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       srec_fpc - four packed code file format


       All  ASCII based file formats have one disadvantage in common: they all
       need more than double the amount of characters as opposed to the number
       of  bytes  to be sent.  Address fields and checksums will add even more
       characters.  So the shorter the records, the more characters have to be
       sent to get the file across.

       The  FPC format helps to reduce the number of characters needed to send
       a file in ASCII format, although it still needs  more  characters  than
       the  actual  bytes  it  sends.  FPC stands for "Four Packed Code".  The
       reduction is accomplished by  squeezing  4  real  bytes  into  5  ASCII
       characters.   In fact every ASCII character will be a digit in the base
       85 number system.  There aren’t enough letters, digits and  punctuation
       marks  available  to  get  85  different characters, but if we use both
       upper case and lower case letters we will manage.   This  implies  that
       the  FPC  is  case  sensitive, as opposed to all other ASCII based file

   Base 85
       The numbering system is in base 85, and is somewhat hard to  understand
       for us humans who are usually only familiar with base 10 numbers.  Some
       of us understand base 2 and base 16 as well, but base 85  is  for  most
       people  something  new.  Luckily we don’t have to do any math with this
       number system.  We just convert a 32 bit number into a 5  digit  number
       in  base  85.   A 32 bit number has a range of 4,294,967,296, while a 5
       digit number in base 85 has a range of 4,437,053,125, which  is  enough
       to do the trick.  One drawback is that we always have to send multiples
       of 4 bytes, even if we actually want to send 1, 2 or 3  bytes.   Unused
       bytes are padded with zeroes, and are discarded at the receiving end.

       The digits of the base 85 numbering system start at %, which represents
       the value of 0.  The highest value of a digit in base 85 is 84, and  is
       represented  by  the  character  ’z’.  If you want to check this with a
       normal ASCII table you will notice that we have used one character  too
       many!   Why?  I don’t know, but for some reason we have to skip the ’*’
       character in the row.  This means that after the ’)’ character  follows
       the ’+’ character.

       We  can  use  normal  number  conversion algorithms to generate the FPC
       digits, with this tiny difference.  We have to check whether the  digit
       is  going  to be equal or larger than the ASCII value for ’*’.  If this
       is the case we have to increment the digit once to stay  clear  of  the
       ’*’.  In base 85 MSD digits go first, like in all number systems!

       The  benefit  of this all is hopefully clear. For every 4 bytes we only
       have to send 5 ASCII characters, as opposed to  8  characters  for  all
       other formats.

       Now  we  take a look at the the formatting of the FPC records.  We look
       at the record at byte level, not at the actual base 85  encoded  level.
       Only  after  formatting the FPC record at byte level we convert 4 bytes
       at a time to a 5 digit base 85 number.  If we don’t have  enough  bytes
       in the record to fill the last group of 5 digits we will add bytes with
       the value of 0 behind the record.

                     |$ | ss | cc | ffff | aaaaaaaa | dddddddd |
       The field are defined-as:--+------+----------+----------+

       $       Every line starts with the character $,  all  other  characters
               are digits of base 85.

       ss      The  checksum.  A one byte 2’s‐complement checksum of all bytes
               of the record.

       cc      The byte‐count.  A one byte value, counting all  the  bytes  in
               the record minus 4.

       ffff    Format code, a two byte value, defining the record type.

               The  address  field.   A  4  byte number representing the first
               address of this record.

               The actual data of this record.

   Record Begin
       Every record begins with the ASCII character "$".  No  spaces  or  tabs
       are allowed in a record.  All other characters in the record are formed
       by groups of 5 digits of base 85.

