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

[Jamie Granger]

Hi Jared,

Ok you're right, it does use the whole 16-bits.  (I was thinking about
char constants, which do use only the low 8-bits of a 16-bit word).

So on the C55 you have to do a "typedef u16_t u8_t", I agree.  However,
I don't think the solution is as simple as doing the typedef plus some
packing/unpacking in the physical layer.  I think that there may also be
some problems buried in the internals of lwip due to using u8_t pointer
dereferencing.

Thanks for your various comments.

I need to look into this further, but I'm actually going away on holiday
for a week now, so will forget all about this until I get back.

Many thanks,
Jamie


-----Original Message-----
From: address@hidden
[mailto:address@hidden On
Behalf Of Grubb, Jared
Sent: 03 September 2009 21:40
To: Mailing list for lwIP users
Subject: Re: [lwip-users] lwIP on TI C55 processor

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
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