using pointers to display characters from arrays

osu1177 · Joined: 10 Feb 2011 Posts: 14

Hi All,
I have spent quite a bit of time trying to educate myself on pointers and how they can be used to display characters from large arrays. I even have (presumably) working code for the screen I am working with. However, the compiler gives me a syntax error, and when I try to simplify things, I only get more errors.

I hate to ask such easy questions, but I am having trouble finding the answers on my own. I am posting here the entirety of the code I am trying to get working, without showing what modifications I have made. Perhaps it is just one thing wrong that I am overlooking.

here is the first line of code which gives me problems:

Ttelmah · Joined: 11 Mar 2010 Posts: 19499

The line as posted, has an extra bracket in front of the FONT6x8 declaration, which probably causes the current error.

However there are other probable problems/limitations.

First question is how the fonts themselves are declared?.
FONT6x8 etc....
Limitation of the PIC, is that it is more complex than a conventional linear address map chip, if it comes to addressing data stored in the ROM. It has two separate memory 'spaces', not a single linear memory space like most chips. This means that if the tables are stored as 'const', you can't simply generate pointers to this. You need to use the ROM declaration, which adds code to handle the extra addressing, and declare the pointers as pointers to ROM.

Second point is that you need to declare the table, and then load the values separately. So (if the tables are in RAM):

osu1177 · Joined: 10 Feb 2011 Posts: 14

Hi Ttelmah,
I removed the first bracket but it still gave me a syntax error...but lets skip that for a moment and perhaps fix the other problems.

I am declaring my fonts before the function declaration. Here is exactly how it appears (
I have omitted two of the font tables):

osu1177 · Joined: 10 Feb 2011 Posts: 14

I forgot to mention, your second piece of advice regarding how to declare the font tables is working fine. Thank you!

osu1177 · Joined: 10 Feb 2011 Posts: 14

Ttelmah,
I have been trying a few things with not too much luck, and now I am trying to make sure I understand you correctly. Correct me if I am wrong, but pointer operations require that the elements of the array be stored in one consecutive, uninterrupted sequence. If a PIC splits this into two locations, than the pointers may produce errors, unless the arrays are declared in such a way as to compensate for this.
So, if that is the case, how can I fix this error, exactly? What kind of declarations do I have to make? Will it also involve redoing some of the pointer-related code?

Right now, I am declaring my arrays, in the same location as before (right before the routine), with this

Ttelmah · Joined: 11 Mar 2010 Posts: 19499

Unfortunately, pointers to ROM, are one of those features that only approximately 50% works. It works find, if you just 'read' a pointer, or if you use a single pointer to something, but arrays of these pointers, or complex usage with arrays pointing to arrays, requires you to be 'sneaky'.

Now, yes, a _single_ pointer to an object, needs to point to an object that is consecutive in memory, _but_ if you are using multiple pointers in an array, these can point to objects scattered all over memory.

I've found that where you have an array of rom* pointers, the system just goes screwy. However if you jast save them as integers, and then cast them to pointers or transfer them one by one into a simple pointer, this then works fine.

So: