1. Bitwise Structures Chapter 9: C programming Language ספטמבר 04
Copyright Meir Kalech

2. What is Bitwise Structure?
The smallest type is of 8 bits (char).
Sometimes we need only a single bit.
For instance, storing the status of the lights in 8 rooms:
We need to define an array of at least 8 chars. If the light of room 3 is turned on the value of the third char is 1, otherwise 0.
Total array of 64 bits.
EXPENSIVE in place and time!!!
ספטמבר 04
Copyright Meir Kalech

3. What is Bitwise Structure?
It is better to define only 8 bits since a bit can also store the values 0 or 1.
But the problem is that there is no C type which is 1 bit long (char is the longer with 1 byte).
Solution: define a char (8 bits) but refer to each bit separately.
Bitwise operators, introduced by the C language, provide one of its more powerful tools for using and manipulating memory. They give the language the real power of a “low-level language”.
ספטמבר 04
Copyright Meir Kalech

4. What is Bitwise Structure?
Accessing bits directly is fast and efficient, especially if you are writing a real-time application.
A single bit cannot be accessed directly, since it has no address of its own.
The language introduces the bitwise operators, which help in manipulating a single bit of a byte.
bitwise operators may be used on integral types only (unsigned types are preferable).
ספטמבר 04
Copyright Meir Kalech

5. Bitwise Operators & bitwise AND | bitwise OR ^ bitwise XOR ~
1’s compliment
<<
Shift left
>>
Shift right
All these operators can be suffixed with =
For instance a &= b; is the same as a = a & b;
ספטמבר 04
Copyright Meir Kalech

6. Bitwise Operators – truth table1
ספטמבר 04
Copyright Meir Kalech

7. Bitwise Operators - Examples
& | ^ ~
>>3
<<3
ספטמבר 04
Copyright Meir Kalech

8. Setting Bits How can we set a bit on or off?
Manipulations on bits are enabled by mask and bitwise operators.
Bitwise OR of anything with 1 results in 1.
Bitwise AND of anything with 0 results in 0.
ספטמבר 04
Copyright Meir Kalech

9. Setting Bits For instance, how can we turn on the light in room #3?
lights:
char lights = 0x0;
char mask = 0x1;
mask <<= 2;
lights |= mask;
mask:
mask:
lights:
ספטמבר 04
Copyright Meir Kalech

10. Setting Bits For instance, how can we turn off the light in room #3?
lights:
char lights = 0x27;
char mask = 0xfb;
lights &= mask;
mask:
lights:
ספטמבר 04
Copyright Meir Kalech

11. Getting Bits How can we know if a bit is on or off?
Manipulations on bits are enabled by mask and bitwise operators.
Bitwise AND of anything with 1 results in the same value.
ספטמבר 04
Copyright Meir Kalech

12. Getting Bits
For instance, how can we check if the light in room #3 is turned on or off?
lights:
char lights = 0x27;
char mask = 0x1;
mask <<= 2;
if(lights & mask)
puts(“turned on”);
else
puts(“turned off”);
mask:
mask:
lights & mask:
ספטמבר 04
Copyright Meir Kalech

13. Bitwise - Example Suppose we have 8 rooms:
Light is on in the rooms requested by the user.
Print which rooms are lighted.
void main() {
unsigned char lights = 0;
set_lights(&lights);
print_lights(lights); }
ספטמבר 04
Copyright Meir Kalech

14. Bitwise - Example void set_lights(unsigned char *lights) {
int j, answer;
unsigned char mask;
for(j=0,mask=1; j<8; j++,mask<<=1)
answer=0;
printf(“lights in room #%d?”, j+1);
scanf(“%d”, &answer);
if(answer)
*lights |= mask; }
lights
mask 1
answer: 1 (yes)
*lights |= mask 1
ספטמבר 04
Copyright Meir Kalech

15. Bitwise - Example void set_lights(unsigned char *lights) {
int j, answer;
unsigned char mask;
for(j=0,mask=1; j<8; j++,mask<<=1)
answer=0;
printf(“lights in room #%d?”, j+1);
scanf(“%d”, &answer);
if(answer)
*lights |= mask; }
lights 1
mask 1
answer: 0 (no)
*lights |= mask 1
ספטמבר 04
Copyright Meir Kalech

16. Bitwise - Example void set_lights(unsigned char *lights) {
int j, answer;
unsigned char mask;
for(j=0,mask=1; j<8; j++,mask<<=1)
answer=0;
printf(“lights in room #%d?”, j+1);
scanf(“%d”, &answer);
if(answer)
*lights |= mask; }
lights 1
mask 1
answer: 1 (yes)
*lights |= mask 1
ספטמבר 04
Copyright Meir Kalech

17. Bitwise - Example void set_lights(unsigned char *lights) {
int j, answer;
unsigned char mask;
for(j=0,mask=1; j<8; j++,mask<<=1)
answer=0;
printf(“lights in room #%d?”, j+1);
scanf(“%d”, &answer);
if(answer)
*lights |= mask; }
lights 1
mask 1
answer: 1 (yes)
*lights |= mask 1
ספטמבר 04
Copyright Meir Kalech

18. Bitwise - Example room #1 is lighted
void print_lights(unsigned char lights) {
int j;
unsigned char mask;
for(j=0,mask=1; j<8; j++,mask<<=1)
if(lights & mask)
printf(“room #%d is lighted”,j+1);
else
printf(“room #%d is NOT lighted”,j+1); }
lights 1
mask 1
lights & mask 1
Output:
room #1 is lighted
ספטמבר 04
Copyright Meir Kalech

19. Bitwise - Example room #2 is NOT lighted
void print_lights(unsigned char lights) {
int j;
unsigned char mask;
for(j=0,mask=1; j<8; j++,mask<<=1)
if(lights & mask)
printf(“room #%d is lighted”,j+1);
else
printf(“room #%d is NOT lighted”,j+1); }
lights 1
mask 1
lights & mask
Output:
room #2 is NOT lighted
ספטמבר 04
Copyright Meir Kalech

20. Bitwise - Example room #3 is lighted
void print_lights(unsigned char lights) {
int j;
unsigned char mask;
for(j=0,mask=1; j<8; j++,mask<<=1)
if(lights & mask)
printf(“room #%d is lighted”, j+1);
else
printf(“room #%d is NOT lighted”, j+1); }
lights 1
mask 1
lights & mask 1
Output:
room #3 is lighted
ספטמבר 04
Copyright Meir Kalech

21. Bitwise - Example room #4 is lighted
void print_lights(unsigned char lights) {
int j;
unsigned char mask;
for(j=0,mask=1; j<8; j++,mask<<=1)
if(lights & mask)
printf(“room #%d is lit”, j+1);
else
printf(“room #%d is NOT lit”, j+1); }
lights 1
mask 1
lights & mask 1
Output:
room #4 is lighted
ספטמבר 04
Copyright Meir Kalech