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#include<stdio.h>
/*
* Set n bits in x starting at position p to rightmost n bits of y.
*/
unsigned setbits(unsigned x, unsigned p, unsigned n, unsigned y) {
p++; // We're actually interested in p+1.
unsigned offset = (p - n); // Calculate offset of y bits being inserted into x.
unsigned lower_x = x & ~(~0 << offset); // Grab LSBs of x to save.
unsigned y_mask = (y & ~(~0 << n)) << offset; // Grab pattern from LSBs of y.
x = x >> p << p; // Just keep MSBs of x.
return x | y_mask | lower_x; // Concatenate MSBs of x with LSB pattern from y with LSBs of x.
}
int main() {
printf("%x\n", setbits(0xaa, 3, 3, 0x33));
printf("%x\n", setbits(0x00, 3, 3, 0xff));
}
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