Written by: Tony Hedstrom email@example.com https://www.angelfire.com/games2/codehut/ Version 0.4 6/05/07 This guide will show you how to find useful ROM addresses and make Game Genie codes using a program called Gens Tracer. I'll explain how to make and use memory traces and assembly trace logs. I originally wrote this for some people in a NHL 94 forum. ------------------------------------------------- Here's a list of what you'll need... 1) Gens Hacking Version (https://thegshi.org/ (download section)). 2) Gens Tracer (https://thegshi.org/ (download section)). 3) A hex editor (your choice or bpsoft.com for free trial version). 4) NHL94 ROM (in bin format). 5) Some basic 68000 assembly language knowledge is helpful, but not absolutely necessary. Here's what that stuff is for... We'll use Gens Hacking Version to find RAM addresses. Some people use Kega Fusion to find RAM addresses. If you'd rather use it, that's fine. We'll use Gens Tracer for memory traces and assembly trace logs. Gens Tracer is really the "magic" program that helps us find useful ROM addresses. The hex editor is for finding ROM addresses, and the NHL94 ROM is what we'll be poking around in trying to find ROM addresses. To try to keep this relatively simple, we'll find the ROM address that determines how much time you get for each period. The game normally lets you select from 5, 10 and 20 minute periods, and there is the "cheat" that lets you have 30 second periods. I'll show you how to find those values in the ROM so you can change them to any value you want, and I'll show you how to stop the clock completely so you'll have infinite time. Not that you'd want infinite time, but mainly to show you how it's done. Since making Game Genie codes is kinda my "thing", I'll also show you how to make Game Genie codes for all this stuff. Okay, lets get to the good stuff... The first thing we need to do is find the RAM address for the clock. This is very easy to do. Just start up Gens Hacking Version, load your NHL94 ROM, and start a game. When the clock starts to count down, click on "Cheats / Search GG codes". Click the "reset" button and then click "OK". Let the clock count down 1 or 2 seconds and click on "Cheats / Search GG codes". Now make sure "less than", "Previous Value" and "1 byte" are all checked, then click on "Search". Click "OK" and go back to the game and let the clock count down a few more seconds. click on "Cheats / Search GG codes" again and click "Search". Keep repeating this until you only have a few RAM addresses left. If you're doing this on your own, you'll need to test each RAM address to find out which one is the one you're looking for. You can do that by highlighting the RAM address and clicking "add cheat" and testing each one until you find the one that you're looking for. For this example, I'll tell you that the RAM address for the clock is FFC469. It's actually FFC468, but for our purposes (and to keep this shorter), FFC469 will work fine. Okay, so we know that the RAM address for the clock is FFC469. Now we need to find out where in the ROM this RAM address is being loaded, modified, etc. That's where Gens Tracer come in. When you set up Gens Tracer, it has a folder called "tracer". In that folder is a text file called "hook_log.txt". Open up that text file (with notepad) and change it so it looks exactly like this... hook_pc1 0 -1 -1 hook_pc2 1 -1 -1 hook_pc3 1 -1 -1 hook_rd1 0 -1 -1 hook_rd2 0 -1 -1 hook_rd3 0 -1 -1 hook_wr1 0 ffc469 ffc469 hook_wr2 0 -1 -1 hook_wr3 0 -1 -1 hook_ppu1 1 -1 -1 hook_ppu2 1 -1 -1 hook_ppu3 1 -1 -1 (There's other stuff down here, but just leave it as is). As you can see, we put our RAM address for the clock in there for a hook on write (hook_wr1). What this does is tells Gens Tracer to make a note every time that RAM address is changed, and what ROM address caused it to change. This is