Re: [Sdcc-user] Calculating IP Checksum

[Rod B. <ro...@bo...>]

Your are not think variable size here.  When doing a 16-bit checksum you set the variable overflow.  This is haw it is done is hardware.  Add all the bytes up and throw away the overflow but make sure you use a 16-bit variable.

If you want to do an 8-bit checksum use a 8-bit variable.  Same for 32-bit checksums.  The overflow is surposed to happen.

(unsigned short)0xfff5 + (unsigned char)0x0f = (unsigned short)0x0004 or something like that.  I haven't had my morning coffee yet.

Regards,
Rod

On Thu, 13 Mar 2003 11:37:23 -0500
"Olaniyi Bajomo" <zu...@em...> wrote:

> WEll yes I have tried to do it in C but it don't work casue I can perform 2's complement addition in C,my main problem is how to deal with an overflow?
> 
> Any ideas?
> 
> thanks 
> 
> 
> 
> >Ever thought of trying it with plain C ?
> >Best regards,
> 
> >Michael
> 
> > -----Urspr=FCngliche Nachricht-----
> > Von: sdc...@li...=20
> 
> > [mailto:sdc...@li...]
>  Im Auftrag von=20
> > Olaniyi Bajomo
> > Gesendet: Donnerstag, 13. M=E4rz 2003 15:45
> > An: sdc...@li...
> 
> > Betreff: [Sdcc-user] Calculating IP Checksum
> >=20
> >=20
> > Hello guys/Gals I have a small problem. I want biuld an=20
> > TCP/IP packet in my code and calculate the checksum for IP=20
> > and the TCP. but for some reason I can't get my checksum=20
> > program to so it. All it those is perfrom 16bit 2's=20
> > complement addition of a buffer that contains the IP pseudo=20
> > header, the TCP header and data. but I think there is a=20
> > problem when ever the summation results in 0xff in any of my=20
> > 8bit result registers as I dont think this is treated as a=20
> > carry or am I wrong? I have the code  attached to=20
> > this email. if anyone has a better idea as to how to do this=20
> > p>lease let me know. thanks
> >=20
> > ------------------------CODE STRIP-----------------------------
> >=20
> > code unsigned int ip_checksum(xdata u_int16_t *Ip_Pak, int=20
> > pckLength) {
> > 	Ip_Pak;
> > 	pckLength;
> > // first 16bit set to r1 r2
> > // second 16bit set to r5 r6
> > // pckLength set to _ip_checksum_PARM_2
> >   _asm
> > =09
> > 	mov r3, _ip_checksum_PARM_2
> > 	mov r4, (_ip_checksum_PARM_2+1)
> > 	mov a ,r3    // divide lenght by 2
> > 	clr c        // clear carry
> > 	rrc a        // shift lower length by 1 to right
> > 	mov r3,a     // save lower
> > 	mov a,r4   =20
> > 	clr c
> > 	rrc a         // shift high length by 1 to right
> > 	mov r4,a      // save higher
> > 	movx a, @dptr
> > 	mov r1,a      // get high byte of 16 bit numer
> > 	inc dptr
> > 	movx a, @dptr
> > 	mov r2,a	   // get lower byte of 16bit number
> > 	dec r3
> >  loop:
> > 	inc dptr
> > 	movx a, @dptr =20
> > 	mov r5, a      // get next number
> > 	inc dptr
> > 	movx a, @dptr
> > 	add a,r2       // add lower bytes
> > 	mov r2,a
> > 	;clr a
> > 	mov a,r1
> > 	addc a,r5     // add carrys
> > 	;add a,r5      // add higher byte
> > 	mov r1,a      // store addition=20
> > 	clr a
> > 	addc a,r2     // add carry to lower byte
> > 	mov r2,a
> > 	djnz r3 , loop
> > 	cjne r4,#0x00,high
> >   finish:
> >      mov a, r1
> > 	 cpl a
> > 	 mov dph,a
> > 	 mov a, r2
> > 	 cpl a
> > 	 mov dpl,a
> > 	 ajmp end
> >   high:
> >     djnz r4,loop
> > 	ajmp finish
> >   end:
> >  _endasm;
> > }
> > ------------------END CODE STRIP-----------------------------
> -- 
> _______________________________________________
> Sign-up for your own FREE Personalized E-mail at Mail.com
> http://www.mail.com/?sr=signup
> 
> 
> 
> -------------------------------------------------------
> This SF.net email is sponsored by:Crypto Challenge is now open! 
> Get cracking and register here for some mind boggling fun and 
> the chance of winning an Apple iPod:
> http://ads.sourceforge.net/cgi-bin/redirect.pl?thaw0031en
> _______________________________________________
> Sdcc-user mailing list
> Sdc...@li...
> https://lists.sourceforge.net/lists/listinfo/sdcc-user
>