive code at all.

  G.I. Joe Loader
  ---------------
  Said to be the most-ripped IRQ loader. Unfortunately this is a
  "captive" fastloader similiar to dreamload (but not restricted
  to disk images because it is file name-based) and there is no
  reliable way to detect if the computer has been reset to switch
  back to the standard protocol. To exit this loader, hold down
  the disk change button until the red LED turns on, just like
  sleep mode.

  Epyx FastLoad Cartridge
  -----------------------
  ONLY the fast loader from this cartridge is supported, no
  disk editor/copier/whatever functions.

  GEOS
  ----
  GEOS 2.0 can be booted from D64 images made from original disks
  as well as D41/71/81 images created using geoMakeBoot (make sure to
  Configure the system for a 1541/1571/1581 before using geoMakeBoot).
  When file-based M-R emulation is disabled, GEOS may detect sd2iec as
  a 1541 or 1581, depending on the version of Configure used. This may
  cause the system to fail to boot, e.g. if sd2iec is detected as a 1581
  while booting from a D64 disk image. It is recommended to set up file-
  based M-R emulation when using GEOS to avoid these problems.

  GEOS 1.3 may or may not work - it boots, but wasn't tested in-depth.
  Gateway seems to work but was not tested beyond booting it from a D64
  image.

  Using the buttons for changing the current disk image is supported,
  but do make sure that you only access disk images that the drive
  type that is selected in GEOS would support (i.e. D64 for a 1541,
  D64/D71 for a 1571 and D81 for a 1581).

  Wheels
  ------
  Wheels can be booted from any disk image type it supports. The correct
  rom emulation file (XR) MUST be set, especially for CMD HD emulation.

  Do not use the disk change feature to change disk images when HD emulation
  is in use - Wheels does not check for disk changes on that drive!
  For other drive types the restrictions on disk image type of GEOS also
  apply to Wheels.

  ELoad Version 1
  ---------------
  This loader was made for EasyProg but may also be used in other programs.
  It detects and supports the sd2iec natively.

JiffyDOS:
=========
The JiffyDOS protocol has very relaxed timing constraints compared to
Turbodisk, but still not as relaxed as the standard Commodore IEC protocol.

x00 files:
==========
P00/S00/U00/R00 files are transparently supported, that means they show
up in the directory listing with their internal file name instead of the
FAT file name. Renaming them only changes the internal name. The XE
command defines if x00 extensions are used when writing files, by
default sd2iec uses them for SEQ/USR/REL files but not for PRG.
Parsing of x00 files is always enabled even when writing them is not.

x00 files are recognized by checking both the extension of the file
(P/S/U/R with a two-digit suffix) and the header signature.

Disk Images:
============
Disk images are recognized by their file extension (.D64, .D41, .D71, .D81,
.DNP) and their file size (must be one of 174848, 175531, 349696, 351062,
819200 or a multiple of 65536 for DNP). If the image has an error info block
appended it will be used to simulate read errors. Writing to a sector with
an error will always work, but it will not clear the indicated error.
D81 images with error info blocks are not supported.

Warning: There is at least one program out there (DirMaster v2.1/Style by
THE WIZ) which generates broken DNP files. The usual symptom is that
moving from a subdirectory that was created with this program back to
its parent directory using CD:_ (left arrow) sets the current directory
not to the parent directory, but to an incorrect sector instead. A
workaround for this problem in sd2iec would require an unreasonable
amount of system resources, so it is recommended to avoid creating
subdirectories with this version of DirMaster. It is possible to fix
this problem using a hex editor, but the exact process is beyond the scope
of this document.

M2I files:
==========
NOTICE: Support for M2I files will be removed in the next release, see
        the deprecation notices at the top of this file for advice.

M2I files are fully supported. sd2iec supports SEQ and USR files in this
format in addition to PRG and DEL which were already implemented in MMC2IEC.
For compatibility reasons the file type is not checked when opening files.
Inside an M2I file the files are always shown as 0 (DEL) or 1 blocks
because calling stat for every file was slowing down the directory listing
too much. For compatibility with existing M2I files the data files do not
use P00 headers even when the file type is SEQ or USR.

REL files:
==========
Partial REL file support is implemented. It should work fine for existing
files, but creating new files and/or adding records to existing files
may fail. REL files in disk images are not supported yet, only as files
on a FAT medium. When x00 support is disabled the first byte of a REL
file is assumed to be the record length.

Changing Disk Images
====================
Because some programs require more than one disk side there is support
for changing the currently mounted disk image with a button connected
to the disk change pin.

If your circuit doesn't have a disk change pin/button you might be able to
add it yourself:
- For the original MMC2IEC:
  Connect a button from PA4 to ground. PA4 is pin 36 on the DIL version
  of the controller or pin 33 on the surface-mount version.
- For Shadowolf's MMC2IEC 1.x PCBs:
  Connect a button from PC4 to ground. PC4 is pin 23 on the DIL version
  of the controller or pin 23 on the surface-mount version.
- Any other circuit without disk change pin on a convenient connector
  somewhere and no button dedicated to that function: Please check
  with the supplier of the board and read config.h in the sources
  to find out how to connect it.

