I found something that bugs a lot of people, the fact HM moves cannot be deleted, can be easily erased on Fire Red.
There are two main routines that check if an attack is an HM or not:
one for the battle routine at 0x80441B8;
one for the in-screen move learning at 0x08125A90.
Both routines check for HMs in different locations.
The first checks if the attack given is part of a non-deletion list at 0x0825e014, ended in FFFF, and searches through it until it reaches the ending value, or finding a valid attack. Here's the code:
Code:
ROM:080441B8 ; =============== S U B R O U T I N E =======================================
ROM:080441B8
ROM:080441B8
ROM:080441B8 Battle_HM_set ; CODE XREF: sub_80CE8DC+A80p
ROM:080441B8 PUSH {R4,LR}
ROM:080441BA LSLS R0, R0, #0x10
ROM:080441BC LSRS R3, R0, #0x10 ;given attack stored
ROM:080441BE LDR R2, =unk_825E014 ;list location
ROM:080441C0 LDRH R0, [R2]
ROM:080441C2 LDR R1, =0xFFFF
ROM:080441C4 CMP R0, R1
ROM:080441C6 BEQ loc_80441EA ;is end of list
ROM:080441C8 MOVS R4, R1
ROM:080441CA ADDS R1, R2, #0
ROM:080441CC
ROM:080441CC loc_80441CC ; CODE XREF: Battle_HM_set+30j
ROM:080441CC LDRH R0, [R2]
ROM:080441CE ADDS R1, #2
ROM:080441D0 ADDS R2, #2
ROM:080441D2 CMP R0, R3
ROM:080441D4 BNE loc_80441E4
ROM:080441D6 MOVS R0, #1 ;is same attack, undeletable
ROM:080441D8 B loc_80441EC
ROM:080441D8 ; ---------------------------------------------------------------------------
ROM:080441DA DCB 0
ROM:080441DB DCB 0
ROM:080441DC off_80441DC DCD unk_825E014 ; DATA XREF: Battle_HM_set+6r
ROM:080441E0 dword_80441E0 DCD 0xFFFF ; DATA XREF: Battle_HM_set+Ar
ROM:080441E4 ; ---------------------------------------------------------------------------
ROM:080441E4
ROM:080441E4 loc_80441E4 ; CODE XREF: Battle_HM_set+1Cj
ROM:080441E4 LDRH R0, [R1]
ROM:080441E6 CMP R0, R4
ROM:080441E8 BNE loc_80441CC ;new end_of_list check
ROM:080441EA
ROM:080441EA loc_80441EA ; CODE XREF: Battle_HM_set+Ej
ROM:080441EA MOVS R0, #0 ;ended list, attack deletable
ROM:080441EC
ROM:080441EC loc_80441EC ; CODE XREF: Battle_HM_set+20j
ROM:080441EC POP {R4}
ROM:080441EE POP {R1}
ROM:080441F0 BX R1
The other searches for them at 0x0845A80C, the TM attack list. It does so by looking over the TMs at position 50+
Code:
ROM:08125A90 ; =============== S U B R O U T I N E =======================================
ROM:08125A90
ROM:08125A90
ROM:08125A90 Check_for_HM ; CODE XREF: ROM:0813939Ep
ROM:08125A90 PUSH {LR}
ROM:08125A92 LSLS R0, R0, #0x10
ROM:08125A94 LSRS R2, R0, #0x10
ROM:08125A96 MOVS R1, #0
ROM:08125A98 LDR R3, =TM_List ;Location for all TM Attacks
ROM:08125A9A
ROM:08125A9A loc_8125A9A ; CODE XREF: Check_for_HM+28j
ROM:08125A9A MOVS R0, R1
ROM:08125A9C ADDS R0, #0x32
ROM:08125A9E LSLS R0, R0, #1
ROM:08125AA0 ADDS R0, R0, R3
ROM:08125AA2 LDRH R0, [R0] ;loads HM required by R1
ROM:08125AA4 CMP R0, R2
ROM:08125AA6 BNE loc_8125AB0
ROM:08125AA8 MOVS R0, #1 ;if equal, undeletable, return 1
ROM:08125AAA B loc_8125ABC
ROM:08125AAA ; ---------------------------------------------------------------------------
ROM:08125AAC off_8125AAC DCD TM_List ; DATA XREF: Check_for_HM+8r
ROM:08125AB0 ; ---------------------------------------------------------------------------
ROM:08125AB0
ROM:08125AB0 loc_8125AB0 ; CODE XREF: Check_for_HM+16j
ROM:08125AB0 ADDS R0, R1, #1
ROM:08125AB2 LSLS R0, R0, #0x18
ROM:08125AB4 LSRS R1, R0, #0x18
ROM:08125AB6 CMP R1, #6 ;maximum TM checking
ROM:08125AB8 BLS loc_8125A9A
ROM:08125ABA MOVS R0, #0 ;not any TM, deletable
ROM:08125ABC
ROM:08125ABC loc_8125ABC ; CODE XREF: Check_for_HM+1Aj
ROM:08125ABC POP {R1}
ROM:08125ABE BX R1
So, how to "fix" it? Well, change 080441D6 to 00 and 08125AA8 to 00 to make no attack undeletable.
If, on the other hand, you have a wish to prevent the player from deleting random attacks, simply repoint the list to a location where your attacks fit (plus the 0xffff part), and change the following addresses:
0x08125A9C to 00 (one byte only)
0x08125AAC to your list pointer reversed
0x08125AB6 to the number of attacks you placed -2 (to a max of 101 undeletable attacks)
If, for some reason, you wish to make all attacks undeletable, change
080441EA to 00
08125ABA to 00
Hope this helps those hacks who want to get rid of HMs.
January 9th, 2010 (2:23 AM).
Edited May 2nd, 2016 by Spherical Ice.
