Memory File I/O Functions

This library contains functions that mimic file input/ouput operations,
but read and write directly to RAM. It is intended to allow for the
design of programs using these functions even on systems that do not
include any sort of filesystem support, but do allow blocks of memory
to be loaded from and saved to external storage. For this reason, each
function has the same signature it's equivalent in the file library.

At the beginning of the program use the directives

  #include <stddef.h02>
  #include <memio.h02>

The following functions are defined:

  m = mopen(z, &a);     Open memory file, using zero-page bytes z and z+1
                        as a pointer and a as the starting address of the
                        file. The starting address must be at least $0100.

                        Returns the the z as the "file pointer", or 0 if an
                        invalid address is specified. 

  r = mclose(m);        Close memory file pointed to by m, by writing an
                        EOF and clearing the memory pointer.

                        Returns 0 if successful or 255 if m contains an 
                        invalid address.

                        Note: Calls mflush() then sets the bytes at`m and
                        m+1 to 0.

  r = mflush(m);        Flush memory file pointed to by m, by writing a
                        NUL character at the memory pointer to 0.
                      
                        Returns 0 if successful or 255 if m contains an 
                        invalid address.

                        Note: The NUL character (ASCII code 0) is use as the
                        EOF marker because many systems initialize RAM to 0,
                        it's the easiest character to test for, and it can
                        never be part of a C style string.

  c = mgetc(m);         Read character from memory file pointed to by m. 
                      
                        Returns character read from file.
                      
                        Note: Returns 255 if m contains an invalid address.
                      
                        Returns a system dependent garbage character if end
                        of file has been reached or any other I/O error. Use
                        meof(f) and merror(f) to check for these conditions.

  r = mputc(m, c);      Write character c to memory file opened to m.
                      
                        Returns 0 if successful or 255 if m contains an 
                        invalid address.

  n, c = mgets(m, &s);  Reads a maximum of 128 characters from keyboard until
                        a newline or EOF character is encountered, storing the 
                        entered characters as null-terminated string s.

                        Returns number of characters entered ( 255 if m 
                        contains an invalid address) and the final character
                        of the returned atring.

                        Note: Any ASCII control character between EOF (code 0)
                        and Space (code 32) is interpreted as a newline. This
                        character will be included as the last character of
                        the string.

  n, c = mputs(m, &s):  Writes up to 128 characters of null-terminated string
                        s to the memory file. Does not write a trailing C/R.

                        Returns number of characters written, or 255 if m
                        contains an invalid address.

  r = mputln(m, &s):    Writes up to 128 characters of null-terminated string
                        s followed by a C/R to the memory file.

                        Returns 0 if successful or 255 if m contains an 
                        invalid address.

                        Note: Calls mputs() followed by mputc().

  l, h = maddr(m);      Returns the address contained in memory pointer m
                        as two bytes in LSB, MSB format.

                        This non-standard function is included for debugging
                        purposes.

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

  setdst(&s);         Set destination string pointer
  setsrc(&s);         Set source string pointer and initialize index

along with the zero page variable pairs
  
  dstlo,dsthi:        Destination string pointer
  srclo,srchi:        Source string pointer

and the assembler constant
  
  EOFCHR              End of File character code`
  RTNCHR              Carriage Return character code