General tips to conserve ROM in large applications

[bignick270]

What are some great ways to conserve ROM in large programs on a PIC16F877A chip?

[PCM programmer]

Here is a post of mine from the old CCS forum on this topic.
These comments apply to 16F PICs.

===================================
I had a similar problem. My 16c77 was 93% full.
I know that the compiler starts to have problems if it
gets to 95%, so I really only had 2% space left.

So I looked at the .LST file. I saw many, many places
where the compiler used ROM very inefficiently.
In the first hour of examining the .LST file, I recovered
400 bytes by re-writing my code. In the next hour I
got back maybe 300 more bytes, and so on. Eventually,
I probably recovered at least 1500 bytes.

1. CCS does pointer operations very inefficiently.

Don't do:
ptr = buffer;
a = *ptr++;
b = *ptr++;
c = *ptr++;

Do this, if possible. It uses a lot less ROM:
a = buffer[0];
b = buffer[1];
c = buffer[2];

-----------------------------------
2. Avoid using CCS string functions. They use up tons of ROM.

If you are calling one string function many times,
then it's not so bad. But if you use a CCS string
function just once, then you are wasting a lot of
ROM for just one function. There is usually a cheaper
way to do it.

----------------------------------
3. Avoid using printf() or lcd_putc() with strings.
If you are sending 5 characters or less, then it is
cheaper to use constant characters.

Don't do:
printf("ABC");

Do:
putc('A');
putc('B');
putc('C');

----------------------------------------------
Code can usually be written more efficiently.
Here was the original way that I sent commands.
Note: I sent a lot more than 3 commands, and they
were spread out all over the program. I am just
using 3 commands together, below, as an example.

puts("$PXXXP"); // Send P command
puts("$PXXXA"); // Send A command
puts("$PXXXR"); // Send R command

Here's a cheaper way to do it. Create a central
routine that sends the command string. Then just
call that routine with the command letter.

send_command('P');
send_command('A');
send_command('R');

void send_command(char command_letter)
{
printf("$PXXX");
putc(command_letter);
putc(0x0d);
putc(0x0a);
}

It might take you a few days, but I will bet that
you can recover 1K to 2K of your ROM space by looking
at the .LST file.

[arunb]

excellent tips indeed.

I think a listing of similar coding tips should be made sticky...

thanks
a