Button activators ("Jokers") for systems with no 'If, Then' code type?

[Lazy Bastard]

I realize that in many cases, this would require a new routine to be written (unless you don't mind hijacking an existing routine and overwriting functionality/remapping buttons), but I'd like to know how one makes a joker for such systems as NES, SNES, etc, in which we have no code types other than simple writes.

I'm also interested in any information on common values for different button presses, as there generally are for systems like PSX, PS2, N64, etc.

Not having jokers is a constant source of irritation in hacking retro systems, and I think if we nailed down a decent way of making them, we could do a lot more in general.

[rimsky82]

Do you mean like my tetris up code?

I don't remember how I found it, but I found the routine that checks for button presses, and simply hijacked where it looks for the up button and wrote a new routine that alters the position of the falling piece in the puzzle. Take a look. At $8926, I overwrote two lines with a jump to an unused location. Then, I executed the lines I overwrote and made the jumps accordingly so no functionality was missing. Then, I continued with the routine for moving the piece up, and finally jumped back to where I began.

Looking back, I maybe could have used less lines by making it a subroutine, but I wrote this one a while back.

I attached an fceux cht file so you don't have to do all of the copy/pasting.

GGXAVOPX
EGXANPLA
NYXEEPES
XTXEOOPK
PXKYENNN
LEKYONNY
ENKYXNNY
LEKYUNNY
GKKYKNNN
LNKYSNNY
OEKYVNNN
SXKYNNNY
VUKNENNY
PXKNONUN
AEKNXNNN
ENKNUNNY
AEKNKNNN
SXKNSNNY
PKKNVNNL
PXKNNNNN
ENSYONNY
ZESYXNNY
VKSYUNNY
PKSYKNNY
GKSYSNNN
ZXSYVNNN
OESYNNNN

[Lazy Bastard]

Cool; thanks for the info

Did you ever make a map of what values it expected per button/button combo?

[rimsky82]

Here's an explanation without having to open an emulator.

00:8924:A5 B6     LDA $00B6 = #$00
00:8926:4C C0 FF  JMP $FFC0    <- Jump to new location
00:8929:EA        NOP
00:892A:A5 B5     LDA $00B5 = #$00
...
01:FFC0:29 03     AND #$03  <- Overwritten instruction (Check for left or right)
01:FFC2:F0 03     BEQ $FFC7 <- If no left or right, continue by branching over the jump
01:FFC4:4C 73 89  JMP $8973 <- else jump to the routine for left/right
01:FFC7:A5 B6     LDA $00B6  <- Get the current button press
01:FFC9:29 08     AND #$08  <- Check for Up Button
01:FFCB:F0 08     BEQ $FFD5  <- if no up button, branch to the end
01:FFCD:A5 41     LDA $0041 <- get current height of piece
01:FFCF:29 FF     AND #$FF  <- Check to see if it's at the top
01:FFD1:F0 02     BEQ $FFD5  <- If so, branch to the end
01:FFD3:C6 41     DEC $0041 <- else, move the piece up
01:FFD5:4C 2A 89  JMP $892A  <- go back to original code

Button values are a bitfield.
$00F7 (bin) 0000 0000 = ABSeSt UDLR

so a value of 0x80 would be button A, or 0x44 would be down and B, or 0xC1 would be A + B + Right, and so on.

[Lazy Bastard]

Ah, interesting. So, for individual buttons:

0x80 = A
0x40 = B
0x20 = Select
0x10 = Start
0x08 = Up
0x04 = Down
0x02 = Left
0x01 = Right

...and combinations are addition operations. Very similar to PSX or N64.

Is there any consistency in this system, among other NES games? It would be nice to determine either one vastly consistent button value set, or several very common sets, and write a quick guide to hacking jokers for NES (then move on to SNES, etc); something similar to this, perhaps:

http://GameHacking.org/?s=faqs&id=10

[Abystus]

01:FFC9:29 08     AND #$08  <- Check for Up Button
01:FFCF:29 FF     AND #$FF  <- Check to see if it's at the top

Is there any reason that a CMP was not used here or was it just preference? Just wondering if doing the AND method of comparison does anything different in the big picture. Also with the value FF compared to the top of the screen is it because when reaching the top of the screen Y (0) would wrap to FF and compare true for the branch (I would assume 0 would be used as the comparison value but again just picking your brain )?

