Rotina para controle de portas paralelas em C, código fonte .c
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#ifdef SCCSID
static sccsid[] = "@(#) lp.c 1.1 91/03/18  19:50:09";  
#endif
/*****************************************************************************
*   Hardware line printer driver.

   This module implements the functionality needed to drive a PC-style
   parallel port.  All function calls are non-blocking.  The function 
   passed to lpt_io() determine what action is taken.  The actions
   are defined in lp.h and again for reference below.  In general, to 
   use a port, the following procedure is used:

   Test for port presence  (lpt_io(IS_PRESENT))
   if so, then (lpt_io(INIT) to initialize the port

   For each character, test lpt_io(IS_BUSY)
   When port is not busy, lpt_io(OUT) with the character

   Optionally, one may retrieve the status of the printer port with
   lpt_io(STAT).

   Finally, if one desires to select or deselect a printer (assert or
   deassert the SELECT line on the interface), call lpt_io(SELECT) with
   the appropriate argument.


   Two test routines are provided in this module.  Defining TEST_1 
   includes a main() and creates a program that outputs 50,000 "*"
   characters to the port and displays transmission statistics.

   Defining TEST_2 creates a program that will output a file specified
   on the command line or redirected into it.


   Following is a table of function calls vs arguments vs returned value:


Function           Port          Byte          Mode      Returned value
------------------------------------------------------------------------------
IN               LPT base port   N/A          IN         Byte Read
OUT               LPT base port   byte to print    OUT         Port Status
INIT            LPT base port    N/A            INIT      Port Status
STAT            LPT base port    N/A            STAT      Port Status
SELECT ASSERT      LPT base port   ASSERT         SELECT      Port Status
SELECT DEASSERT      LPT base port   DEASSERT      SELECT      Port Status
IS_BUSY            LPT base port    N/A            IS_BUSY      0 if not busy
IS_ACK            LPT base port    N/A            IS_ACK      0 if not ACK
IS_PRESENT         LPT base port    N/A            IS_PRESENT   0 if not present

***************************************************************************/

/**************************************************************************/
/*&&&&&&&&&&&&&&&&&&&&&&*/
#define TEST_2
/*&&&&&&&&&&&&&&&&&&&&&&*/
/**************************************************************************/

#if defined(TEST_1) || defined(TEST_2)
#include <stdio.h>
#include <time.h>
#endif

#include <bios.h>
#include "lp.h"

/* function codes */
/********************************************************/
/* these are defined in lp.h and are here for reference 
#define IN 1
#define OUT 2
#define INIT 3
#define STAT 4
#define SELECT 5
#define IS_BUSY 6
#define IS_ACK 7
#define IS_PRESENT 8
***********************************************************/

/********************************************************/
/* subfunction codes for function SELECT
   Again, these are defined in lp.h
#define ASSERT   100
#define DEASSERT 101
****************************************************/

/***************************************************************************

 port architecture.

Each lpt port starts at a base address as defined below.  Status and 
control ports are defined off that base.

                write                           read
=============================================================================
Base   data to the printer is latched.         Read latched data

base+1   not defined                             Read printer status lines
                                    Bits are as follows:
                                    bit 7   busy      0x80
                                    bit 6    ack         0x40
                                    bit 5   paper out   0x20
                                    bit 4   select in   0x10
                                    bit 3   error      0x08
                                    2,1,0   Not used

base+2   write control bits                  read the same control bits
      Bits are defined as follows:         Normally reads the latched
      bit 0   *strobe         0x01         bits written to same port
      bit 1   *auto feed      0x02
      bit 2   init printer   0x04
      bit 3   *select out      0x08
      bit 4   turn on irq7 on ACK hi-2-lo toggle   0x10
      5,6,7   not used

******************************************************************************/
/********************************************/
/* defined in lp.h and are here for ref only
#define LPT1 0x3bc
#define LPT2 0x378
#define LPT3 0x278
**********************************************/

#ifdef TEST_1
main()
{
   unsigned status;
   unsigned lpt_io();
   unsigned int i;
   time_t start_time, end_time;

   status = lpt_io(LPT1, 0, INIT);
   printf("sending 50,000 chars\n");
   start_time = time(NULL);

   for (i=0;i<50000;i++) {
      while ( status=lpt_io(LPT1,0,IS_BUSY) ) /* spin while busy */
         ;
      status = lpt_io(LPT1, '*', OUT);
      if (!(i%1000))
         printf("*");
   }
   end_time = time(NULL);
   printf("\n50,000 chars in %ld seconds or %ld chars/sec\n",
      end_time-start_time,
      50000L / (end_time-start_time) );

   exit(0);

