**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: 2024/7/16 01:43, |

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