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Need some help #locating some data in the wrong endian

 
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James
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Need some help #locating some data in the wrong endian
PostPosted: Fri Jun 02, 2006 1:27 pm     Reply with quote

I posted this in another thread but its offtopic now and I can't really figure it out. I did search, but found very little on endians. Also, I'm locating a struct to my data for a good reason, its a little odd but it does exactly what I want, so onto my problem:

Code:

int data[8];

struct struct_spi {
 int16 block1;
 int16 block2;
 int bitmapped;
};

struct struct_spi mySPIstruct
#locate mySPIstruct = data


Which works great! Except for one thing. The values in data[] come from the source in the wrong endian. So, say my

data = 0x11,0x22,0x33,0x44,0x55

What I want is:
mySPIstruct.block1 = 0x1122
mySPIstruct.block2 = 0x3344
mySPIstruct.bitmapped = 0x55

but ofcourse when I run this I get:
mySPIstruct.block1 = 0x2211
mySPIstruct.block2 = 0x4433
mySPIstruct.bitmapped =0x55

Is there a way to use locate and structs to solve this without swapping variables at runtime? Just adjusting the struct, not screwing up the order of values in data[] or any new structs that I locate in the same spot.

Thank You Everyone,
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Fri Jun 02, 2006 2:04 pm     Reply with quote

I don't see how it can be done, because int16 inherently stores data
in Lo-Hi format. It seems to me that you're looking for some pragma
that will automatically swap the bytes for you, by inserting appropriate
ASM code. I don't think it exists. I think that, in your routine that
loads the array, you need to add code to do the byte swapping right
after the array is loaded.
Guest








PostPosted: Fri Jun 02, 2006 2:59 pm     Reply with quote

Hmm... Problem with that is since the two share memory locations I'll be changing data[] too.

On second thought I agree that I can't do what I wanted to. Unless there is someway to specify the location to each half of an int16.


The whole reason I wanted to get away from this is that I'm always pulling data in and sometimes I need them swaped, was trying to save from doing the very repetitive:

Code:

  temp  = (int16)data[0] << 8;
  temp += data[1];


And if I define a struct I can access my data by meaningful name. Maybe I'll just do both, swap and use the struct for fun. Oh well.

Maybe there is a better way to swap then what I was doing ?
ckielstra



Joined: 18 Mar 2004
Posts: 3680
Location: The Netherlands

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PostPosted: Fri Jun 02, 2006 5:17 pm     Reply with quote

Code:
  temp  = (int16)data[0] << 8;
  temp += data[1];
On a PIC18 this takes 14 bytes, 7 clock cycles.

Shorter is
Code:
temp = make16(data[0], data[1]);
8 bytes, 4 clock cycles.
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