Pointer Functions

This library contains functions for basic pointer access and manipulation.

These functions are intended to allow sequential reading and writing of
individual bytes to arbitrary locations in memory. 

A pointer consists of any two consecutive bytes in Zero Page and is 
referenced using the single byte zeto page address (0-254).

Note: There is no concept of a null pointer in C02. A pointer containing
the value 0 simply points to the first byte of memory. 

Usage: at the beginning of the program use the directives

  #include <pointer.h02>

The following application functions are defined:

  ptr, msb, lsb =       Pointer Set: Set the contents of the pointer at
    ptrset(zp, &addr);  location zp to the address &addr.
    
                        Returns the zsro-page pointer location, address
                        high byte, and address low byte.

  ptr, msb, lsb =       Pointer Address: Retrieves the contents of the 
    ptradr(zp);         pointer at location zp.
    
                        Returns the zsro-page pointer location, address
                        high byte, and address low byte.

  ptr, b =              Pointer Put: Stores byte of b at the address 
    ptrput(zp, b);      pointed to by the pointer at location zp, then
                        increments the pointer contents.

                        Returns the zsro-page pointer location and value
                        written to address.

  b = ptrget(zp);       Pointer Get: Retrieves the contents of the byte 
                        at the address pointed to by the pointer at 
                        location zp, then increments the pointer contents.

  ptrinc(zp);           Pointer Increment: Increases the contents of the
                        pointer at location zp by one.

  ptrdec(zp);           Pointer Decrement: Decreases the contents of the
                        pointer at location zp by one.

  ptr, msb, lsb =       Pointer Add: Adds the 16-bit value specified by &addr 
    ptradd(zp, &addr);  or msb, lsb to the contents of the pointer at 
    ptradd(zp,msb,lsb); location zp.

  ptr, msb, lsb =       Pointer Add: Subtracts the 16-bit value specified by  
    ptrsub(zp, &addr);  &addr or msb, lsb to the contents of the pointer at 
    ptrsub(zp,msb,lsb); location zp.

  r =                   Pointer Compare: Compares the contents of the pointer
    ptrcmp(zp, &addr);  at location zp to the address &addr.

                        Returns 255 if the contents are less than the address
                        (pointing to a byte lower in memory), 0 if the contents
                        are equal to the address (pointing to the same byte),
                        and 1 if the contents are greater than the address 
                        (pointing to a byte higher in memory).

                        These results can be evaluated using the C02 unary
                        comparator ! or the test-operators :+ or :-.

Note: This library expects the following to be defined: 

the transient variables  

  temp0, temp1, temp2   Temporary storage