Beefy Boxes and Bandwidth Generously Provided by pair Networks
Perl Monk, Perl Meditation
 
PerlMonks  

comment on

( [id://3333]=superdoc: print w/replies, xml ) Need Help??
Lines 21 thru 23 of the following example demonstrate operations on individual bits, specified by their position, of a status word. The difficulty arises when we need to input or output this word. We need the bytes in the order that our hardware expects them ("endianness") and we want the position 0 to refer to the most significant bit.

I created a disc file which contains nothing but a 16-bit status word and verified the exact contents of that file with the utility "xxd" (came packaged with vim). My example reads that status word, makes a few changes, and writes it back out to another file. Again I use xxd to verify that I made the changes that I intended.

OK, I admit that lines 11 thru 19 and 25 thru 33 were developed empirically for this use of vec on my 64-bit intel machine. They are not portable to other environments but should work, without change, on any application on this or similar machine.

use strict; use warnings; my $stats; open my $DEVICE_IN, "<", 'stat.dat' or die "Cannot open input device: $!"; $stats = <$DEVICE_IN>; close $DEVICE_IN; $stats = pack( 'b16', reverse( unpack( 'B16', $stats ) ) ); vec($stats, 2, 1) = 0; # Reset bit 2 vec($stats, 6, 1) = 1; # Set bit 6 vec($stats, 10, 1) ^= 1; # Toggle bit 10 $stats = pack( 'b16', reverse( unpack( 'B16', $stats ) ) ); open my $DEVICE_OUT, ">", 'newstat.dat' or die "Cannot open output device: $!"; print $DEVICE_OUT $stats; close $DEVICE_OUT;

Here is the content of the two data files (displayed by xxd) after this example runs

C:\Users\Bill\forums\monks>xxd stat.dat 00000000: f531 .1 C:\Users\Bill\forums\monks>xxd newstat.dat 00000000: d711 .. C:\Users\Bill\forums\monks>

Note: There is no guarantee that traditional bit masking will eliminate this problem in I/O. It might, but testing is still required.

Bill

In reply to Re^3: How can I set a bit to 0 ? by BillKSmith
in thread How can I set a bit to 0 ? by bartender1382

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":



  • Are you posting in the right place? Check out Where do I post X? to know for sure.
  • Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
    <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>
  • Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
  • Want more info? How to link or How to display code and escape characters are good places to start.
Log In?
Username:
Password:

What's my password?
Create A New User
Domain Nodelet?
Chatterbox?
and the web crawler heard nothing...

How do I use this?Last hourOther CB clients
Other Users?
Others admiring the Monastery: (6)
As of 2024-03-28 19:54 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?

    No recent polls found