Changes between Version 2 and Version 3 of howto/bitfields_in_microblaze

Timestamp:

Feb 18, 2015, 9:54:26 AM (9 years ago)

Author:

chunter

Comment:

--

howto/bitfields_in_microblaze

@@ -34 +34 @@
 
  * A [http://en.wikipedia.org/wiki/Union_type union] is used between to designate that {{{raw_value}}} and the proceeding struct occupy the same space.
- * The {{{__attribute__ ((__packed__))}}} 
+ * The {{{__attribute__ ((__packed__))}}} struct attribute informs the compiler that that each member of the structure should be placed such that the memory required is minimized. This minimizes the chances that the bit-field will be automatically padded to an unexpected length when it is placed into a larger structure with other elements.
  * By convention, {{{raw_value}}} is the same size as the proceeding struct (a single byte). If the code does not need to interpret the individual bits in the bit-field, it can instead access the fully byte itself by using {{{raw_value}}}.
  * In the struct definition, the {{{ :X }}} notation is used to tell the compiler that the element is {{{X}}} bits wide.
 
+{{{bitfield_example_type}}} defines three bit-fields: A, B, and C. B is a 6-bit integer that is surrounding by single-bit values A and C. Let's first define a function that will print this bit-field for us. This function will print the entire byte in hexadecimal and will then print the fields A, B, and C as decimal values. We'll use this function in the coming examples:
+
+{{{
+#!div style="font-size: 90%"
+  {{{#!C
+void print_bitfield_example_type(bitfield_example_type my_bitfield){
+        xil_printf("bitfield_example_type Contents:\n");
+        xil_printf(".raw_value      = 0x%02x\n", my_bitfield.raw_value);
+        xil_printf(".A              = %d\n", my_bitfield.A);
+        xil_printf(".B              = %d\n", my_bitfield.B);
+        xil_printf(".C              = %d\n", my_bitfield.C);
+}
+  }}}
+}}}
+
+
+== Case 1: Manually Initializing a Bit-field ==
+
+One way to create and initialize a bit-field is to manually declare a local variable and assign values to the bit-field's elements.
+
+{{{
+#!div style="font-size: 90%"
+  {{{#!C
+        bitfield_example_type my_bitfield;
+        my_bitfield.raw_value = 0; //clear out the entire bitfield
+        my_bitfield.A              = 1;
+        my_bitfield.B              = 60;
+        my_bitfield.C              = 0;
+        print_bitfield_example_type(my_bitfield);
+  }}}
+}}}
+
+The above code creates a {{{bitfield_example_type}}} named {{{my_bitfield}}} and sets the A, B, and C elements to 1, 60, and 0 respectively. It then calls the previously-defined print function to verify the contents of the bit-field. The output of that print function is the following:
+
+{{{
+#!div style="font-size: 90%"
+
+
+}}}
+
+
+== Case 2: Using a Constructor Function to Initialize a Bit-field ==
+
+
+----
+= Example: Interpretation of Byte Array =
 
 = Example: Register Access =
-
-= Example: Interpretation of Byte Array =