One of the C language’s strengths is that it can venture down into the depths of a computer’s inner workings. It loves to play in those dark, subterranean pools of raw data, bits, ports, and other primitive places that few high-level languages dare to tread.

Specifically, the C language is great at manipulating bit fields.

A bit field is where a byte, word, or larger chunk of data is manipulated one or more bits at a time. For example, an 8-bit byte (char) may represent 8 different binary values or toggles. This approach is more efficient than using 8 separate int variables to express on-off conditions.

You find bit fields used in legacy programs, dating back to when memory or storage was tight. Still, even when you have gigabytes available, sometimes packing a dozen or so conditions into a single int makes sense. In the online world, bit fields are used frequently because it takes less time to transmit a 32-bit word than it does to transmit 32 words.

An example of a bit field is the way colors are mapped in computer graphics. Three colors — red, green, and blue — are represented by three bits:

100 = Red
010 = Green
001 = Blue

As you run through the permutations of binary values 001 through 111, you encounter the basic PC text graphics colors, as shown in the table below.

When the red/green/blue bits combine, they create the intermediate colors, such as blue and green (011) make cyan. Blue and red (101) create magenta. All bits off (000) is black. All bits on (111) is white.

A fourth bit on the original PC (1000) set color intensity. When that bit is on, a second set of 8 colors is available, from gray (1000 or “bright black”) through bright white (1111). The color “bright brown” (1110) is yellow.

The text color bit field on the original PC was 8-bits wide in total. The first four bits set the background color, the second four set the text color.

As an early PC programmer, I eventually committed the bit field values to memory. I still remember that 0x04 is the text color red. In fact, the common acronym RGB helps me remember in which order the bits lie in the field: red, green, and blue.

PC text color is just one example of a bit field. In computerdom you’ll discover many bit fields. The C library uses bit fields in many functions and constants, but often macros are used to help manipulate them, so you don’t really get exposed directly. Regardless, bit fields allow for a lot of information to be contained in a tiny space.

As an example, suppose you’ve written a computer game. In the game, you use a bit field to track the status of a starship. The bit field looks like this:

Bit 0 – Transporter status
Bit 1 – Warp Drive
Bit 2 – Shields
Bit 3 – Phasers

Forgetting for a moment about Paramount Studios and their legions of lawyers defending copyrights, you need to code your game so that it can check the status of each of those bits, set a bit, and reset a bit. I’ll go over that process in next week’s Lesson. Before then, here are some bit field tidbits to ponder:

• Bits are referenced by their position within the byte, 0 through the width of the byte/word/long word.
• When looking at a binary representation of a bit field, bit 0 is on the far right.
• Generally speaking, a byte (char) is 8-bits wide.
• A word (short int) is 16-bits wide.
• A long word (int) is 32-bits wide.
• To set a bit is to assert its value to 1.
• To reset (or unset) a bit is to assert its value to 0.
• The value 1 is on or TRUE; the value 0 is off or FALSE.