Ubuntu Manpage: srec_fpc - four packed code file format

NAME

       srec_fpc - four packed code file format

SYNOPSIS

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 may be used 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 formats.

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.

Records
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 │
--

Example

       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!
              D81400000000B000576F77212044696420796F7520726561
              431400000000B0106C6C7920676F207468726F7567682061
              361400000000B0206C6C20746861742074726F75626C6520
              591100000000B030746F207265616420746869733F000000
              00000000
       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.
              $kL&@h%%,:,B.\?00EPuX0K3rO0JI))
              $;UPR'%%,:<Hn&FCG:at<GVF(;G9wIw
              $7FD1p%%,:LHmy:>GTV%/KJ7@GE[kYz
              $B[6\;%%,:\KIn?GFWY/qKI1G5:;-_e
              $%%%%%
       As you can see the length of the lines is clearly shorter than the original ASCII lines.

AUTHOR

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

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

NAME

SYNOPSIS

Example

SEE ALSO

AUTHOR