Re: [lwip-users] lwIP on TI C55 processor

[Grubb, Jared]

You say that "char" is 16-bits, but only uses the lower 8. I don't  
think that's true. For example:

char x = 1000;
int y = x;
printf("%d", y);

I bet that you see "1000" get printed out. (I've never used C55, but  
this is my hunch)

In C, the type "char" is defined as being the minimally-addressable  
unit in the architecture and sizeof(char)==1 byte. For most  
architectures "byte" means "8 bits", but for the C55, "byte" means "16  
bits". (This is one reason the RFC's for IPv4/6 all use the word  
"octet" rather than "byte" -- "byte" is ambiguous, "octet" always  
means 8 bits)

Therefore the type "u8_t" is impossible on the C55 (and likewise for  
u8_t*).

For example:

    char x[4] = "abc";

should look like 00 65 00 66 00 67 00 00 in memory and be 8 octets  
long (but sizeof(x)==4).

If your PHY layer reads that variable x and transmits only the lower 8  
bits of every "byte" (skipping over the upper 8 bits completely), then  
you can use LwIP on the C55 just fine (do a "typedef u16_t u8_t").  
Every packet will take twice as many bytes as it would on a  
byte==octet system, but at least it works.

But, if that is not the case, I think you'll have to write a custom IP  
stack possibly from scratch, and I think it will be very difficult.

Jared

On 3 Sep 2009, at 08:21, Jamie Granger wrote:

> Chris,
>
> Yes, the C55 does allow packing 8-bit values to/from 16-bit words as  
> you
> suggest.  This is all just 16-bit operations, and shifting and  
> masking,
> which are fine.  In fact there is a "char" type on the C55, but it  
> is a
> 16-bit value which uses only the lower 8-bits.
>
> So I think, in many places in lwip where an 8-bit pointer is used to
> access memory, I would have to replace it with some packing/unpacking
> similar to your example.  It looks like it might be a lot of work -
> which is why I was asking if anyone's already done it.
>
> Thanks,
> Jamie
>
>
>
> -----Original Message-----
> Jamie,
>
> Does the C55 provide any way of handling 8 bits, or of packing 16 bits
> down to 8?
>
> For instance:
>
> int16 a,b,c;
>
>        c = (a<<8) + (b & 0x00ff);
>
> where the 8 bit values are the bottom 8 bits of the 16 bit a and b.
>
> Much of the data encoding in TCP/IP etc is 8 bit values packed into 16
> bit locations. If you can manage the pack and unpack, then there is
> hope.
>
> Regards,
>
> Chris.
> -- 
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