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8bitworkshop/presets/vcs/random.wiz
2021-03-04 08:49:48 -06:00

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import "banks";
// Random Number Library
//
// Uses a Galois Linear Feedback Shift Register to generate random numbers.
// http://en.wikipedia.org/wiki/Linear_feedback_shift_register#Galois_LFSRs
//
// [random.lo] and [random.hi] must be defined in RAM somewhere.
namespace random {
in ram {
var lo : u8;
var hi : u8;
}
// The polynomial value is taken from here:
// http://users.ece.cmu.edu/~koopman/lfsr/16.txt
let LSFR_POLYNOMIAL = 0x845F;
// Get a pseudorandom 16-bit number.
// Precondition:
// random.lo, random.hi are currently seeded to some non-zero value.
// Return:
// random.lo, random.hi = result
func next() {
random.hi = random.hi >>> 1;
random.lo = random.lo >>>># 1;
if carry {
random.lo = a = random.lo ^ LSFR_POLYNOMIAL as u8;
random.hi = a = random.hi ^ (LSFR_POLYNOMIAL >> 8) as u8;
}
}
}