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
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
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 $, all other characters
are digits of base 85.
ss The checksum. A one byte 2’s‐complement checksum of all bytes
of the record.
cc The byte‐count. A one byte value, counting all the bytes in
the record minus 4.
ffff Format code, a two byte value, defining the record type.
The address field. A 4 byte number representing the first
address of this record.
The actual data of this record.
Every record begins with the ASCII character "$". 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.
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.
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
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
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
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.
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
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.
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
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
For extra points: Who invented this format? Where is it used?