
*** X64 (X64 and VICE emulator image files)
*** Document revision 1.1

  This file type, created by Teemu Rantanen, is used on the X64 emulator (a
UNIX-based emulator) which has been superceeded by VICE. Both Vice and  X64
support the X64 file standard, with Vice also supporting  the  regular  D64
and T64 files.

  X64 is not a specific type of file, but rather  encompasses  *all*  known
C64 disk types (hard disk, floppies, etc). An X64 is created by  prepending
a 64-byte header to an  existing  emulator  image  (1541,  1571,  etc)  and
setting specific bytes which describe what  type  of  image  follows.  This
header has undergone some revision, and this description file is  based  on
the 1.02 version, which was the last known at the time of writing.

  The most common X64 file you will  see  is  the  D64  variety,  typically
174912 bytes long (174848 for the D64 and 64 bytes for the header, assuming
no error bytes are appended). The header layout (as used in 64COPY)  is  as
follows:

      00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F        ASCII
      -----------------------------------------------   ----------------
0000: 43 15 41 64 01 02 01 23 00 00 00 00 00 00 00 00   C.Ad...#תתתתתתתת
0010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   תתתתתתתתתתתתתתתת
0020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   תתתתתתתתתתתתתתתת
0030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   תתתתתתתתתתתתתתתת
0040: XX XX XX                     <- standard C64 image starts here....

   Bytes:$00-03: This is the "Magic header" ($43 $15 $41 $64)
             04: Header version major ($01)
             05: Header version minor ($01, now its up to $02)
             06: Device type represented ($00)
                  $00=1540 See note below...
                  $01=1541 (Default)
                  $02=1542
                  $03=1551
                  $04=1570
                  $05=1571
                  $06=1572
                  $08=1581
                  $10=2031,4031
                  $11=2040,3040
                  $12=2041
                  $18=4040
                  $20=8050
                  $21=8060
                  $22=8061
                  $30=SFD-1001
                  $31=8250
                  $32=8280
             07: Maximum tracks in image (only in version 1.02 or greater)
                  1540/41/70:  35
                        1571:  35
                        1581:  80 (Logical single-sided disk)
             08: Number of disk sides in image. This value must be $00  for
                 all 1541 and 1581 formats.
                  $00=No second side
                  $01=Second side
             09: Error data flag. I assume that if this  location  has  any
                 non-zero value in it, this will indicate the existance  of
                 the error bytes. For a description of the error bytes, see
                 the D64 topic dealing with extended formats.
          0A-1F: Unused, set to $00
          20-3E: Disk image description (in ASCII or ISO Latin/1)
             3F: Always set to $00
            40-: Standard C64 file begins here

  The first four bytes used for the device type at  position  $06  ($00  to
$03) are functionally the same, and are compatible with  older  version  of
X64 files. Some old X64 files might have $00 for the device  type  (instead
of $01), but it makes no real difference.

  As most instances of X64 files will be strictly 1541 images, bytes  08-3F
are set to zero, and some versions of the  X64  emulator  don't  use  bytes
08-3F.

  There is no advantage for PC users to use this format since virtually  no
PC emulator that I know of uses them, and for the most  part,  it  provides
the same functionality as a D64 file.

  In order to read a generic X64 file, first you must determine that it  is
an X64, and then determine the type of file it  contains.  In  effect,  you
have  to  double-decode  the  file,  which  makes  support  a  little  more
difficult. Also, you would have to be able to work with *all* of the  types
of drives that X64 supports, a daunting task.

  Its only advantage is that  is  encompasses  all  of  the  standard  disk
formats on the C64. If other disk types  were  common  (like  1581  or  CMD
disks), then this format might be more popular.

