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General Question Re: Memory Models and Abstract Pointers

Andrew E. Kalman

Hi All.

I'm becoming more familiar with C51's various memory models, but I could use some confirmation on one issue.

Here's a code snippet:

char CODE_B = 'B';
char CODE_C = 'C';

...

void TaskShow( void )
{
    OStypeMsgP msgP;
    
    InitPORT();
    PORT = 0x00;
    
    for (;;) {
        OS_WaitMsg(MSG_UPDATE_PORT_P, &msgP, OSNO_TIMEOUT, TaskShow1);
        
        if ( *(char *)msgP == CODE_C ) {
            PORT &= ~0xFE;
            PORT |= ( counter >> 8 ) & 0xFE; 
        }
        else
            PORT ^= 0x01; 
    }
}

where OStypeMsgP is void *.

For the SMALL memory model, this code works fine. For the LARGE memory model (with variables in XDATA), I must change the line that dereferences the pointer to
if ( *(char xdata *)msgP == CODE_C ) {

Makes sense to me ...

1) Could someone just verify that this is the intended behavior of variables that do not have an explicit memory type? I.e. if I do not add an explicitly declared memory type (e.g. data|xdata|idata|pdata|...) to the variable's declaration, then will it automatically be in the selected memory model's default memory area?

2) From what I can see, this means that "abstract pointers" are always of type void *, and one must simply dereference them properly, using the explicitly declared memory type in the dereferencing cast. Correct?

Thanks,

Parents Reply Children
  • 0 in reply to Dan Henry

    I just don't want to turn Keil's website into a Salvo discussion forum, they have their own already.

    Regards,

    - Mark

  • 0 in reply to Mark Odell

    I just don't want to turn Keil's website into a Salvo discussion forum, they have their own already.

    Agreed. This is Keil's web space, after all.

    I'm pretty happy with the way we do message pointers -- anything more explicit invariably leads to cross-platform portability problems and increased maintenance headaches. It does work well, once users get the hang of it. And it can be quite efficient.

    Thanks again, Mark, for your help and interest in my original query. It has helped me in my understanding of the pointer situation with Cx51.

    Regards,