LOloadzero·Mar 14, 2025
I think at least some of the NOPs are due to this
From my C transliteration:
https://github.com/loadzero/si78c/blob/master/si78c.c
// xref 0462
//
// This code is using the shot counter as an index into the ROM
// (where some code resides), for the purposes of random number generation.
//
// For the saucer direction logic, only bit 0 of each bytecode is used.
//
// The 256 bytes used reside at 0800 -> 08FF
//
// If you check bit 0 of each byte in that ROM section, you will find that there is no bias,
// and there are exactly 128 0's and 128 1's.
//
// It seems unlikely that this was an accident, I think Nishikado deliberately constructed
// the ROM this way, and used some well placed NOPs to achieve fair balance.
//
// E.g. these NOPs
//
// 0854: 00 00 00
// 0883:
Read the rest
00 00 00
//
// This information can be exploited to the player's advantage.
//
// If using the shot counting trick to get high scores, the
// expected saucer direction for the first 6 saucers (if counting),
// will be as follows:
//
// [22,37,52,67,82,97]
// [0, 1, 1, 0, 1, 1]
// [L, R, R, L, R, R]