16-bit FCS Computation Method

[Whammo]

require io

$f0b8 constant good

: ftab 8 0 do dup 1 and if
1 rshift $8408 xor
else 1 rshift then loop ;

: tomem $ff 0 do
i ftab $c000 i 2* + ! loop ;

: fcstab 2* $c000 + @ ;

: fcs ( fcs c -- newfcs c )
swap 2dup xor $ff and fcstab
swap 8 rshift xor swap ;

: rfgc ( -- fcs )
parse-name 8 8 open ioabort
clrchn 8 chkin ioabort
chrin chrin 2drop $ffff
begin chrin fcs drop readst until
clrchn 8 close ;


: aoop ( fcs -- msb lsb )
dup invert 8 rshift
swap invert $ff and ;

: coip ( msb lsb fcs -- fcs )
swap fcs drop swap fcs drop ;


: pppfcs16 ( fcs caddr u -- fcs caddr )
0 do tuck i + c@ fcs drop swap
loop ;

: addfcs16 ( caddr u -- )
dup >r $ffff -rot pppfcs16
over invert $ff and
over r@ + c!
swap invert 8 rshift
swap r> 1+ + c! ;

: tryfcs ( caddr u+2 -- )
$ffff -rot
pppfcs16 drop good = if ." good"
else ." bad" then cr ;


: fcstest \ fcstest filename
rfgc dup aoop rot coip u. ;

[jkotlinski]

What is FCS? Forth Check Sum?

…

On Thu, 15 Jul 2021 at 14:06, Whammo ***@***.***> wrote: require io $f0b8 constant good \ rfc1662 c.2. : ftab 8 0 do dup 1 and if1 rshift $8408 xorelse 1 rshift then loop ; \ computes FCS table entry for byte : tomem 256 0 doi ftab $c000 i 2* + ! loop ; \ places FCS table in memory : fcstab 2* $c000 + @ ; \ address table entry per byte and stack : fcs ( fcs c -- newfcs c ) swap 2dup xor $ff and fcstab swap 8 rshift xor swap ; \ calculates checksum given the previous checksum and the current byte : rfgc ( -- fcs ) \ Read File Get Checksum --- rfgc filename parse-name 8 8 open ioabort clrchn 8 chkin ioabort chrin chrin 2drop $ffffbegin chrin fcs drop readst until clrchn 8 close ; : aoop ( fcs -- msb lsb ) \ Add On OutPut inverted FCS as byte stream method dup invert 8 rshift swap invert $ff and ; : coip ( msb lsb fcs -- fcs ) \ Calculate On InPut method swap fcs drop swap fcs drop ; : pppfcs16 ( fcs caddr u -- fcs caddr ) \ checksum memory range0 do tuck i + c@ fcs drop swaploop ; : addfcs16 ( caddr u -- ) \ init checksum, do memory, append checksum dup >r $ffff -rot pppfcs16 over invert $ff and over r@ + c! swap invert 8 rshift swap r> 1+ + c! ; : tryfcs ( caddr u+2 -- ) \ check memory checksum $ffff -rot pppfcs16 drop good = if ." good"else ." bad" then cr ; — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#356>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAY34O2CBNV2342VPTXQTGDTX3FMXANCNFSM5ANNALYQ> .

[Whammo]

PPP Frame Check Sequence RFC 1662

[Whammo]

It's a checksum that returns one value ($f0b8) when it's correct. Anything else is incorrect.