endianness : Re: timer.c and ckinit.c Re: Timer resolution et al : Read_timer()

Sun Mar 14 11:27:48 UTC 2004

On Sat, 13 Mar 2004, sashti srinivasan wrote:

>  This lines store two bytes on variable clicks.
>    clicks = (msb << 8) | lsb;
>
>  My doubt is, since i386 is little endian, why this
>  line is written like this instead of:
>    clicks = (lsb << 8) | msb;
>
>     I guess whatever is written already is correct,
>  something is wrong with my understanding.  Very
>  sorry if my doubt is a trivial one.

The endianness of 'clicks' is irrelevant, because the compiler handles
that for you. The endianness of msb and lsb are already handler in the
fact that ther _sre_ msb and lsb .. the order those two data appear in
memory will change with endianness, but you don't see that in the above
code. You should see it if you look at the definitions of msb and lsb.

ATB,
	Gordon.
-- 
Gordon Scott				  http://www.gscott.co.uk

		Linux ... Because I like to *get* there today.

Hey, I'm looking for a new job!           See ->  http://www.gscott.co.uk

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Date:	Sun, 14 Mar 2004 14:25:21 +0100
From:	Gregory Matus <gregmatu at poczta.onet.pl>
Reply-To: Gregory Matus <gregmatu at poczta.onet.pl>
Message-ID: <12600.040314 at poczta.onet.pl>
To:	rtems-users <rtems-users at rtems.com>
Subject: Re: Timer resolution et al : Implementation of Read_timer()
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit

Hallo all,
I send patch to clock/ckinit.c file of i386/pc386 based bsps.
It contains procedure clock_read giving (I hope) microsecond accuracy.
I know it is ugly, sorry.

It fills timeval struct as follows:
tv_sec  - contains number of ticks(rtems ticks not timer0) from big-bang
tv_usec - microseconds elapsed from last tick.
I don't want to create new struct an I used what was available.
I think that maybe it will be beter if functions clock_read will return
microsecs from clock statrt as uint64, but I'm not sure.

Also I attached trivial test/example application
based on hallo example.

Gregory Matus

****** ckinit.c patch ******

38a39
> #include <sys/time.h>
61a63,96
> /* needed by Clock_read, filled by clockOn()  */
> static   rtems_unsigned32  microseconds_per_isr;
> 
> /*-------------------------------------------------------------------------+
> |         Function: Clock_read
> |      Description: read current 'time' elapsed since clock start with
> |                   microsecond precision
> | Global Variables: 
> |        Arguments:  
> |          Returns: parg->tv_sec  is Clock_driver_ticks since clock start
> |                   parg->tv_usec is microsecs elapsed since last
> |                                 Clock_driver_tick occurance
> +--------------------------------------------------------------------------*/
> void Clock_read(struct timeval *parg)
> {
>   unsigned long clock_ticks_elapsed;
>   unsigned long clock_isrs_elapsed;
>   unsigned long timer0_ticks;
>   unsigned32 level;
>   unsigned8 msb,lsb;
> 
>   _CPU_ISR_Disable(level);
>     outport_byte(TIMER_MODE, TIMER_SEL0 | TIMER_LATCH);
>     inport_byte(TIMER_CNTR0, lsb);
>         inport_byte(TIMER_CNTR0, msb);
>         clock_ticks_elapsed = Clock_driver_ticks;
>         clock_isrs_elapsed = Clock_isrs_per_tick - Clock_isrs;
>       _CPU_ISR_Enable(level); 
> 
>       timer0_ticks = (msb << 8) | lsb;
>       parg->tv_sec = clock_ticks_elapsed;
>       parg->tv_usec = (clock_isrs_elapsed * microseconds_per_isr 
>       + TICK_TO_US(US_TO_TICK(microseconds_per_isr) - timer0_ticks));
> }
122,123c157,158
<   rtems_unsigned32  microseconds_per_isr;
< 
---
> /*  rtems_unsigned32  microseconds_per_isr;
> */

*********** test.c *************

/*
 *  Simple test program -- simplified version of sample test hello.
 */

#include <bsp.h>

#include <stdlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>

extern void Clock_read(struct timeval *);

rtems_task Init(
  rtems_task_argument ignored
)
{
        struct timeval start, end;
  struct timeval diff;

  printf( "\n\n*** HELLO WORLD TEST ***\n" );
  printf( "Hello World\n" );
        Clock_read(&start);
        printf("foo\n");
/*      rtems_task_wake_after(144);
*/
        Clock_read(&end);
        diff.tv_sec = end.tv_sec - start.tv_sec;
        diff.tv_usec = end.tv_usec - start.tv_usec;
        if ( diff.tv_usec < 0 )
                diff.tv_usec = -diff.tv_usec;
        printf("Diff is ticks %ld and useks %ld\n", diff.tv_sec, diff.tv_usec);
  printf( "*** END OF HELLO WORLD TEST ***\n" );
  exit( 0 );
}

/* configuration information */

#define CONFIGURE_TEST_NEEDS_CONSOLE_DRIVER

#define CONFIGURE_TEST_NEEDS_CLOCK_DRIVER
#define CONFIGURE_RTEMS_INIT_TASKS_TABLE

#define CONFIGURE_MAXIMUM_TASKS 1

#define CONFIGURE_INIT

#include <confdefs.h>

/* end of file */ 