Avoid creating multiple copies of code in memory

Question

I'm new to developing on embedded systems and am not used to having very little program memory (16kB in this case) to play with. I would like to be able to create global variables, arrays, and functions that I can access from anywhere in the program while only existing in one place in memory. My current approach is to use static class members and methods that I can use by simply including the header file (e.g. #include "spi.h").

What is the best approach for what I'm trying to do?

Here is an example class. From what I understand, variables such as _callback and function definitions like call() in the .cpp will only appear in spi.o so they will appear only once in memory, but I may be mixed up.

spi.h:

#ifndef SPI_H_
#define SPI_H_

#include "msp430g2553.h"

class SPI {
public:
    typedef void (*voidCallback)(void);

    static voidCallback _callback;
    static char largeArray[1000];
    static __interrupt void USCIA0TX_ISR();
    static void call();

    static void configure();
    static void transmitByte(unsigned char byte, voidCallback callback);
};

#endif /* SPI_H_ */

spi.cpp:

#include "spi.h"

SPI::voidCallback SPI::_callback = 0;
char SPI::largeArray[] = /* data */ ;

void SPI::configure() {
    UCA0MCTL = 0;
    UCA0CTL1 &= ~UCSWRST;                   
    IE2 |= UCA0TXIE;

}

void SPI::transmitByte(unsigned char byte, voidCallback callback) {
    _callback = callback;
    UCA0TXBUF = byte;
}

void SPI::call() {
    SPI::_callback();
}

#pragma vector=USCIAB0TX_VECTOR
__interrupt void SPI::USCIA0TX_ISR()
{
    volatile unsigned int i;
    while (UCA0STAT & UCBUSY);
    SPI::call();
}

to simply include the header and start using the "SPI" just use Singleton pattern , it is a common in embedded that you have one peripheral and only one object to deal with hardware no more objects allowed. , also you may use factory pattern to allow for example a certain number of objects (SPI channels) — Alfeky
– Alfeky, Commented Jun 19, 2013 at 9:26

Pete Becker · Accepted Answer · 2013-06-17 20:55:51Z

10

The data members and the member functions of the class you wrote will only be defined once in memory. And if they're not marked static, the member functions will still only be defined once in memory. Non-static data members will be created in memory once for each object that you create, so if you only create one SPI object you only get one copy of its non-static data members. Short version: you're solving a non-problem.

answered Jun 17, 2013 at 20:55

Pete Becker

77.1k8 gold badges82 silver badges171 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Michael Dorgan · Accepted Answer · 2013-06-17 21:14:57Z

1

As per Pete, static won't affect code doubling up, only member vars. In your example, there is 0 difference between static non static memory usage except perhaps for the _callback var (which you call out as an error.) And that one variable would only double up if the class were created more than once.

If you want code to not exist in memory when not in use, look into overlays or some sort of dynamic linking process. DLL type code will probably be major overkill for 16K, but overlays with compressed code might help you out.

Also, beware of extra linked in code from libraries. Closely examine your .map files for code bloat from innocuous function calls. For instance, a single printf() call will link in all sorts of vargs stuff if it is the only thing using it. Same for software floating point (if you don't have a FP unit by default.)

answered Jun 17, 2013 at 21:14

Michael Dorgan

12.5k3 gold badges36 silver badges63 bronze badges

2 Comments

JustcallmeDrago Over a year ago

Sorry, the "call out as an error" was a leftover from another question I asked. I updated the question with an example large array to illustrate what I'm talking about.

Michael Dorgan Over a year ago

In your new case, there still is no reason to declare it static except to make it a singleton type object for (dubious) safety's sake. Again, you are better of studying your map file to see where you bloat is coming from and fighting things from there.

Collectives™ on Stack Overflow

Avoid creating multiple copies of code in memory

2 Answers 2

Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related