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chedzoy

Fast Rom Data Lookup For Small Arrays

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Hi all,

 

In the old days (of pure asm) if you had a small byte data array you could write a subroutine which just added the array index to the program counter making it jump to a retlw which returned the data. Then make sure the whole routine was within one page (eg org 0x300).

 

The new general rom data lookup (rom char *x = "data":rolleyes: is great, but it has quite a big code overhead.

 

Maybe the compiler could optimise it for char arrays of less than 253 ish bytes.

 

At the moment I am trying to get round this by forcing the lookup subroutine into a single page by putting it in using #pragma DATA 0700, (raw code values); the calling it from a asm{} block in my code.

It would be a bit easyer if we could have a way of forcing a block of asm{} into a particular location.

 

What Im doing at the moment is..

 

char decode(char t)

{

 

asm

{

movlw 0x0F

andwf _t, W

call 0x0700

movwf _t

}

return t;

}

 

 

#pragma DATA 0x0700, 0x0782, 0x343F, 0x340C, 0x345B, 0x345E, 0x346C, 0x3476, 0x3477, 0x341C, 0x347F, 0x347E, 0x347D,

0x3467, 0x3433, 0x344F, 0x3473, 0x3471;

 

 

This is intended to do a simple decode for a seven segment display (and its not been tested it yet !)

 

 

PS is this all BoostC related.

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chedzoy,

 

The current implementation of rom data is total universal, it will work crossing codepage boundarys etc. It doesn't try to be minimalist which it could be in some cases.

 

Short strings (I think its less than 12 characters) are currently best left stored without the "rom".

 

Improving this is on the todo list, so you should see it in later versions.

 

Regards

Dave

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