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| -rw-r--r-- | lib8tion.h | 52 |
1 files changed, 52 insertions, 0 deletions
@@ -524,6 +524,58 @@ LIB8STATIC int8_t avg7( int8_t i, int8_t j) #endif } +// mod8: Calculate the remainder of one unsigned 8-bit +// value divided by anoter, aka A % M. +// Implemented by repeated subtraction, which is +// very compact, and very fast if A is 'probably' +// less than M. If A is a large multiple of M, +// the loop has to execute multiple times. However, +// even in that case, the loop is only two +// instructions long on AVR, i.e., quick. +LIB8STATIC uint8_t mod8( uint8_t a, uint8_t m) +{ +#if defined(__AVR__) + asm volatile ( + "L_%=: sub %[a],%[m] \n\t" + " brcc L_%= \n\t" + " add %[a],%[m] \n\t" + : [a] "+r" (a) + : [m] "r" (m) + ); +#else + while( a >= m) a -= m; +#endif + return a; +} + +// addmod8: Add two numbers, and calculate the modulo +// of the sum and a third number, M. +// In other words, it returns (A+B) % M. +// It is designed as a compact mechanism for +// incrementing a 'mode' switch and wrapping +// around back to 'mode 0' when the switch +// goes past the end of the available range. +// e.g. if you have seven modes, this switches +// to the next one and wraps around if needed: +// mode = addmod8( mode, 1, 7); +// See 'mod8' for notes on performance. +LIB8STATIC uint8_t addmod8( uint8_t a, uint8_t b, uint8_t m) +{ +#if defined(__AVR__) + asm volatile ( + " add %[a],%[b] \n\t" + "L_%=: sub %[a],%[m] \n\t" + " brcc L_%= \n\t" + " add %[a],%[m] \n\t" + : [a] "+r" (a) + : [b] "r" (b), [m] "r" (m) + ); +#else + a += b; + while( a >= m) a -= m; +#endif + return a; +} // scale8: scale one byte by a second one, which is treated as |