Changes between Version 3 and Version 4 of howto/bitfields_in_microblaze

Timestamp:

Feb 18, 2015, 10:20:13 AM (9 years ago)

Author:

chunter

Comment:

--

howto/bitfields_in_microblaze

@@ -66 +66 @@
         my_bitfield.B              = 60;
         my_bitfield.C              = 0;
-        print_bitfield_example_type(my_bitfield);
+        print_bitfield_example_type( my_bitfield );
   }}}
 }}}
@@ -74 +74 @@
 {{{
 #!div style="font-size: 90%"
+  {{{
+bitfield_example_type Contents:
+.raw_value      = 0x79
+.A              = 1
+.B              = 60
+.C              = 0
+  }}}
+}}}
 
-
-}}}
+The above print verifies that the bit-field behaved the way we expected.
 
 
 == Case 2: Using a Constructor Function to Initialize a Bit-field ==
+
+A second way to initialize a bit-field is to use a constructor. For the sake of argument, suppose that every use of {{{bitfield_example_type}}} required the C element to be set to 1. Rather than trying to remember this requirement with every declaration of the bit-field, we can use a simple constructor function that enforces this requirement while still allowing us to set the A and B fields independently. Here is our constructor:
+
+{{{
+#!div style="font-size: 90%"
+  {{{#!C
+bitfield_example_type constructor(bitfield_example_type bitfield_argument){
+        bitfield_example_type my_bitfield;
+
+        my_bitfield   = bitfield_argument;
+        my_bitfield.C = 1;
+
+        return my_bitfield;
+}
+  }}}
+}}}
+
+The following code shows how to use this constructor and then print the contents of the bit-field:
+
+{{{
+#!div style="font-size: 90%"
+  {{{#!C
+        bitfield_example_type my_bitfield;
+        my_bitfield =  constructor( (bitfield_example_type){ .B = 27 } );
+        print_bitfield_example_type( my_bitfield );
+  }}}
+}}}
+
+The above code uses an extremely useful syntax called a [https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html designated initializer]. We can create a {{{bitfield_example_type}}} directly in the argument of our call to {{{constructor}}} by explicitly naming the fields we want assigned. In this case, we have set B to 27 and have chosen not to set either A or C. This will create a {{{bitfield_example_type}}} where both A and C are set to 0 but B is set to 27. Our constructor, however, should explicitly set the C field to be 1 according to our requirements. Here is the resulting print:
+
+{{{
+#!div style="font-size: 90%"
+  {{{
+bitfield_example_type Contents:
+.raw_value      = 0xB6
+.A              = 0
+.B              = 27
+.C              = 1
+  }}}
+}}}
+
+This provides a concise way of initializing a bit-field where some fields are dynamic and need to change while other fields are required to be set a certain way every time.