ARRAY of BITS in C

Projects Programming
16

Well now, this is puzzling. Case 1 uses a bit field:

struct myByte {
int foo : 1;
};

volatile myByte bar;

void setup ()
  {
  Serial.begin (115200);
  bar.foo = 1;
  Serial.println (bar.foo);
  }

void loop () {}

Compiling for Uno:

Binary sketch size: 2,312 bytes (of a 32,256 byte maximum)

Case 2 uses "and" and "or":

volatile byte bar;

void setup ()
  {
  Serial.begin (115200);
  bar |= 1;
  Serial.println (bar & 1);
  }

void loop () {}

Compiled for Uno:

Binary sketch size: 2,302 bytes (of a 32,256 byte maximum)

OK, it saved 8 bytes. But now change Case 1 to be:

struct myByte {
byte foo : 1;
};

volatile myByte bar;

void setup ()
  {
  Serial.begin (115200);
  bar.foo = 1;
  Serial.println (bar.foo);
  }

void loop () {}

(The bit field "foo" changed from int to byte).

Now I get:

Binary sketch size: 2,198 bytes (of a 32,256 byte maximum)

In this particular case, using the bit-field saved 114 bytes of program memory.