**Bit copy**: To set or clear a bit within a byte variable based upon a bit in the same
or other byte variable try:

(byte2 &= ~BITMASK2), (byte1 & BITMASK1) ? (byte2 |= BITMASK2) : 0;

Which could be encapsulated into a macro:

#define setmask(b1, m1, b2, m2) (((b2) &= ~(m2)), ((b1) & (m1)) ? ((b2) |= (m2)) : 0)

which you would use thusly:

setmask(byte1, BITMASK1, byte2, BITMASK2);

The result for the PIC is:

bcf _byte2,3 btfsc _byte1,2 bsf _byte2,3

Which is very effecient. If you don't want to clear the bit first (e.g. if it was a port pin) then it would be:

(!(byte1 & BITMASK1) ? (byte2 &= ~BITMASK2) : 0), (byte1 & BITMASK1) ? (byte2 |= BITMASK2) : 0;

PIC - Bit operations@

**Sign Extension**: Extends the sign of a n value by invoking with `SIGNEX(x, n-1)`

#define SIGNEX(v, sb) ((v) | (((v) & (1 << (sb))) ? ~((1 << (sb))-1) : 0))Better, from:

https://graphics.stanford.edu/~seander/bithacks.html#VariableSignExtend

unsigned b; // number of bits representing the number in x int x; // sign extend this b-bit number to r int r; // resulting sign-extended number int const m = 1U << (b - 1); // mask can be pre-computed if b is fixed x = x & ((1U << b) - 1); // (Skip this if bits in x above position b are already zero.) r = (x ^ m) - m;The code above requires four operations, but when the bitwidth is a constant rather than variable, it requires only two fast operations, assuming the upper bits are already zeroes.

**Count bits set**: Counting bits set by lookup table

//table where value at addr is the number of bits set for that number static const unsigned char BitsSetTable256[256] = { #define B2(n) n, n+1, n+1, n+2 #define B4(n) B2(n), B2(n+1), B2(n+1), B2(n+2) #define B6(n) B4(n), B4(n+1), B4(n+1), B4(n+2) B6(0), B6(1), B6(1), B6(2) }; unsigned int v; // count the number of bits set in 32-bit value v unsigned int c; // c is the total bits set in v // Option 1: c = BitsSetTable256[v & 0xff] + BitsSetTable256[(v >> 8) & 0xff] + BitsSetTable256[(v >> 16) & 0xff] + BitsSetTable256[v >> 24]; // Option 2: unsigned char * p = (unsigned char *) &v; c = BitsSetTable256[p[0]] + BitsSetTable256[p[1]] + BitsSetTable256[p[2]] + BitsSetTable256[p[3]]; // To initially generate the table algorithmically: BitsSetTable256[0] = 0; for (int i = 0; i < 256; i++) { BitsSetTable256[i] = (i & 1) + BitsSetTable256[i / 2]; }

file: /Techref/language/ccpp/bittwid.htm, 3KB, , updated: 2020/2/8 20:59, local time: 2024/5/29 08:39, |

©2024 These pages are served without commercial sponsorship. (No popup ads, etc...).Bandwidth abuse increases hosting cost forcing sponsorship or shutdown. This server aggressively defends against automated copying for any reason including offline viewing, duplication, etc... Please respect this requirement and DO NOT RIP THIS SITE. Questions?<A HREF="http://www.ecomorder.com/techref/language/ccpp/bittwid.htm"> Efficient bit twiddling in C</A> |

Did you find what you needed? |

## Welcome to ecomorder.com! |

## Welcome to www.ecomorder.com! |

.