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8bitworkshop/presets/nes/bcd.c

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#include "neslib.h"
word bcd_add(word a, word b) {
2019-03-16 00:34:17 +00:00
register word c, d; // intermediate values
c = a + 0x0666; // add 6 to each BCD digit
d = c ^ b; // sum without carry propagation
c += b; // provisional sum
d = ~(c ^ d) & 0x1110; // just the BCD carry bits
d = (d >> 2) | (d >> 3); // correction
return c - d; // corrected BCD sum
}