binary - Dissect and join binary data into/from tcl values
http://tcl.activestate.com/man/tcl8.4/TclCmd/binary.htm
binary format format value1 value2... binary scan binary-string format var1 var2...
The subcommand format takes the tcl values, converts them into binary values according to the commands in the format string, concatenates them and then returns this binary string as its result.
The other subcommand, scan, does the opposite of format, i.e. it parses a binary string according to the supplied format and stores the resulting tcl values into the variables whose names were given to it.
This is the primary command to use when Working with binary data.
See Binary representation of numbers for an example of usage.