Newsgroups: algonquinc.academic.courses.dat.2343.01f.announce
From: idallen@freenet.carleton.ca (Ian! D. Allen)
Organization: The National Capital FreeNet, Ottawa, Ontario, CANADA
Subject: Right and Left shift
Date: Wed, 5 Sep 2001 18:43:35 +0000 (UTC)

I see evidence that I mis-labelled right and left shift in some
classes this week.  Here's the truth with a few examples each:

Left Shift (LSH) 1 bit:   x = x << 1;

    Before:  00001000    ( =  8 decimal)
    After:   00010000    ( = 16 decimal )

    Before:  10010010
    After:   00100100    ( one bit is "lost" off the top)

Right Shift (RSH) 1 bit:  x = x >> 1;

    Before:  00001000    ( = 8 decimal )
    After:   00000100    ( = 4 decimal )

    Before:  01001001
    After:   00100100    ( one bit is "lost" off the bottom)

A real C program using shifts and masking:

/* C Program to decode the Unix return status of a command.
 * If the command completes normally, the exit status is zero.
 * If you interrupt it, you see a non-zero signal status.
 * Compile and run this on a Unix system, e.g. ACADAIX:
 *
 *      $ cc thisfile.c
 *      $ ./a.out
 *      'sleep 5' terminated with exit status 0 and signal 0
 *      $ ./a.out
 *      ^C (Interrupt)
 *      'sleep 5' terminated with exit status 0 and signal 2
 *
 * -IAN! idallen@ncf.ca
 */
#include <stdio.h>

#define COMMAND "sleep 5"

        int
main( void ){
        int status = system(COMMAND);

        int exitstatus = status >> 8;
        int signalstatus = status & 0xFF;

        printf("'%s' terminated with exit status %d and signal %d\n",
                COMMAND, exitstatus, signalstatus);

        return status;
}
-- 
-IAN!  Ian! D. Allen   Ottawa, Ontario, Canada   idallen@ncf.ca
       Home Page on the Ottawa FreeNet: http://www.ncf.ca/~aa610/
       College professor at: http://www.algonquincollege.com/~alleni/
       Board Member, TeleCommunities CANADA  http://www.tc.ca/