Alright thanks, Ill try it again when I get home from work.

Sure. Why not? It would be a good exercise for you to write some new code that reduces all prices and hp amounts by 1/2 on load. Remember shifting right divides binary numbers by 2. That's a simple way to do even division.

Yes. you can do that, but ideally, you'd want to use a cross assembler which would be able to assemble your code right into the ROM in one step. I haven't done any NES hacking in years though, so I can't recommend a specific  tool to do this.

Remember though, if you're adding instructions, you'll need to find some free space in the ROM. For example, say you want to add a single shit instruction right after it loads the price value. It's not as simple as just 'adding' the instruction. You can't do that(unless you disassembled and reassembled the entire ROM which is unlikely).

 Instead, you'd strategically replace an instruction or two and jump a new routine in a new spot in the ROM. There you'd do your shift, and take care fo the instructions you replaced earlier and return back to the original game code.

You could always convert the instruction you're looking for into its opcode (see here) and then search for it in a hex editor.  For example, look at:


      Mnemonic   Addressing Mode       Opcode
      ---------------------------------------
      LDA        #aa                   A9
      LDA        $aa                   A5
      LDA        $aa,X                 B5
      LDA        $aaaa                 AD
      LDA        $aaaa,X               BD
      LDA        $aaaa,Y               B9
      LDA        ($aa,X)               A1
      LDA        ($aa),Y               B1


lda ($26),y would be correspond to

LDA        ($aa),Y               B1

so, your command in hex would be "B1 26".  Keep in mind that something like lda $1234 would be "AD 34 12" and not "AD 12 34".

Hope that helps.

Big chunks of 00s or FFs are likely candidates.
But some games might also use junk data to hide free space.
When I looked at STED (making a hack to show EXP on the data menu), there wasn't much free space.
I vaguely remember seeing some text that looked like debug menu text near the end of the program ROM. If there's no way to access it, then I suppose that could be considered free space.

I was able to use your tips to find free space in the rom, and add some code in there to cut shop prices in half.  The game crashed at the moment before the prices were listed.  The LDA instruction that loads the price into memory was replaced by a jump to the free space with the instructions:lda load price into memory, logical shift right, and a jump back into the main code. Is there something I missed?

Yep, you're going to want to use a debugger and look at your code being executed. It'll show you what's happening. It's most likely doing something you didn't intend or expect with the code you wrote.

Care to post the last little bit of your log? It sounds like you're trying to jump from one bank into another bank without swapping it in.