Not realy nice yet just quick and splashes of blood.
   * The scanning could be replaced with "binary scan".
   * Unix <> Dos path handling is nonexistent.
   * The DateTimes in the archive format are ignored.
   * Files are extracted to uppercase names.

   * Files split across multiple archive parts are patched
together in the right way if one expands into an empty
filepath.

   * Start with dumping all files from the disk set into the current directory.

The archive format description is taken from "Technical Documentation for FreeDOS Backup & Restore" [http://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/dos/backup/brtecdoc.htm]

 #!/usr/bin/tclsh

 set ptype U
 set pre_path ".\\restore\\"
 set do 1
        set idx 0

 while {1} {
        set bidx [ format %03d [ incr idx ] ]
        set backup  backup.$bidx
        set control control.$bidx

        set stfd [ open $backup r ]
        fconfigure $stfd -encoding binary -translation binary

        set ctrlfd [ open $control r ]
        fconfigure $ctrlfd -encoding binary -translation binary
        set index [ read $ctrlfd ]

        set len [string length $index] ; puts stderr indexlen:$len

        set pos 0

        while {$pos < $len} {
                set act [string index $index $pos ]
                scan $act %c val
                puts -nonewline stderr [format "0x%02x " $val]
                switch -- $act  \
                  \x8b {
                        puts stderr "Header"
                        set headerentry [ string range $index $pos [ incr pos 0x8b ] ]
                        set hd_signature [ string range $headerentry 1 7 ]
                        scan [ string range $headerentry 9 9 ] %c hd_backno
                        set hd_filler    [ string range $headerentry 10 137 ]
                        scan [ string range $headerentry 138 138 ] %c hd_lastdisk
                        puts stderr [format "\t\"%s\" %d %d\n" $hd_signature $hd_backno $hd_lastdisk]
                } \x46 {
                        puts stderr "Dir Entry"
                        set direntry [ string range $index $pos [ incr pos 0x46 ] ]
                        set dir_path [ string range $direntry 1 63 ]
                        set dir_path [ string trimright ${pre_path}$dir_path \x0 ]
                        set dir_fcnt [ string range $direntry 64 65 ]
                        binary scan  $dir_fcnt s dir_fcnt
                        set dir_dummy [ string range $direntry 66 69 ]
                        puts stderr [format "\t\"%s\" %d \n" $dir_path $dir_fcnt ]
                        if {$do} {
                                if { $ptype== "U" } {
                                        set dir_path [ string map [ list \\ /] $dir_path ]
                                }
                                puts stderr "trying: mkdir $dir_path"
                                catch [ file mkdir $dir_path ]
                        }
                } \x22 {
                        puts stderr "File Entry"
                        set fileentry [ string range $index $pos [ incr pos 0x22 ] ]
                        set file_name [ string range $fileentry 1 12 ]
                        set file_name [ string trimright $file_name \x0 ]
                        scan [string range $fileentry 13 13 ] %c file_split
                        set file_size [ string range $fileentry 14 17 ]
                          binary scan  $file_size i file_size

                        set file_cidx [ string range $fileentry 18 19 ]
                          binary scan  $file_cidx s file_cidx
                        set file_coffs [ string range $fileentry 20 23 ]
                          binary scan  $file_coffs i file_coffs
                        set file_clen [ string range $fileentry 24 27 ]
                          binary scan  $file_clen i file_clen
                        scan [ string range $fileentry 28 28 ] %c file_attrib
                        set file_dummy [ string range $fileentry 29 29 ]
                        set file_dtime [ string range $fileentry 30 33 ]
                          binary scan  $file_dtime i file_dtime
                        puts stderr [format "\t\"%s\" %d %d %08x:%08x %02x %08x\n" \
                                        $file_name $file_size $file_cidx \
                                        $file_coffs $file_clen $file_attrib \
                                        $file_dtime \
                        ]
                        if {$do} {
                                seek $stfd $file_coffs start
                                set file_cont [ read $stfd $file_clen ]
                                puts stderr "trying: open [ file join $dir_path $file_name ]"
                                set fd [ open [ file join $dir_path $file_name ] a ]
                                fconfigure $fd -encoding binary -translation binary
                                puts -nonewline $fd $file_cont
                                close $fd
                        }
                } \x00 {
                        puts stderr "End of File"
                        incr pos
                }
                puts -nonewline stderr next? ; # gets stdin
        }
        close $stfd
        close $ctrlfd

 }