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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
  • 0 in reply to Andrew E. Kalman

    Andrew, I'm afraid I don't have much to add here. I'd have done the message object a bit differently to avoid the 8051 data space specifier conundrum but as I'm sure you're well aware, there's not much you can do with a typedef once you've declared it except use it.

    For the message object I'd simply forgo the pointer version and supply the base type letting user create a pointer from it. E.g.

    typedef enum MsgTypes
{
    MSG_COMPLEX,
    MSG_A,
    MSG_B,
    MSG_C,
    NUM_MSGS
} MsgTypes;

typedef struct o_complex
{
    int  var;
    char arr[5];
} o_complex;

typedef struct o_msg
{
    MsgTypes            type;
    union MsgBody
    {
        o_complex       complex;
        unsigned long   apple;
        unsigned int    butter;
        unsigned char   cheese;
    } body;
} o_msg;

int main(void)
{
    /* Pointer resides in IDATA but points only to DATA space
    */
    o_msg idata * data pMsg;

    pMsg->type        = MSG_A;
    pMsg->body.cheese = 'a';

    /* blah, blah, blah...
    */
    for (;;);

    return 0;
}

    If you think I might be of any further assistance, maybe we should take it off-line, :-)

    Regards,

    - Mark

Reply
  • 0 in reply to Andrew E. Kalman

    Andrew, I'm afraid I don't have much to add here. I'd have done the message object a bit differently to avoid the 8051 data space specifier conundrum but as I'm sure you're well aware, there's not much you can do with a typedef once you've declared it except use it.

    For the message object I'd simply forgo the pointer version and supply the base type letting user create a pointer from it. E.g.

    typedef enum MsgTypes
{
    MSG_COMPLEX,
    MSG_A,
    MSG_B,
    MSG_C,
    NUM_MSGS
} MsgTypes;

typedef struct o_complex
{
    int  var;
    char arr[5];
} o_complex;

typedef struct o_msg
{
    MsgTypes            type;
    union MsgBody
    {
        o_complex       complex;
        unsigned long   apple;
        unsigned int    butter;
        unsigned char   cheese;
    } body;
} o_msg;

int main(void)
{
    /* Pointer resides in IDATA but points only to DATA space
    */
    o_msg idata * data pMsg;

    pMsg->type        = MSG_A;
    pMsg->body.cheese = 'a';

    /* blah, blah, blah...
    */
    for (;;);

    return 0;
}

    If you think I might be of any further assistance, maybe we should take it off-line, :-)

    Regards,

    - Mark

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