C2PB

[mark pruitt]

I've found a fair amount of information about CRC for DTC/DTE communications.
Most of it is written in C or C+. The below psuedo code creates a look up
table. The help file makes it easy to find corresponding PB data types, but I'm
not familiar enough with C syntax to get anything more from this code than it looks
like two "FOR/NEXT" loops (and I'm not real sure about that). If someone could
translate this to PowerBASIC syntax I would very much appreciate it.

unsigned short a = 0
unsigned short b[256]={0}
unsigned short c[8] = {0x0001, 0x0002, 0x0003, 0x0004, 0x0005, 0x0006, 0x0007, 0x0008}

{
int d, e, f
for (d=0; d<256; d++)
{
for (b[d]=0, f = 0x0000, e=0; e<8; e++)
{
if(d & f)b[d]^=c[e];
f<<=1;
}
}
a=0;
Return 0;
}



------------------
Mark Pruitt
[email protected]



[This message has been edited by mark pruitt (edited June 11, 2003).]

[Kev Peel]

I think this is a pretty close translation, the variables a, b and c are probably global, d, e and f are local. I put the code in a PbMain() function for testing. Note also, I started the arrays at zero as this is what C does. You'll have to test whether the results are correct, though!

Global a        As Integer
Global b()      As Integer
Global c()      As Integer


Function PBMain

 Dim d As Long, e As Long, f As Long

 ReDim b(0:255)
 ReDim c(0:7)
 
 c(0) = 1
 c(1) = 2
 c(2) = 3
 c(3) = 4
 c(4) = 5
 c(5) = 6
 c(6) = 7
 c(7) = 8

 For d = 0 To 255

     For e = 0 To 7
        If (d And f) Then b(d) = b(d) Or c(e)
        Shift Left f, 1
     Next e

 Next d
 a = 0
 Function = 0

End Function

------------------
Kev Peel
KGP Software (Custom Programming Solutions), UK.

[mark pruitt]

Thanks Kev! As soon as I saw you had posted, I thought of your header
conversion tool. Do you know if it would give the same results?

------------------
Mark Pruitt
[email protected]

[Kev Peel]

Hi Mark, no unfortunately the hdr2inc program only converts equates, function declarations and UDT's to PowerBASIC. I've always wanted to write a CPP2PB translator but that would mean writing a "psuedo-compiler" to be accurate enough

------------------
Kev Peel
KGP Software (Custom Programming Solutions), UK.

[Cecil L. Williams]

Without seeing the whole code, variables a, b and c might not be
global. They may only have file scope in which case you've got
a possible prob somewhere down the line.

If the variables were global, the "extern" or "extern C"
definition would be present.

C/C++ arrays are ALWAYS indexed at zero(0).

Hope this helps,
Cecil

------------------

[Paul Noble]

I think 'unsigned short' should strictly be a WORD in PowerBASIC, though I'm not sure if that
makes any difference to these calcs. Mind you, if they really are globals, it could be significant.

Also, I think the first line inside the loop needs to be an XOR, not an OR, ie:

If (d And f) Then b(d) = b(d) xor c(e)

--Paul



------------------
Zippety Software, Home of the Lynx Project Explorer
http://www.zippety.net
mailto[email protected][email protected]</A>

[mark pruitt]

I finally had a chance to try this out. Special thanks to Kev Peel for the initial
translation, and Paul, for pointing out "XOR" instead of "OR". I changed variable
names, commented, etc. to make the code more meaningful.

    'CRC-16 look up table
    '
    'initial translation from C by Kev Peel
    '
    'tweaked by Mark Pruitt
    '
    'feel free to use, modify, etc
    '
    '**********************************************************************************************************************


    #COMPILE EXE

    FUNCTION PBMAIN ()

    GLOBAL CrcReg%           'global variable to hold current value of CRC Register
    GLOBAL table% ()         'global look up table
    LOCAL c% ()              'increments a counter from 0 to 7
    LOCAL d$                 'added for testing, can be commented out

    LOCAL i??, j??, mask??

    REDIM table%(0 TO 255)

    REDIM c%(0 TO 7)
            c%(0) = &h1001   'put hex values here to create custom CRC table
            c%(1) = &h2002
            c%(2) = &h4004
            c%(3) = &h8008
            c%(4) = &h1111
            c%(5) = &h2222
            c%(6) = &h4444
            c%(7) = &h9008

    FOR i?? = 0 TO 255             'first "FOR" loop; increments table%()

            table%(i??) = 0        'initial settings for each element of table%()

            mask?? = &h0001

                FOR j?? = 0 TO 7    'second "FOR" loop; this one creates the data for table%()

                IF(i?? AND mask??) THEN table%(i??) = table%(i??) XOR c%(j??)
            SHIFT LEFT mask??, 1
                    NEXT j??
                        d$ = STR$(table%(i??)) 'this line and next line added for testing only
                           PRINT d$
                NEXT i??

            CrcReg% = 0           'set global variable to initial value
            FUNCTION = 0          'function is done
            WAITKEY$              'added for testing purposes; can be commented out
            END FUNCTION


------------------
Mark Pruitt
[email protected]

[This message has been edited by mark pruitt (edited June 12, 2003).]