From c9adeb731014f88d857be2e98072807b2934f1f5 Mon Sep 17 00:00:00 2001 From: Adam Carpenter Date: Mon, 22 Jul 2019 13:36:12 -0400 Subject: Finished 2-6. --- ch2/2-6.c | 18 +++++++++++------- 1 file changed, 11 insertions(+), 7 deletions(-) diff --git a/ch2/2-6.c b/ch2/2-6.c index ee6770d..0932206 100644 --- a/ch2/2-6.c +++ b/ch2/2-6.c @@ -1,15 +1,19 @@ #include +/* + * 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) { - // set n bits in x starting at p to rightmost n bits of y - unsigned offset = (p + 1 - n); - unsigned y_mask = y & ~(~0 << n); - unsigned bot = x & ~(~0 << offset); - unsigned top = (x >> offset << offset) & y_mask; - unsigned result = top & bot; - return result; + 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)); } -- cgit v1.2.3