[rimsky82]

01:FFC9:29 08     AND #$08  <- Check for Up Button
01:FFCF:29 FF     AND #$FF  <- Check to see if it's at the top

Is there any reason that a CMP was not used here or was it just preference? Just wondering if doing the AND method of comparison does anything different in the big picture. Also with the value FF compared to the top of the screen is it because when reaching the top of the screen Y (0) would wrap to FF and compare true for the branch?

If I used a CMP for the up btn, then it would have to be the up button alone. With the AND, the player could also be pressing any of the other buttons. However the second could have used a CMP, I guess, but makes the same result (yet opposite) as far as the zero flag goes anyway.

Its not actually a screenYpos, but the position within the puzzle. If I didn't have that check, the piece would rise above the puzzle bounds and glitch the game.

Now that I think about it, the AND FF was pointless, since I was checking for a zero byte anyway. Have to remember I made this code after starting out... heh.

[Abystus]

If I used a CMP for the up btn, then it would have to be the up button alone. With the AND, the player could also be pressing any of the other buttons. However the second could have used a CMP, I guess, but makes the same result (yet opposite) as far as the zero flag goes anyway.

Its not actually a screenYpos, but the position within the puzzle. If I didn't have that check, the piece would rise above the puzzle bounds and glitch the game.

Now that I think about it, the AND FF was pointless, since I was checking for a zero byte anyway. Have to remember I made this code after starting out... heh.

Ah good point I wasn't taking in account for multiple buttons pressed at the same time which a compare would completely ignore. Also I noticed the instruction you replaced used an AND instead of a compare anyways so no points off for following the original flow of code. I've never been a friend of bitwise operations for the most part however I can now see the method to their madness and may attempt to use them more in the future.

[lee4]

SNES button values from Megaman X found by dcx2 at wiird

00 = NO button press
01 = Start
02 = Select
04 = R
08 = L
10 = X
20 = A
40 = Y
80 = B

[Lazy Bastard]

Sweet; good to know.

Hopefully we can cross reference a few more games for both NES and SNES, and get a baseline.

[Abystus]

SNES button values from Megaman X found by dcx2 at wiird

00 = NO button press
01 = Start
02 = Select
04 = R
08 = L
10 = X
20 = A
40 = Y
80 = B

Maybe these values change by developer as trying the above values in searches while holding the various buttons did not produce any results in MK2. I would assume for testing/protocol purposes on their end developers would try to make all their games follow the same control input layout. I may test this on another Capcom game when I get home from work to see if there is any pattern.

[Helder]

I was thinking of this myself, perhaps trying various Capcom games would net the same results where maybe nintendo developed games might be a different set. If this is the case we would have to make a table with different developers and their button values. I also remember some emulator or a plugin that I think it was DarkSerge who told me about it years ago that has code support like the PSX/N64 GS.

[nolberto82]

To find button addresses and values what I do is use a debugger. For example for the NES I put a breakpoint on read on 0x4016 and find where the register writes to. Nintendo games from what I have seen use the same values for all their games. Other companies may use different values. I wonder if it is possible to add support for jokers to retro emulators is one of those things that is really needed.

[lee4]

those values from Megaman X are for game-in button mapping.


I did my own testing

Here from SNES9X emulator universal values and addess will be different each game.

Example

Street Fighter 2 Turbo Button Register Activator (2-byte mode) [SNES9X]
P1_____A7A6
7E00A6:0000

0000 = NO Buttons
0010 = R
0020 = L
0040 = X
0080 = A
0100 = Right
0200 = Left
0400 = Down
0800 = Up
1000 = Start
2000 = Select
4000 = Y
8000 = B

[lee4]

Streets of Rage 3 on the Gens210mk1 emulator

FFF704
00 = no button
01 = up
02 = down
04 = left
08 = right
10 = B
20 = C
40 = A
80 = Start

FFF706
E0 = no button
E1 = Z
E2 = Y
E4 = X
E8 = Mode (6 buttons)