Home
  Software
    Reminders
      SHR

Inconsistencies in the SHR (>>) operation

A thread in the PIC/SX mailing list caused some discomfort about the workings of the shift right operation in different languages. Until now, I was convinced that the correct way to interpret that operation was in the 'logical' context, i.e., a clean bit operation, ignoring any significance of the MSb (=sign).

It seems it isn't that simple. It isn't even simple: some languages, like C, don't even specify how it will work: The C99 standard says it is 'implementation-dependent'. So C-compilers can differ in the way they do it.

Below are some texts I ran, using different languages/implementations.

Comparison

Language Compiler/Version shift result divide/2 Comments
C gcc 4.2.3 -8 -8 Test with 'int' type


2147483640 2147483640 Test with 'unsigned' type
Pascal fpc 2.2.0 2147483640 -8 Test with 'integer' type


2147483640 2147483640 Test with 'longword' type
Tcl tclsh 8.5 -8 -8
Perl perl 5.8.8 2147483640 -8
Python python 2.5.2 -8 -8
MySQL mysql 5.0.20 9223372036854775800 -8 The large number is because of the default type
Javascript Firefox 3.0.7 -8 -8
PHP php 5.2.9-dev -8 -8

Warning: mysql_connect(): No such file or directory in /home/jcoppens/jcoppens.com/php/std_lib.php on line 1579
Could not connect to the database manager:
Error reported: No such file or directory


(c) John Coppens ON6JC/LW3HAZ correo