MC Bug (MAP Change Bug) , which was used current published tas (
https://youtu.be/HzZKAnFbEbQ?t=3849 ), though there are some improved any% tases on nicovideo, is well-known by Japanese geeks of this game. However, address information is rare. So I'll add some.
7E1313 Each time you change MAPs, it is added by one.
7E13AF, 7F8E41, 7F8E51, 7FA4D5 and 7FB836 also changes, though their effects are unknown.
7FB001, 7FB041, 7FB0C1, ... , 7FB281 (0x7FB001+0x80*n (n=0, 1, ... , 0xB)) also changes, which determines NPCs speed (if the values are 0, they don't move).
According to the author of a tas on nicovideo, Emperor/Empress
class character and the number of the party members and the last NPC you spoke to or the last enemy you touched also affect.
Some Japanese walkthrough site notes that the
class ally character you spoke to last and the last spoken NPC who teaches you fusion magic affect a bug character you can invite. I'll add addresses. 7FFF0A is the class you spoke to last and 7FFF0B is the last fusion magic teacher.
edit: The bug's result is different from not the class but the each ally character.
7E138A and 7E138B are important. If you fix the both values and you speak to an NPC or press start, text dialogue is glitched. Sometimes you warp at random or a battle starts.
Further research of this bug is necessary for bug limit 100% or all events without save corruption category. It is
the only way a promising way to cope with the both Salamander and Umbrology (and I personally think using the bug for other purposes for these category should be prohibited). Well-known way, which was caused by entering Shipwreck has a defect. Event flags are rewinded.
edit: You can also get the Ancient Magic Tome by other serious bugs, Counter Bug and Wraith Form Bug.
Counter Bug: This bug is triggered by an ally(=A) who is casted revive got defeated by another ally's(=B) counter type attack from a multiple target attack by a confused or charmed ally(=A). It corrupts RAM. One of the result is that you can find Ancient Magic Tomes and many other equipments and items in the warehouse.
Wraith Form Bug: This bug is caused by an ally who cannot have any magic after he/she cast Wraith Form cast the spell. It also corrupts RAM. Equipments and items in the warehouse are rewritten, though Wraith Form itself can be learned when you give up Salamander.
These two bugs can cause serious multiple results. Currently, they are not controllable.
Another way to get the Ancient Magic Tome is subframe reset. By the way, any% tas and rta with subframe reset are already completed and you can watch them on nicovideo.
Note: All above serious bugs are not available in the remastered version.