Bit Fields & Bitwise Operations

1. Bit Fields & Bitwise Operations Professor Hugh C. LauerCS-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 BjarneStroustrup, and from C: How to Program, 5th and 6th editions, by Deitel and Deitel) Bit Fields & Bitwise Operations

2. Bitwise Operations Especially ECE-2801 and ECE-3803 • 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 Bit Fields & Bitwise Operations

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 unsignedchar, short, int, long, longlong Bit Fields & Bitwise Operations

4. b a 1 1 1 0 1 1 1 0 1 0 0 0 1 0 0 0 Examples unsigned int c, a, b; c = a & b; c = a | b; c = a ^ b; c = ~a; c = a << 2; c = a >> 3; Bit Fields & Bitwise Operations

5. b a a 1 0 1 1 1 1 1 1 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 1 0 1 1 1 1 1 0 1 1 1 1 1 0 0 0 0 0 1 0 0 0 0 1 0 Right Shift is Tricky unsigned int c, a; c = a >> 3; signed int c, a, b; c = b >> 3; c = a >> 3; Bit Fields & Bitwise Operations

6. Two Approaches Much more frequent in real world! • Traditional C • Use #define and a lot of bitwise operations • Modern • Use bit fields Bit Fields & Bitwise Operations

7. Traditional C definition of bit fields #define EMPTY 01 #define JAM 02 #define LOW_INK 16 #define CLEAN 64 Clean Low ink Paper jam Empty paper Example – Printer Status Register Bit Fields & Bitwise Operations

8. 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 */ Clean Low ink Paper jam Empty paper Printer Status Register (continued) Bit Fields & Bitwise Operations

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 ANDand shift to extract • Need to shift and OR to insert Bit Fields & Bitwise Operations

10. An integer field (traditional style) #define COUNT (8|16|32|64|128) int c = (status & COUNT) >> 3; status |= (c << 3) & COUNT; count Clean Low ink Paper jam Empty paper Printer Status Register (cont.) Bit Fields & Bitwise Operations

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 Bit Fields & Bitwise Operations

12. structstatusReg {unsigned intemptyPaperTray :1;unsigned intpaperJam :1; :2; unsigned intlowInk :1; :1;unsigned intneedsCleaning :1; :1; }; Clean Low ink Paper jam Empty paper Modern Bit-field Definitions Bit Fields & Bitwise Operations

13. structstatusReg {unsigned intemptyPaperTray :1;unsigned intpaperJam :1; :1;unsigned int count :5; :1;unsigned intlowInk :1; :1;unsigned intneedsCleaning :1; :1; }; count Clean Low ink Paper jam Empty paper Printer Status Register (cont.) Bit Fields & Bitwise Operations

14. Modern Bit-fields (continued) structstatusReg s; if (s.empty && s.jam) ...; while(! s.lowInk) ...; s.needsCleaning = true; s.Jam = false; int c = s.count; s.count -= 1; Bit Fields & Bitwise Operations

15. Questions about Bit Fields? Bit Fields & Bitwise Operations