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large tables, small ram, no ROM pointers...

 
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guest
Guest
large tables, small ram, no ROM pointers...
PostPosted: Fri Apr 23, 2004 1:47 am     Reply with quote
I'm porting some code that relies on pointers to data held in ROM. The flavour of the code is:

Code:

const char * data_ptr;

const char the_data1 = {0, 0, ...};
const char the_data2 = {1, 2, ...};

some_func(i) {
    switch(i) {
        case A : data_ptr = the_data1; break;
        default: data_ptr = the_data2; break;
        }
     do_action(data_ptr);
     }


The tables are biggish, and the code is going to be difficult to change to using indexed access by name.

Any suggestions? Am I missing something here?
Ttelmah
Guest
Re: large tables, small ram, no ROM pointers...
PostPosted: Fri Apr 23, 2004 2:02 am     Reply with quote
guest wrote:
I'm porting some code that relies on pointers to data held in ROM. The flavour of the code is:

Code:

const char * data_ptr;

const char the_data1 = {0, 0, ...};
const char the_data2 = {1, 2, ...};

some_func(i) {
    switch(i) {
        case A : data_ptr = the_data1; break;
        default: data_ptr = the_data2; break;
        }
     do_action(data_ptr);
     }


The tables are biggish, and the code is going to be difficult to change to using indexed access by name.

Any suggestions? Am I missing something here?

Use numbers.
You cannot have 'pointer' to ROM, but if the_data1, is an array, you can simply code 'data_ptr', as an integer, that is the address in the array.
You can join your arrays together (provided they fit inside a ROM page), and just hand the 'start point' in the larger array to the function.

Best Wishes
Ttelmah
Guest
Re: large tables, small ram, no ROM pointers...
PostPosted: Fri Apr 23, 2004 2:07 am     Reply with quote
The tables are biggish, and the code is going to be difficult to change to using indexed access by name.

Any suggestions? Am I missing something here?[/quote]
Use numbers.
You cannot have 'pointer' to ROM, but if the_data1, is an array, you can simply code 'data_ptr', as an integer, that is the address in the array.
You can join your arrays together (provided they fit inside a ROM page), and just hand the 'start point' in the larger array to the function.

Best Wishes[/quote]

Add to this, that you can use a 16bit 'number', and use the top byte, as an 'index' to which array to use. So if the value is 0x00xx then the subroutine can use 'data1', while if it is 0x01xx, it can select 'data2' etc..
Remember also, that you don't have to structure your array usin the const declarations (which add a significant overhead with the 'fetch' header attached to each array). Instead use a #ROM declaration to store the array. and the read_program_eeprom call to fetch the data from an address.

Best Wishes
guest
Guest
thanks...
PostPosted: Fri Apr 23, 2004 9:17 pm     Reply with quote
Thanks, useful suggestions. I'll try the #ROM (or #ORG??) approach first.
guest
Guest
read_program_memory() on 16C745?
PostPosted: Sat Apr 24, 2004 1:41 am     Reply with quote
I'm getting the message:

*** Error 12 "C:\9.c" Line 268(1,20): Undefined identifier read_program_memory

How do I access ROM based tables again? Micro is PIC16C745...

I've tried read_program_eeprom() also with same result.
dyeatman



Joined: 06 Sep 2003
Posts: 1934
Location: Norman, OK

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Read_Program_Memory Problem
PostPosted: Sat Apr 24, 2004 7:35 am     Reply with quote
What version are you using?

If you are using 3.190 this is a bug that cropped up. 3.189 seems to work fine though...
Ttelmah
Guest
Re: read_program_memory() on 16C745?
PostPosted: Sat Apr 24, 2004 7:55 am     Reply with quote
guest wrote:
I'm getting the message:

*** Error 12 "C:\9.c" Line 268(1,20): Undefined identifier read_program_memory

How do I access ROM based tables again? Micro is PIC16C745...

I've tried read_program_eeprom() also with same result.

Change the chip...
Unfortunately, the ability to use the 'read_program_memory' function, requires that the _chip_ supports this capability. Generally, only 'F' designated chips have this ability (it is part of the support to read/write the flash memory). In chips without this ability, the only way to store 'numbers' into the ROM, is to code each byte, using a RETLW function, and then use a table based jump to the code. This implicitly limits the maximum table size, requiring it to fit in a page of ROM (including the access code), and wastes a significant amount of space.
Why these modern 'USB' chips are not 'F' designs, is odd...

Best Wishes
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