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TI Emulator!                                                   4/15/1994
v 5.0

written by Edward Swartz
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     This file describes the format of the emulated TI files used in TI
Emulator! v5.0.

     First, the filename of a TI file is practically the same as on the
TI, but it has changes to accomodate MS-DOS restrictions.  Of the ten
characters, the last two will be put into the extension if necessary.
Also, several illegal DOS characters -- <>=,;:*?[]()/\!~ -- will be
changed by adding 80h to their ASCII values.  This may make it difficult
to type the filename from DOS... but these ARE TI files.
     For your convenience, the most common TI filename characters that
are illegal in DOS, '-' and '/', will be changed into '­' and '¯'.  The
'­' character is IBM ASCII 173 and '¯' is code 175.  Just type the code
on the numeric keypad with the Alt key held down to get these
characters.

     The actual format of the file tries to follow the sectorized format
used on the TI.  All sectors are 256 bytes long.  All files, though,
start with a 128-byte record describing the file attributes.  These
first 128 bytes of a TI Emulator! file are copied directly from the TI's
File Descriptor Record (FDR) for that file.  Those bytes are, if you
want to know:

byte 00-09 : Filename.  Unused chars are spaces.
     10-11 : unused
     12    : File Status Flags (the file type)
          
               Bits:
               0 = program (1) / data (0)         (1h)
               1 = internal (1) / display (0)     (2h)
               2 = unused                         (4h)
               3 = write protected                (8h)
                  In the TI DSR, this bit write-protects the file.         
                  Under TI Emulator!, this is implemented using the DOS    
                  write-protect bit.  If the bit happens to be set, it     
                  will be ignored.
               4 = unused                         (10h)
               5 = unused                         (20h)
               6 = unused                         (40h)
               7 = variable (1) / fixed (0)       (80h)

               Putting these together gives these types:
               >00 = DIS/FIX       >01 = PROGRAM
               >02 = INT/FIX       >80 = DIS/VAR
               >82 = INT/VAR       

               On newer systems, the 10h bit is used in combination        
               with backups. (Like the archive bit in DOS files.)  TI      
               Emulator! doesn't use this bit.

     13    : Number of records per sector.  = 256/Record Length,
             even for variable files.
     14-15 : Number of Sectors Used.  In TI byte order.
     16    : EOF marker for variable and program files.
     17    : Record length (0 in program files)
     18-19 : Number of fixed records OR number of sectors used by          
             variable files.  In Intel byte-order.
     20-27 : unused (date/time in newer systems)
     28-255: unused in TI Emulator! v5.0 but really encoded pointers to
             the sectors used by the file.

     
     Then, the sectors in the file follow directly afterward in one
contiguous block.  A TI Emulator! file should always be (n*256)+128
bytes long, where n is the value in bytes 14-15.

     Under the TI Device Service Routine (DSR), no records are ever
broken over a sector boundary.  So, a fixed file with record length of
129 bytes will waste 127 bytes a sector.  TI Emulator! follows this
standard.  (It's wasteful, but it's compatible!)

     In variable length record files, each record consists of a length
byte and the data.  A length of zero is legal, and the record is only
the zero byte.  Up to 255 records may be stored in one sector (all
length 0, of course).  In a sector, all records directly follow each
other except when a record cannot fit in the remaining space in a
sector.  The byte where the new record would have gone is set to >FF to
signify end-of-sector.  (It is called the end-of-file marker, but this
really only signifies the end of the sector.)  To detect end-of-file, a
combination of the end-of-sector marker and the <Number of Sectors Used>
record are used.  An EOF marker must appear in a sector, so the maximum
number of bytes in a sector that can be used in a non-program file is
255.
     In fixed length record files, only <Number of Records Per Sector>
records will be stored in one sector.  Less than this number of records
may only appear in the last sector.  When a new sector is allocated, the
first record always starts at byte 0, and others follow contiguously.
The longest record length is 255 bytes; however, this doesn't mean that
all the bytes in a sector can't be used:  a fixed file with a length of
64 characters can have exactly four records in each sector.

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