Used memory test

Mortenc · Joined: 22 Feb 2007 Posts: 55

Hello super-forum.
I'am going to selecting a processor for a project and have a program in a size about 3-4 kB. We have tried to compile the C-program with processor PIC16F688 and with PIC18F1320.
PIC16F688 result = 3142 bytes
PIC18F1320 result = 4842 bytes
We thought that the PIC18 will use less program memory because of it�s structure
(no page shift needed an so on)
(The compiler is a CCS type but not the newest one)

Do you know why this bad result with PIC18?
or
have you any experience of this type of test?

Ttelmah · Guest

First question. Do you use interrupts?. If so, the handlers for these will be significantly larger (there are a lot more registers that have to be saved...).
Second question. How old is the compiler?. Basically, the early PIC18 releases, very much used 16 code, with tweaks. Extra optimisations for the 18 were latter added and improved. Code sizes for 18 projects markedly dropped as the V3 compilers developed.
Third question. Do you use arithmetic?. The internal hardware multiply, is one of the biggest savings, but if not used, then improvements from here won't be large.
Now, it is worth also realising that some of the 'improved' instructions, don't save anything in space. For instance, the MOVFF instruction, which allows data to be moved from one place to another, without using W. Though it is a single instruction, it uses two words of instruction memory...The same applies with the direct memory accessing. Instructions to use this are larger. Finally, there will be significantly more 'initialisation' overhead on the 18 chip. Generally, every extra peripheral, increases the amount of code needed for this.
Because of all these, the bigger chips, will tend to use memory more slowly as new functions are added, but use more memory for smaller programs.

Best Wishes

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

Mortenc · Joined: 22 Feb 2007 Posts: 55

Dear ckielstra,

Many thanks to you,
It was just the different in the list file "words" in PIC16F688
and "bytes in PIC18F1320.
Sorry my bad knowledge of C yet.
I was also wondering why one normal assembly instruction was counting 2 on the programlines in the list file of PIC18, but only 1 program line in the PIC16. (why this confussion?)

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

Mortenc · Joined: 22 Feb 2007 Posts: 55

Well, thanks again,
I like this forum - It has been a good day - I learn something.