::: Area #68 (Comms............ [Acorn])
Message: #2561 (Read 17 times, has 0 replies, 1500 bytes)
Date   : Tue Nov 11 08:27:50 1997
From   : Matthew Page of fidonet#2:2504/112
To     : Jonathan Harston
Subject: Re: XModem-CRC Calculation

Hi,

As it happens I have just finished writing some Xmodem code for a project
I'm working on. So here is the snippet of CRC checking code. You will need
to call the PROCass first. You then pass the whole of the block to FNcrc
giving it's length.

DEF FNcrc(block%,length%)
    B%=block%:C%=length%
=USR(crc)

DEF PROCass
    LOCAL pass%
    DIM code% 1024
    FOR pass%=8 TO 10 STEP 2
    P%=code%
    L%=code%+1024
    [OPT pass%
    .crc
     MOV     R3,#0
     MOV     R4,R1          
     MOV     R5,R2             
    .crcl1
     LDRB    R2,[R4],#1     
     EOR     R3,R3,R2,LSL #8    
     MOV     R1,#8       
    .crcl2
     TST     R3,#&8000       
     MOV     R3,R3,LSL#1       
     EORNE   R3,R3,#&1000        
     EORNE   R3,R3,#&0021      
     SUBS    R1,R1,#1            
     BNE     crcl2                
     SUBS    R5,R5,#1            
     BNE     crcl1                
     MOV     R1,#&FF       
     ORR     R1,R1,#&FF00
     AND     R0,R3,R1
     MOV     PC,R14
    ]
    NEXT
ENDPROC

I hope this helps.

See Ya Round
(Y)atthew
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