Arbitrary code execution in Red/Blue using the "8F" item

[Evie the Bird Mother 🌸 ☽]

Pigdevil2010 made a simpler version of the ws m (hex: 63) bootstrap code for Yellow and annotated it. He/she was going to post it on the forums, but had some trouble registering.

Here it is:

Notes: Though it's normally impossible for DA94 (end of list terminator, FF) to be E9, using ws m will change it to this value. This is perfectly safe. Only problem is if you withdrew a Pokémon and DA93 (20th Pokémon) was not FF, but this is not possible in normal gameplay.

(You must have exactly 19 Pokémon in the box.)

When you deposit a 20th Pokémon, after using w sm, DA94 gets changed back to FF again.


Pokémon:

Butterfree
Voltorb
Gyarados
Spearow
Golduck
Poliwrath
Voltorb
Pikachu
Clefairy
Golduck
Venomoth
Metapod
Tangela
Nidoking
Haunter
Flareon
Parasect
Growlithe
Voltorb

Which would make the RAM and ASM looked like this:
; Initial hl = DA7F
$DA7F <- 13 || inc de
$DA80 <- 7D || ld a, l ; a = 7F
$DA81 <- 06 ||
$DA82 <- 16 || ld b, 16 ; b = 16
$DA83 <- 05 || dec b ; b = 15 (since Mew is unobtainable)
$DA84 <- 80 || add a, b ; a = 94
$DA85 <- 6F || ld l, a ; l = 94
$DA86 <- 06 ||
$DA87 <- 54 || ld b, 54 ; b = 54
$DA88 <- 04 || inc b ; b = 55 (since Raichu is unobtainable in Yellow)
$DA89 <- 80 || add a, b ; a = E9
$DA8A <- 77 || ld (hl), a ; $DA94 <- E9
$DA8B <- 7C || ld a, h ; a = DA
$DA8C <- 1E ||
$DA8D <- 07 || ld e, 7 ; e = 7
$DA8E <- 93 || sub e ; a = D3
$DA8F <- 67 || ld h, a ; h = D3
$DA90 <- 2E ||
$DA91 <- 21 || ld l, 21 ; l = 21
$DA92 <- 06 ||
$DA93 <- FF || ld b, ff ; rst 38 prevention
$DA94 <- E9 || jp (hl) ; finally jumps to $D321!

[luckytyphlosion]

Is it possible to rewrite ROM data with arbitrary code? If not, how many different sections of RAM can you use to use 8F/w sm?

[Evie the Bird Mother 🌸 ☽]

Is it possible to rewrite ROM data with arbitrary code? If not, how many different sections of RAM can you use to use 8F/w sm?

Not possible; you can never rewrite the ROM (read only memory) without doing something like editing it with a hex editor, even though Game Genie makes temporary patches to the ROM. If you try to write to the ROM with w sm, nothing will happen. The Game Boy/machine alone cannot write to ROM.

You can write to VRAM (0x8000-0x9FFF), RAM (0xC000-0xCFFF), WRAM (0xD000-0xFEFF) and RAM (2) (0xFF80-0xFFFF) with wsm (haven't tested 8F on Red/Blue but I imagine things would work the same). Writing to SRAM and I/O apparently didn't work. I've never looked up what I/O is, but I'm a bit surprised at SRAM not being changed; as it is for data that is saved.

[Nerator]

If you try to write to the ROM with w sm, nothing will happen.

To be perfectly precise, if i'm not mistaken, if you'll try to write something to ROM0($0000-$3FFF), then the game will switch ROM banks in $4000-$7FFF. For example if you'll execute

Code: [Select]

ld [$2000],a
then game will switch to bank, which number is in register a. For Red/Blue last bank is $2B i believe, for Yellow it's $3F. Not sure, what will happen, if we'll try to write to switchable ROM ($4000-$7FFF), or how it could be used at all.

EDIT:
Actually, what i whote above is not completely right. For the game to switch banks, we need to write in $2000-$2FFF area. Also we can switch the RAM banks (switchable RAM is at $A000-$BFFF) by writing in $4000-$5FFF area. It seems, that RAM contains 16 banks (0-F)
EDIT2:
Aslo i found in diassemby of Red many tries to write values to $6000 and $0000 usually 0 or 1. Have no idea what these are for.

[pigdevil2010]

I can finally registered! Thanks Torchickens!

So, since I have a shorter bootstrapping code for w sm. Here is also my shorter bootstrapping code for 8F.

You must have exactly 5 Pokemon in a party, these are:
Pidgey with 233 HP
Parasect
Onix
Tentacool
Kangaskhan

Which would make the RAM and ASM looked like this:

Code: [Select]

; Initial hl = D163
$D163 <- 05 || dec b
$D164 <- 24 || inc h    ; h = D2
$D165 <- 2e ||
$D166 <- 22 || ld l, 22 ; l = 22
$D167 <- 18 ||
$D168 <- 02 || jr 2     ; pc = D16B
$D16B <- 24 || inc h    ; h = D3
$D16C <- 00 || nop
$D16D <- e9 || jp hl    ; pc = D322


[Evie the Bird Mother 🌸 ☽]

Well done! No more specific remaining PP, or moves. 

To get a Pidgey with that much HP at level 100 from being 'fresh' (just Rare Candied), use six HP Ups. This will always give it a max of 237 HP (because HP DVs don't exist in Generation I/II). Five HP Ups will give it 234 HP, but problem is the Pokémon on Route 1 tend to deal 2 or 3 damage. I don't know if it's possible for them to deal 1 HP, but you'd probably have to have stat experience/good DVs on the Pidgey's Defense.

[pigdevil2010]

Five HP Ups will give it 234 HP, but problem is the Pokémon on Route 1 tend to deal 2 or 3 damage. I don't know if it's possible for them to deal 1 HP, but you'd probably have to have stat experience/good DVs on the Pidgey's Defense.

Just make it poisoned, heal it to 234 HP, then walk 4 steps.

[Evie the Bird Mother 🌸 ☽]

Five HP Ups will give it 234 HP, but problem is the Pokémon on Route 1 tend to deal 2 or 3 damage. I don't know if it's possible for them to deal 1 HP, but you'd probably have to have stat experience/good DVs on the Pidgey's Defense.

Just make it poisoned, heal it to 234 HP, then walk 4 steps.

Oops, I forgot about poison. Thanks for mentioning it.

[gskw]

When I write Z80 ASM, how do I turn it into hex codes?

[Wack0]

When I write Z80 ASM, how do I turn it into hex codes?

http://iimarck.us/etc/asmopcodes.txt can help, or you can use a compiler and get the compiled result out of the object file.

[gskw]

Thanks. I think I'm going to write my own assembler to make stuff easier...

EDIT: The compiler is live online!

[Wack0]

Thanks. I think I'm going to write my own assembler to make stuff easier...

EDIT: The compiler is live online!

Heh, thanks. I've been meaning to write one myself actually.

[gskw]

Why wouldn't we make the code jump into the PC items so we can get more space?

[pigdevil2010]

Why wouldn't we make the code jump into the PC items so we can get more space?

Yeah, I once have an idea about doing this too.

[PokeGlitchFanatic]

Gosh.The  first gen was screwed up big time.