#ifndef _MC6821_H_
  #define _MC6821_H_
  
  /*
   * This file describes the memery mapping of the MC6821 PIA.
   * The unions describe overlayed registers. Which of them is used is
   * determined by bit 2 of the corresponding control register.
   * this files expects the PIA_REG_PADWIDTH to be defined the numeric
   * value of the register spacing.
   *
   * Data came from MFC-31-Developer Kit (from Ralph Seidel,
   * zodiac@darkness.gun.de) and Motorola Data Sheet (from 
   * Richard Hirst, srh@gpt.co.uk)
   *
   * 6.11.95 copyright Joerg Dorchain (dorchain@mpi-sb.mpg.de)
   *
   */
  
  #ifndef PIA_REG_PADWIDTH
  #define PIA_REG_PADWIDTH 255
  #endif
  
  struct pia {
  	union {
  		volatile u_char pra;
  		volatile u_char ddra;
  	} ua;
  	u_char pad1[PIA_REG_PADWIDTH];
  	volatile u_char cra;
  	u_char pad2[PIA_REG_PADWIDTH];
  	union {
  		volatile u_char prb;
  		volatile u_char ddrb;
  	} ub;
  	u_char pad3[PIA_REG_PADWIDTH];
  	volatile u_char crb;
  	u_char pad4[PIA_REG_PADWIDTH];
  };
  
  #define ppra ua.pra
  #define pddra ua.ddra
  #define pprb ub.prb
  #define pddrb ub.ddrb
  
  #define PIA_C1_ENABLE_IRQ (1<<0)
  #define PIA_C1_LOW_TO_HIGH (1<<1)
  #define PIA_DDR (1<<2)
  #define PIA_IRQ2 (1<<6)
  #define PIA_IRQ1 (1<<7)
  
  #endif