   Checksum field
       This field is a one byte 2’s‐complement checksum of the entire  record.
       To  create  the checksum make a one byte sum from all of the bytes from
       all of the fields of the record:

       Then take the 2’s‐complement of this sum to create the final  checksum.
       The 2’s‐complement is simply inverting all bits and then increment by 1
       (or using  the  negative  operator).   Checking  the  checksum  at  the
       receivers  end  is  done  by  adding  all  bytes together including the
       checksum itself, discarding all carries, and the result  must  be  $00.
       The padding bytes at the end of the line, should they exist, should not
       be included in checksum.  But it doesn’t really matter if they are, for
       their influence will be 0 anyway.

   Byte Count
       The  byte  count  cc  counts  the number of bytes in the current record
       minus 4.  So only the number of address bytes and the  data  bytes  are
       counted  and  not the first 4 bytes of the record (checksum, byte count
       and format flags).  The byte count can have any value from 0 to 255.

       Usually records have 32 data bytes.  It is not recommended to send  too
       many data bytes in a record for that may increase the transmission time
       in case of errors.  Also avoid  sending  only  a  few  data  bytes  per
       record, because the address overhead will be too heavy in comparison to
       the payload.

   Format Flags
       This is a 2 byte number, indicating what format is represented in  this
       record.   Only a few formats are available, so we actually waste 1 byte
       in each record for the sake of having multiples of 4 bytes.

       Format code 0 means that the address field in  this  record  is  to  be
       treated as the absolute address where the first data byte of the record
       should be stored.

       Format code 1 means that the address field in this record  is  missing.
       Simply  the  last  known  address  of the previous record +1 is used to
       store the first data byte.  As if the FPC  format  wasn’t  fast  enough
       already ;‐)

       Format  code  2  means  that  the address field in this record is to be
       treated as a relative address.  Relative to what is not  really  clear.
       The relative address will remain in effect until an absolute address is
       received again.

   Address Field
       The first data byte of the record is stored in the address specified by
       the  Address field aaaaaaaa.  After storing that data byte, the address
       is incremented by 1 to point to the address for the next data  byte  of
       the record.  And so on, until all data bytes are stored.

       The  length  of  the  address  field  is  always 4 bytes, if present of
       course.  So the address range for the FPC format is always 2**32.

       If only the address field is given, without any data bytes, the address
       will be set as starting address for records that have no address field.

       Addresses between records are non sequential.  There may be gaps in the
       addressing  or the address pointer may even point to lower addresses as
       before in the same file.  But every time  the  sequence  of  addressing
       must be changed, a format 0 record must be used.  Addressing within one
       single record is sequential of course.

   Data Field
       This field contains 0 or more data bytes.  The actual  number  of  data
       bytes  is  indicated  by  the byte count in the beginning of the record
       less the number of address bytes.  The first data byte is stored in the
       location indicated by the address in the address field.  After that the
       address is incremented by 1 and the next data byte is  stored  in  that
       new location.  This continues until all bytes are stored.  If there are
       not enough data bytes to obtain a multiple of 4 we use 0x00 as  padding
       bytes at the end of the record.  These padding bytes are ignored on the
       receiving side.

   End of File
       End of file is recognized if the first four bytes  of  the  record  all
       contain  0x00.   In  base  85  this will be “$%%%%%”.  This is the only
       decent way to terminate the file.

   Size Multiplier
       In general, binary data will expand in sized by approximately 1.7 times
       when represented with this format.


       Now  it’s time for an example.  In the first table you can see the byte
       representation of the file to be transferred.  The 4th row of bytes  is
       not a multiple of 4 bytes.  But that does not matter, for we append $00
       bytes at the end until we do have a multiple of 4 bytes.  These padding
       bytes are not counted in the byte count however!
       Only  after  converting  the bytes to base 85 we get the records of the
       FPC type file format presented in the next table.  Note that  there  is
       always a multiple of 5 characters to represent a multiple of 4 bytes in
       each record.
       As you can see the length of the lines  is  clearly  shorter  than  the
       original ASCII lines.



       This man page was taken from the above Web page.  It was written by San
       Bergmans <>

       For extra points: Who invented this format?  Where is it used?