Steve Baldwin <PICLIST@MITVMA.MIT.EDU> wrote:

> For example. All the books make out that beginners can't understand
> pointers but if you think of
>
> char  *TEXT
> as
> TEXT:   DB      'My string', 0
>
> then TEXT has some meaning. You know that TEXT is the address of the
> 'M' and that if you increment it, it will be the address of (point
> to) the 'y'.

Steve:

I'm sure that you understand the concept of pointers, but your
example -- not to put too fine a point on it -- sucks.

In your C statement, "*text" is a VARIABLE, but in your assembly
statement, "TEXT" is a CONSTANT... You CAN"T "increment it", and it
DOESN'T "point to the 'y'".

A more accurate example, if you really do want to point to a constant
array of characters stored in the PIC's program-space, might be:

    Think of:

        char *text;
        text = "My string";

    as equivalent to:

        TEXT    EQU     [any register]

        LOOKUP: MOVWF   PC
        STRING: DT      'My string', 0

                MOVLW   STRING
                MOVWF   TEXT

    And think of:

        char x;
        x = *text;

    as equivalent to:

        X       EQU     [any register]

                MOVF    TEXT,W
                CALL    LOOKUP

                MOVWF   X

This example, of course, is subject to a few restrictions having to
do with the placement of the "STRING" table in memory... If you
wanted to show an example that dealt with character arrays in RAM,
it would be:

    Think of:

        char *text;
        char string[] = "My string";

        text = string;

    as equivalent to:

        TEXT    EQU     [any register]
        STRING  EQU     TEXT+1

                MOVLW   "M"
                MOVWF   STRING
                MOVLW   "y"
                MOVLW   STRING+1
                ....
                MOVLW   "n"
                MOVWF   STRING+7
                MOVLW   "g"
                MOVWF   STRING+8
                MOVLW   0
                MOVWF   STRING+9

                MOVLW   STRING
                MOVWF   TEXT

    And think of:

        char x;
        x = *text;

    as equivalent to:

        X       EQU     [any register]

                MOVF    TEXT,W
                MOVWF   FSR

                MOVF    INDF,W
                MOVWF   X

-Andy

=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering - Vista, California
=== http://www.geocities.com/SiliconValley/2499