similiar to breakpoints. Save the changes you made to your "hook_log.txt" file and close it. Open up Gens Tracer, and load your NHL94 ROM. As soon as you see the "NHL Hockey 94" title screen, press the "," key on your keyboard. This starts the memory tracer. Now keep pressing the "Start" button (usually the "Enter/Return" key on your keyboard) until the game starts. Wait until the clock counts down about 3 or 4 seconds and press the "," key again. This stops the memory tracer. Exit Gens Tracer. The memory trace that you just made for RAM address FFC469 has been saved to a text file called "hook.txt". When you open up "hook.txt", this is what you should see... [01:730A] W32 = 00000000 [FFC468] [00:7830] W16 = 0258 [FFC468] [00:7830] W16 = 0258 [FFC468] [01:5DFC] W16 = 0257 [FFC468] [01:5DFC] W16 = 0256 [FFC468] [01:5DFC] W16 = 0255 [FFC468] [01:5DFC] W16 = 0254 [FFC468] TRACE STOPPED There's some very useful info in this file. A quick breakdown: The ROM addresses are on the far left side, followed by how many bits, followed by the value, and then the RAM address at the far right side. The ROM addresses listed are not the exact ROM addresses we're looking for, but they're usually pretty close. Next I'll show you how to use the info in the "hook.txt" file to find the ROM addresses we're looking for. Here's the memory trace we just made: [01:730A] W32 = 00000000 [FFC468] [00:7830] W16 = 0258 [FFC468] [00:7830] W16 = 0258 [FFC468] [01:5DFC] W16 = 0257 [FFC468] [01:5DFC] W16 = 0256 [FFC468] [01:5DFC] W16 = 0255 [FFC468] [01:5DFC] W16 = 0254 [FFC468] TRACE STOPPED Here's what each of those lines mean... [01:730A] W32 = 00000000 [FFC468] This one just sets the value to 00000000. nothing we need. [00:7830] W16 = 0258 [FFC468] This one tells us which ROM address moves the value 0258 to our RAM address for the clock. 0258 is a hex number. 0258 = 600 in decimal. There are 60 seconds in a minute, so 600 divided by 60 = 10 minutes. 10 minutes is the amount of time that is on the clock when the game started. We can use the ROM address in this line to help us find the actual ROM address we're looking for (the ROM address to change the amount of time you start the game with). [01:5DFC] W16 = 0257 [FFC468] This one tells us the ROM address that is subtracting one number from the clock value for every second that goes by. We can use this ROM address to help us find the instruction that is causing the clock to count down. By changing this instruction, we can stop the clock (infinite time). This is where a little 68000 assembly knowledge can be helpful. [01:5DFC] W16 = 0256 [FFC468] Same as above, except one second less. [01:5DFC] W16 = 0255 [FFC468] Same as above, except one second less. And so on. After you've done several of these memory traces, you can quite often just open up the ROM with a hex editor and go to the ROM address listed and figure out what you need to know. For example, I can open up my NHL94 ROM with a hex editor, go to ROM address $015DFC, and instantly see what instruction I need to change to have infinite time. But since learning how to read raw 68000 assembly is something most people don't want to do, here is how you can make an assembly trace log that will show you everything that's going on in the CPU, and send it all to a text file in a "relatively" easy to read format. When you first see your assembly trace log, it's easy to be overwhelmed by all the crap in there, but don't let it scare you off. After you get used to it, it's not really that hard to understand. An assembly trace log is getting down into the real nuts and bolts of how a Genesis game works. If you can learn how to "read" it, you can find all kinds of very useful ROM addresses (and Game Genie codes). Okay, here's how to make an assembly trace log. Open up your NHL94 ROM using Gens Tracer. Get to the first option screen (the one where