To use this functionality, create a text file that lists the file
names of all disk images you want to swap between, one per line.
The file names are parsed in the same way as the CD command, so you
can include a path to the image if desired.

Examples:
=== example 1 ===
FOO.D64
BAR.D64
BAZ.D64
=== end of example 1 ===

=== example 2 ===
//NEATGAME/:DISK1A.D64
//NEATGAME/:DISK1B.D64
//NEATGAME/:DISK2A.D64
//NEATGAME/:DISK2B.D64
=== end of example 2 ===

The swap list is enabled by sending "XS:filename" over the command channel
with filename being the name of the image list you created, parsed
in the same way as any other file name.

After sending XS the first image in the list is automatically
mounted. To switch to the next image in the list, push the button. If
the new image was mounted successfully both LEDs will blink twice.
When you've reached the last image in the list pushing the button will
mount the first image again. All of this is completely compatible with
normal image mounting/unmounting, so you can unmount the disk image
any time you want and resume the mount cycle later by pushing the
button.

Due to the way this feature is implemented you are not limited to a
swap list containing just D64 images, M2I and even FAT directories
will work too.  FIXME: Does that still work?

If you press the button when no list has been set before or when the
previous list was cleared by sending XS the software will look for a
file called AUTOSWAP.LST in the current (FAT-)directory and use this
as the current swap list until you deactivate it or manually change
the directory (otherwise an AUTOSWAP.LST in the new directory would be
ignored until you send XS, killing the nice "it just works" feeling).

FIXME: Integrate the following into the preceding

The second disk change button is on
- PA5 for LarsP
- PC3 for Shadowolf's MMC2IEC 1.x
- PC2 for Shadowolf's sd2iec 1.x ("Reserve" on the header)
- PG4 for uIEC

Either of those buttons will trigger the use of AUTOSWAP.LST.
If a swap list is already active, the first button will switch
to the next image in the list, the second button will switch
to the previous image in the list and pushing both buttons
together will switch to the first image in the list.

The confirmation blink is red+green followed by
green for "next", by red for "previous" and by
red+green for "first".

Sleep Mode:
===========
If you hold the disk change button down for two seconds, sd2iec will
enter "sleep mode". In this mode it doesn't listen to the bus at all
until you hold down the disk change button for two seconds again
which resumes normal operation. Sleep mode allows you to keep
sd2iec connected to the serial bus even when you load something
from a different drive that uses a fast loader that doesn't
work with more than one device on the bus.

While sleep mode is active, the red LED will be on and the green LED
will be off.

Card detection test:
====================
Because some SD slots seem to suffer from bad/unreliable card detect
switches a test mode for this has been implemented on the units that
have SD card support. If you hold down the PREV button during powerup,
the red (dirty) LED will reflect the card detect status - if the LED
is on the card detect switch is closed. Please note that this does not
indicate successful communication with the card but merely that the
mechanical switch in the SD card slot is closed.

On units with two sd2iec-controlled LEDs, the green (busy) LED will
indicate the state of the write protect switch - if the LED is lit,
the write protection is on. Due to the way the write protect notch
works on SD cards, the indication is only valid when the card is fully
inserted into the slot.

To exit from the diagnostic mode, power-cycle the device or push the
NEXT button once.

Other important notes:
======================
- When you hold down the disk change (forward) button during power
  on the software will use default values instead of those stored
  in the EEPROM.
- File overwrite (@foo) is implemented by deleting the file first.
- File sizes in the directory are in blocks (of 254 bytes), but
  the blocks free message actually reports free clusters. It is
  a compromise of compatibility, accuracy and code size.
- If known, the low byte of the next line link pointer of the directory
  listing will be set to (filesize MOD 254)+2, so you can calculate the
  true size of the file if required. The 2 is added so it can never be
  mistaken for an end marker (0) or for the default value (1, used by at
  least the 1541 and 1571 disk drives).
- If your hardware supports more than one SD card, changing either one
  will reset the current partition to 1 and the current directory of
  all partitions to the root drive. Doing this just for the card that
  was changed would cause lots of problems if the number of partitions
  on the previous and the newly inserted cards are different.

Compilation notes:
==================
sd2iec requires avr-libc version 1.6.x.

sd2iec is set up to be compiled in multiple configurations, controlled by
configuration files. By default the Makefile looks for a file named
'config', but you can override it by providing the name on the make
command line with "make CONFIG=filename".

An example configuration file named "config-example" is provided with
the source code, as well as abridged files corresponding to the
release binaries. If you want to compile sd2iec for a custom hardware
you may have to edit config.h too to change the port definitions.