Returns a random number in A in the range -C to +C (signed). The random byte is produced from the seed at B5FE (set from the R register in 6C27), the counter at $B602 and a byte fetched from memory.

The random bytes come from memory itself. A 16-bit counter at $B602 is increased on every call and used as an address; the byte found there (and one a little further on) is mixed with the previous result. So the program code, the tables and even the ROM serve as the game's random number table, and the R register seed from the start of the game (6C27) decides where the sequence begins. A result equal to the previous one is thrown away.

The way it narrows the random byte down to the range is unusual: B is set to 2*C, and then the random byte is halved (SRL A) until it is no more than B. Finally C is subtracted. Because 0-255 has to be halved several times, the upper half of the range gets far more than its share: with C=10, values 0-10 of the byte (results -10 to 0) mostly come only from the original byte being 0-10 directly, while 11-20 are hit by many different starting values. For C=10 the results come out approximately: each of -10 to -1: 1/256; 0: 16/256; +1 to +5: 31/256 each; +6 to +10: 15/256 each.