you can change Play Mode, Per Length, etc). Press the "/" key on your keyboard. This starts the assembly trace log. Now press your "Start" button 3 times so that you're on the ice and ready to start playing. Wait for the clock to count down a few seconds and then press the "/" key again. This stops the assembly trace log. Exit Gens Tracer. Your newly created assembly trace log has been sent to a file called "Trace.log" Keep in mind that the Genesis CPU is operating at about 4Mhz, so the Trace.log files can be very big. Remember that everytime you make a new memory trace file, or a new assembly trace log, the old one will be overwritten. So if you want to keep any, make sure you rename it. Now lets open up our new Trace.log file and take a look at it. I recommend using something like MS Word to open it with. We'll start off by figuring out how to make the game have infinite time. When you have the file opened, use the "Find" option and type in 01:5DFA. Here's how I came up with that number... Look at the memory trace we made: [01:730A] W32 = 00000000 [FFC468] [00:7830] W16 = 0258 [FFC468] [00:7830] W16 = 0258 [FFC468] [01:5DFC] W16 = 0257 [FFC468] [01:5DFC] W16 = 0256 [FFC468] [01:5DFC] W16 = 0255 [FFC468] [01:5DFC] W16 = 0254 [FFC468] TRACE STOPPED See where it shows the time starting to count down... [01:5DFC] W16 = 0257 [FFC468] [01:5DFC] W16 = 0256 [FFC468] [01:5DFC] W16 = 0255 [FFC468] [01:5DFC] W16 = 0254 [FFC468] 0257 seconds 0256 seconds 0255 seconds 0254 seconds The ROM address shown for each time a second is subtracted is always the same: 01:5DFC, so we know that the instruction we're looking for is near by that ROM address. The way that this memory trace works, you usually need to subtract 2 from the ROM address listed. So to get the ROM address we need to look for in our Trace.log file, we just need to subtract 2 from 01:5DFC = 01:5DFA. That's how I came up with the number to put in the "Find" box. Alright, back to the Trace.log file... type in the ROM address we're looking for (01:5DFA) into the "Find" box and click on "Find Next". Your first match should look like this: 01:5DFA 53 78 SUBQ.W #1,($C468) .... A0=FFFFBDEC A1=FFFFCA32 A2=FFFFC6CE A3=FFFFB74A A4=FFFFB892 A5=FFFFB8B4 A6=FFFFC060 A7=FFFFFF62 D0=FFFF0100 D1=00000122 D2=00000000 D3=00000183 D4=00000006 D5=0000000C D6=0000000A D7=00000001 XNZvC This tells us exactly what we need to know to give us infinite time. SUBQ.W #1,($C468) means that it is subtracting 1 from RAM address $C468. If you remember, FFC468 is our RAM address for time. All we have to do to get the clock to stop counting down (inf time), is figure out how to kill that SUBQ.W instruction. We can easily do this by changing the 5378 opcode shown above to a branch opcode (6002) instead. This will make the game jump right over the subtraction part. We can test this out by making a quick Game Genie code... Just use a Game Genie conversion program (you can get one from my web site): 015DFA:6002 = AKRT-CA94 015DFA is the ROM address, and 6002 is the replacement opcode. Because it is beyond the scope of this guide, I'm not going to go into how I came up with 6002 for the opcode to kill the subtraction. This is where some 68000 assembly knowledge is helpful. Here's a quick tip... using a branch opcode (usually 6002 or 6004) is the most common way to kill many opcodes (subtraction, addition, etc). 