}

#endif

#ifdef TEST_2
/* this version outputs a file to lpt1 */
main(argc, argv)
int argc;
char **argv;
{
   unsigned status;
   unsigned lpt_io();
   long int i=0L;
   time_t start_time, end_time;
   int character;
   int busy_flag=0;

   status = lpt_io(LPT1, 0, INIT);
   start_time = time(NULL);

   if (argc > 1) {
      if (freopen(argv[1], "rb", stdin) == (FILE *) NULL) {
         cprintf("Error, file %s open failed\n", argv[1]);
         exit(1);
      }
   }

   while ( (character = fgetchar()) != EOF) {

      while ( status=lpt_io(LPT1,0,IS_BUSY) ){ /* spin while busy */
         if (!busy_flag) {
            gotoxy(70,25);
            cputs("BUSY    ");
            busy_flag=1;
         }
      }
      if (busy_flag) {
         gotoxy(70,25);
         cputs("PRINTING");
         busy_flag=0;
      }
      status = lpt_io(LPT1, character, OUT);
      i++;
   }
   end_time = time(NULL);

   gotoxy(70,25);cputs("        ");
   gotoxy(1,24);
   cprintf("%ld chars in %ld seconds or %ld chars/sec",
      i,
      end_time-start_time,
      i / (end_time-start_time) );

   exit(0);

}

#endif

/*
*   The meaning of life and the bits returned in the status byte
*   NOTE:  Important - the sense of all bits are flipped such that
*   if the bit is set, the condition is asserted.
*
*Bits----------------------------
*   7   6   5   4   3   2   1   0
*   |   |   |   |   |   |   |   +-- unused
*   |   |   |   |   |   |   +------ unused
*   |   |   |   |   |   +---------- unused
*   |   |   |   |   +-------------- 1 = i/o error
*   |   |   |   +------------------ 1 = selected
*   |   |   +---------------------- 1 = out of paper
*   |   +-------------------------- 1 = acknowledge
*   +------------------------------ 1 = not busy
*
*/


unsigned int
lpt_io(port,byte,mode)
   unsigned port;
   unsigned byte;
   int mode;
{
   unsigned i,j,status;
   long unsigned otime;


   switch (mode) {  /* test for valid commands */

   case OUT:
      outportb(port,byte);   /* send the character to the port latch */

      outportb(port+2, 0x0d); /* set strobe high */
      outportb(port+2, 0x0d); /* do it again to kill some time */
      outportb(port+2, 0x0c); /* set strobe low */
      inportb(port+1);  /* pre-charge the line if +busy is floating*/
      status = (inportb(port+1) & 0x00f8) ^ 0x48;
      return(status);

   case  IN:
      return(inportb(port));

   case IS_BUSY:   /* this checks the busy line */
      return ( (inportb(port+1) & 0x80) ^ 0x80 );   /* zero if not busy */
      /* note that we flip the sense of this bit because it is inverted
         on the port */

   case IS_ACK:   /* this checks the ack line */
      return ( inportb(port+1) & 0x60 );   /* zero if ACK not asserted */

   case SELECT:

      switch (byte) {

      case ASSERT:
         i = inportb(port+2);       /* get the control bits */
         outportb(port+2, i | 0x8);   /* mask bit 3 ON and output */
         return ( (inportb(port+1) & 0xf8) ^ 0x48 );

      case DEASSERT:
         i = inportb(port+2);       /* get the control bits */
         outportb(port+2, i & ~0x8);   /* mask bit 3 OFF and output */
         return ( (inportb(port+1) & 0xf8) ^ 0x48 );

      default:
         return(~0);    /* error */
      }

   case INIT:
      otime = biostime(0,0L);    /* get the timer ticks */
      outport(port+2, 0x08);       /* set init line low */

      /* wait for the next timer transition */
      while ( otime + 1 > biostime(0,0L)) ;
      outportb(port+2, 0x0c);    /* set init line high */
                           /* and select printer */
      /* fall thru */
   case STAT:
      return( ((inportb(port+1) & 0xf8) ^ 0x48) );

   case IS_PRESENT:   /* test to see if the port is present */
   outportb(port,0x55);
      if (inportb(port) == 0x55) {
         return(~0);
      }
      return(0);

   default:
      return(~0);      /* error, all bits set */
   }
}

/************** end of file ******************/

Entendendo a função fork()

Rotina para controle de portas paralelas em C. (biblioteca LP.h)

Usando shared memory

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