Template Functions for C02 Programs

This module provides functions to read from and write to extended
memory.

Extended memory is RAM that is not directly addressable by the 6502.
Read and write operations use a 24-bit logical address, which is
comprised of an 8-bit bank and 16-bit offset and automatically updated 
after each operation.

At the beginning of the program use the directives

  #include <stddef.h02>
  #include <xmemory.h02>

The following functions are defined:

  b,d = xgetad();       Returns the current logical extended bank b
                        and offset d.

                        Note: the physical address stored in XMBANK 
                        and XMADDR is converted to a logical bank and
                        offset, which are returned.

  xsetad(b,d);          Sets the current logical extended bank to b
                        and offset to d.

                        This extended address is used for all read and
                        write functions except for the page operations.

                        Note: the logical address is converted to a
                        physical address stored in XMBANK and XMADDR.

  c = xgetc();          Reads a byte from the current extended address,
                        increments the address, and returns the byte 
                        as c.

  xputc(c);             Writes byte c to the current extended address,
                        then increments the address.
                          
  w = xgeti();          Reads a word starting at the current extended 
                        address, adds 2 to the address, and returns the 
                        word as w.

  xputi(w);             Writes word w at the current extended address,
                        then adds 2 to the address.
  
  c,w = xgetl();        Reads a long starting at the current extended 
                        address, adds 3 to the address, and returns the 
                        high byte of the long in c and the low word in w.

  xputl(c,w);           Writes the long consisting of the byte c and
                        word w at the current extended address, then
                        adds 3 to the address.
  
  xread(n,r);           Reads n bytes starting at the current extended
                        address into array r, then adds n to the address.

  xwrite(n,r);          Writes n bytes of array r starting at the current
                        extended address, then adds n to the address.

  xswap(n,r);           Swaps n bytes of array r with n bytes starting at 
                        the current extended address, then adds n to the 
                        address.

  xload(w);             Reads word w bytes into local memory starting at
                        the address specified by a call to setdst() from
                        extended memory starting at the current extended
                        address, then adds w to the address.

  xsave(w);             Writes word w bytes from local memory starting at
                        the address specified by a call to setsrc() into
                        extended memory starting at the current extended
                        address, then adds w to the address.

  xchng(w);             Swaps word w bytes in local memory starting at
                        the address specified by a call to setdst() with
                        extended memory starting at the current extended
                        address, then adds w to the address.

  xsetpg(b,p)           Sets the current extended address to bank b and
                        offset p * 256. This ensures that the address is
                        on a page boundary for subsequent page read and
                        write operations.
                        
                        Note: The page operation are the fastest of the
                        block read and write functions since the code
                        can use indirect addressing.

  xrpage(n,p);          Copies n 256 byte pages starting at the current 
                        extended bank and page to local memory starting 
                        at address p * 256, then updates the extended
                        address accordingly.
                        
  xwpage(n,p);          Copies n 256 bytes from local memory starting 
                        at address p * 256 to extended memory starting at
                        current bank and page, then updates the extended
                        address accordingly.

  xspage(n,p);          Swaps n 256 bytes from local memory starting at
                        address p * 256 with extended memory starting at
                        current bank and page, then updates the extended
                        address accordingly.

Note: This library expects the following functions to be defined:

  ext();                External function description

along with the zero page variables
  
  zpage0,zpage1:        Zero page variable pair

the external variables
  
  exvar0,exvar1:        External variable pair

and the constants
  
  #CNSTNAME             Constant description
