1. Bit Fields & Bitwise Operations
Professor Hugh C. Lauer CS-2303, System Programming Concepts
(Slides include materials from The C Programming Language, 2nd edition, by Kernighan and Ritchie, Absolute C++, by Walter Savitch, The C++ Programming Language, Special Edition, by Bjarne Stroustrup, and from C: How to Program, 5th and 6th editions, by Deitel and Deitel)
CS-2303, A-Term 2012
Bit Fields & Bitwise Operations

2. Bit Fields & Bitwise Operations
See §2.9 and §6.9 in K&R
Many situations, need to operate on the bits of a data word –
Register inputs or outputs
Controlling attached devices
Obtaining status
Especially ECE-2801 and ECE-3803
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3. Bitwise Operations in Integers
Corresponding bits of both operands are combined by the usual logic operations.
& – AND
Result is 1 if both operand bits are 1
| – OR
Result is 1 if either operand bit is 1
^ – Exclusive OR
Result is 1 if operand bits are different
~ – Complement
Each bit is reversed
<< – Shift left
Multiply by 2
>> – Shift right
Divide by 2
Apply to all kinds of integer types:– Signed and unsigned char, short, int, long, long long
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4. Bit Fields & Bitwise Operations
Examples
unsigned int c, a, b; c = a & b; c = a | b; c = a ^ b; c = ~a; c = a << 2; c = a >> 3; 1 a 1 b
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5. Bit Fields & Bitwise Operations
Right Shift is Tricky
unsigned int c, a;
c = a >> 3;
signed int c, a, b;
c = b >> 3; 1 a 1 b 1 a
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6. Bit Fields & Bitwise Operations
Two Approaches
Traditional C
Use #define and a lot of bitwise operations
Modern
Use bit fields
Much more frequent in real world!
CS-2303, A-Term 2012
Bit Fields & Bitwise Operations

7. Example – Printer Status Register
Empty paper
Paper jam
Low ink
Clean
Traditional C definition of bit fields
#define EMPTY 01
#define JAM
#define LOW_INK 16
#define CLEAN 64
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8. Printer Status Register (continued)
Empty paper
Paper jam
Low ink
Clean
Traditional bit fields (continued)
char status;
if (status == (EMPTY | JAM)) ...;
if (status == EMPTY || status == JAM) ...;
while (! status & LOW_INK) ...;
int flags |= CLEAN /* turns on CLEAN bit */
int flags &= ~JAM /* turns off JAM bit */
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9. Traditional Bit Definitions
Used very widely in C
Including a lot of existing code
No checking
You are on your own to be sure the right bits are set
Machine dependent
Need to know bit order in bytes, byte order in words
Integer fields within a register
Need to AND and shift to extract
Need to shift and OR to insert
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10. Printer Status Register (cont.)
Empty paper
Paper jam
Low ink
Clean
count
An integer field (traditional style)
#define COUNT (8|16|32|64|128)
int c = (status & COUNT) >> 3;
status |= (c << 3) & COUNT;
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11. “Modern” Bit-Field Definitions
See Kernighan & Ritchie, §6.9
Like a struct, except
Each member is a bit-field within a word
Accessed like members of a struct
Fields may be named or unnamed
Machine-dependent
Order of bits in word
Size of word
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Bit Fields & Bitwise Operations

12. Modern Bit-field Definitions
Empty paper
Paper jam
Low ink
Clean
struct statusReg { unsigned int emptyPaperTray :1; unsigned int paperJam :1; :2; unsigned int lowInk :1; :1; unsigned int needsCleaning :1; :1; };
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13. Printer Status Register (cont.)
Empty paper
Paper jam
Low ink
Clean
count
struct statusReg { unsigned int emptyPaperTray :1; unsigned int paperJam :1; :1; unsigned int count :5; :1; unsigned int lowInk :1; :1; unsigned int needsCleaning :1; :1; };
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14. Modern Bit-fields (continued)
struct statusReg s; if (s.empty && s.jam) ...; while(! s.lowInk) ...; s.needsCleaning = true; s.Jam = false; int c = s.count; s.count -= 1;
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15. Questions about Bit Fields?
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