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I/O Programming

  When programming memory mapped I/O devices, the addresses of relevant device registers are needed. These are typically set up as compiler/assembler preprocessor directives, and may be stored in a seperate include file, or simply declared in the source file.

The following code from HLAB6 illustrates the assembly language programming of the parallel port:

/*    porta1s.s
      test of port a parallel i/o
*/
	
      .text
      .even
      .global     main
main:
      move.b      #0xff,%d1          /* set port A */
      move.b      %d1,PADDR          /* for output */
      move.b      #0b10101011,%d1
      move.b      %d1,PADAT          /* write something to port A*/
      rts

MBARx      =      0x10000000     /* Module Base Address value */
PADDR      =      MBARx+0x01C5   /* Port A Data Direction Register;
                                                          8-bit, R/W */				    	
PADAT      =      MBARx+0x01C9   /* Port A Data Register, 8-bit, R/W */

The following C program from HLAB6 performs the same function:

/* porta1c.c
   test of sbc port a */


#define  MBARx   0x10000000         /* Module Base Address value */
#define  PADDR  (* (char *)  (MBARx+0x01C5))  
                                   /* Port A Data Direction Register;
                                                           8-bit, R/W */	
#define  PADAT  (* (char *)  (MBARx+0x01C9))  
                                   /* Port A Data Register, 8-bit, R/W */

int   main() {
  
   PADDR = 0xff;          /* set port A for output */
   PADAT = 0xaa;          /* output something */

   return  0;
}

Observe carefully how the port addresses are setup in each case, and in particular, the type casting used in the C code (see Clements, Chapter 3, page 176).

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