Related Signature Analysis-Type Stuff

For those interested, here's another checksum algorithm designed by BT in
the UK, called a Frame Check Sequence number (FCS).

It's not quite as secure as a CRC, but it is very close and it is much, much
easier to calculate. (You can make out the algorithm from the code).

I implemented it in Z80 assembly using a Forth cross-compiler (i.e. it's all
backwards) for some embedded stuff I was working on.

A CRC on the other hand, is much harder to calculate. See the comparable
listing at the end. It also shows my start value as 8408.

I'm a bit confused myself now, actually, as the CRC code seems to process
the whole 16 bit old CRC in one go (or at least it would if the Z80 was a
true 16 bit processor), which is a bit different from running 1 bit at a
time. I'll hae to do some testing...when I get some time!

Phil.

\ --------------------------------------------------------------------------
-
\ FCS
\
\ Descripton: Piece of Z80/H64180 code to implement the FCS algorithm as
\ defined by British Telecom New Networks Technical Forum -
\ Document CP(83)/12 04/02/83. Call with the old FCS and data
\ byte to be FCS'd and a new FCS is returned. Note the 'exx'
\ at the start and end of the code - MPE Z80 forth crashes if
\ you use the standard BC registers. You should be able to
\ remove these if you are using any other compiler.
\ To parse a complete buffer of information, you need an
\ external loop which calls this function for each data byte.
\ FCS = File Check Sequence
\
\ Parameters: fcs_c0c1 - Old FCS to modify.
\ char - Value to add to FCS.
\
\ Returns: new_fcs_c0c1 - New FCS.
\
\ --------------------------------------------------------------------------
-

Code FCS \ fcs_c0c1 char --- new_fcs_c0c1 ;

        EXX

        BC POP \ Contains new data byte in C
        DE POP \ Contains old FCS, D=c0, E=c1

        A, D LD \ Calc new c0, c0=c0+char
        A, C ADD
       NC, fcs1 JP \ If result > $ff then
        A INC \ add 1 to result
L: fcs1 D, A LD \ Store back to c0

        A, E LD \ Calc new c1, c1=c1+c0
        A, D ADD
       NC, fcs2 JP \ If result > $ff then
        A INC \ add 1 to result
L: fcs2 E, A LD \ Store back to c1

        DE PUSH

        EXX
        NEXT,

End-Code

************ CRC CODE STARTS HERE ***********
HEX

$8408 equ CRC-Seed \ Good initial value for CRC calculation.

Code CRC-16 \ crc char --- new_crc ;

    EXX

    BC POP \ Contains new data byte in C
    DE POP \ Contains old CRC
    HL, # 0 LD \ Define Work Area

    A, E LD \ XOR bottom 8 bits of old CRC with data byte
    A, C XOR
    E, A LD

    A, H LD \ Swap bottom 8 bits of CRC with top 8 bits of
    H, E LD \ Work Area
    E, A LD

    A, D LD \ Swap top 8 bits of CRC with bottom 8 bits
    D, E LD
    E, A LD

    PE, L1 JP \ If parity of XOR even, miss out the CRC XOR

    A, D LD \ XOR the CRC with C001H
    A, # C0 XOR
    D, A LD

    A, E LD
    A, # 01 XOR
    E, A LD

L: L1

    SCF \ Right shift the CRC and then XOR it with work area
    CCF

    H RR
    L RR

    A, D LD
    A, H XOR
    D, A LD

    A, E LD
    A, L XOR
    E, A LD

    SCF \ Right shift the CRC and then XOR it with work area
    CCF \ again

    H RR
    L RR

    A, D LD
    A, H XOR
    D, A LD

    A, E LD
    A, L XOR
    E, A LD

    DE PUSH \ Shove the new CRC back on the stack

    EXX
    NEXT,

End-code
Received on Wed May 08 16:54:23 2002