6002 will "jump" 2 bytes, and 6004 will "jump" 4 bytes. You'll usually want to jump to the next opcode. NEVER jump to an operand (this can cause the game to freeze). Anyways, if you try our new Game Genie code (AKRT-CA94) with your favorite emulator, or on a real Genesis, you'll see that you now have infinite time. A little short cut... Gens emulator will let you enter Game Genie codes in raw format, so instead of converting the ROM address and opcode into a Game Genie code, Gens lets you enter it like this: 015DFA:6002. Saves a little time. You can easily hack infinite time into your ROM by going to ROM address $015DFA and changing the 5378 to 6002. Of course you'll also need to hack the master code too (or else you'll get a blank screen). All right, now I'll show you how to find the ROM addresses for period lengths so you can adjust how much time you start each period with to any value you want. Use the same memory trace and assembly trace log as before. Our memory trace looked like this: [01:730A] W32 = 00000000 [FFC468] [00:7830] W16 = 0258 [FFC468] [00:7830] W16 = 0258 [FFC468] [01:5DFC] W16 = 0257 [FFC468] [01:5DFC] W16 = 0256 [FFC468] [01:5DFC] W16 = 0255 [FFC468] [01:5DFC] W16 = 0254 [FFC468] TRACE STOPPED The ROM address we're interested in here is 00:7830 because that's the ROM addresses that loaded the initial value of 0258. And remember that 0258 was a hex number which equals 600 in decimal, which equals 10 minutes (which is how much time we started the period with). Just like before, we need to subtract 2 from the ROM address listed in the memory trace to get the actual ROM address that we'll be searching for in the trace.log file. So 00:7830 - 2 = 00:782E. If you're not sure how to do hex math, just use your windows calculator. So 00:782E is the ROM address we'll search for in our trace.log file. Open up your trace.log file (with MS Word or ?), open up the "Find" window and type in our ROM address: 00:782E. Click "Find Next" and you'll notice that your first match isn't on the far left side of the page. For this example, we're only interested in matches on the far left side of the page. So click "Find Next" again. Your second match should be on the far left side of the page. This is what we're looking for. Here's what it should look like: 00:782E 31 C0 MOVE.w D0,($C468) .... A0=0000785E A1=FFFFBFAA A2=FFFFCA32 A3=FFFFB84A A4=FFFFB88A A5=FFFFD404 A6=00000000 A7=FFFFFFFA D0=C29D0258 D1=0000FFFF D2=FFFF0000 D3=00000080 D4=0000022E D5=0000000C D6=00005311 D7=0000000A xNzvC Here's what the first line means... 00:782E 31 C0 MOVE.w D0,($C468) 00:782E is the ROM address of the instruction (opcode). 31 C0 is the opcode. MOVE.w D0,($C468) means it is moving the value in register D0 to RAM address $C468. RAM address $C468 (FFC468) is the amount of time on the clock. You may be asking... what the hell is register D0? Pay close attention here, because this is an important part of being able to read an assembly trace. In my "snippet" above from our trace.log file, there are a total of 16 registers shown. Registers are kinda like storage areas. There are 2 types of registers... The "A" registers are for Addresses, and the "D" registers are for Data. There are 8 "A" registers (A0 through A7) and 8 "D" registers (D0 through D7). All 16 registers are always listed for every opcode in our trace.log files. Get to know them, love them,... be one with the registers. Okay, I'm getting carried away, but registers are pretty important. We'll need to read the registers to find the ROM addresses for the values for the period lengths. From our trace.log snippet above, here's part of the first line: MOVE.w D0,($C468) We know that it "MOVEs" the value in register D0 to RAM address $C468. If you look at register D0 (above), this is what you'll see: D0=C29D0258. See the "0258" in there? That's where our number of seconds for each period is coming from. All we have to do is figure out where that value is coming from, and we'll find our ROM addresses. Let me explain one more thing before we move on... you may have noticed that the value in register D0 was C29D0258. Why does the game only use 0258 instead of the entire value of C29D0258? The answer is because the "MOVE" instruction has a "w" after it (MOVE.w), and the w means Word. A "word" is 2 bytes. Here is the list of the letters used and what they mean... B = Byte (1 byte) W = Word (2 bytes) L = Long word (4 bytes) Moving on... we know that register D0 cantains the value that we're interested in (0258). Here's how we figure out where that value comes from. This gets a bit complicated, so go drink a big cup of coffee so you'll be extra alert. To figure out where the value in register D0 is coming from, we need to read the assembly trace log backwards. Here is the part of the trace.log that we'll need to find what we're looking for... 00:7858 30 30 MOVE.w $00(A0,D0),D0 .... A0=0000785E A1=FFFFBFAA A2=FFFFCA32 A3=FFFFB84A A4=FFFFB88A A5=FFFFD404 A6=00000000 A7=FFFFFFF6 D0=C29D0002 D1=0000FFFF D2=FFFF0000 D3=00000080 D4=0000022E D5=0000000C D6=00005311 D7=0000000A xnzvc 00:785C 4E 75 RTS .... A0=0000785E A1=FFFFBFAA A2=FFFFCA32 A3=FFFFB84A A4=FFFFB88A A5=FFFFD404 A6=00000000 A7=FFFFFFF6 D0=C29D0258 D1=0000FFFF D2=FFFF0000 D3=00000080 D4=0000022E D5=0000000C D6=00005311 D7=0000000A xnzvc 00:7818 0C 78 CMPI.W #$0003,($C466) .... A0=0000785E A1=FFFFBFAA A2=FFFFCA32 A3=FFFFB84A A4=FFFFB88A A5=FFFFD404 A6=00000000 A7=FFFFFFFA D0=C29D0258 D1=0000FFFF D2=FFFF0000 D3=00000080 D4=0000022E D5=0000000C D6=00005311 D7=0000000A xnzvc 00:781E 6D 00 BLT #$000E [00:782E] .... A0=0000785E A1=FFFFBFAA A2=FFFFCA32 A3=FFFFB84A A4=FFFFB88A A5=FFFFD404 A6=00000000 A7=FFFFFFFA D0=C29D0258 D1=0000FFFF D2=FFFF0000 D3=00000080 D4=0000022E D5=0000000C D6=00005311 D7=0000000A xNzvC 00:782E 31 C0 MOVE.w D0,($C468) .... A0=0000785E A1=FFFFBFAA A2=FFFFCA32 A3=FFFFB84A A4=FFFFB88A A5=FFFFD404 A6=00000000 A7=FFFFFFFA D0=C29D0258 D1=0000FFFF D2=FFFF0000 D3=00000080 D4=0000022E D5=0000000C D6=00005311 D7=0000000A xNzvC For right now, the only thing we're interested in is register D0. You'll notice that if you start at the bottom (00:782E), the value in D0 is C29D0258. Reading backwards (moving upwards one section), the value in D0 at ROM address 00:781E is still C29D0258. No change in D0 so move upwards to the next section. D0 is still the same, so move upwards again. When we finally get to the very top section (00:7858), you'll see that the value in register D0 has changed. Here's what that section looks like: 00:7858 30 30 MOVE.w $00(A0,D0),D0 .... A0=0000785E A1=FFFFBFAA A2=FFFFCA32 A3=FFFFB84A A4=FFFFB88A A5=FFFFD404 A6=00000000 A7=FFFFFFF6 D0=C29D0002 D1=0000FFFF D2=FFFF0000 D3=00000080 D4=0000022E D5=0000000C D6=00005311 D7=0000000A xnzvc The value in D0 has changed to C29D0002. The 0258 part is gone. So we know that this is the line that loaded the number of seconds (0258) into register D0. This line of code will tell us what we need to know. Drum roll please... MOVE.w $00(A0,D0),D0 I know it may seem a little cryptic, but here's what it means: see where it says (A0,D0)... that means you take the 2 byte value in D0 (0002) and add it to the ROM address in register A0 (remember, the A registers are for Addresses). A0=0000785E so 0000785E + 0002 = 00007860. If you go to ROM address 00007860 with your hex editor, this is the value you'll see: 0258. BINGO!! We found it! This is the value that the game uses for 10 minute periods. You can change that value to any number you want with a Game Genie code, or by hacking the ROM. A value of 0001 would give you 1 second periods, 001E would give you 30 second periods, etc. Just remember it uses hex values. And remember it will only work when you select 10 minute periods (the default time). Here's a couple Game Genie codes: 007860:01A4 = YV6A-ACDA 7 minute periods. Only works when you leave the period length at 10 minutes. 007860:003C = HV6A-AADA 1 minute periods. Only works when you leave the period length at 10 minutes. Here's some "extra" info. If you look at the ROM address (with your hex editor) where we found our value for the 10 minute periods, here is what you'll see around it: 012C 0258 04B0 001E These are all the values for the other period lengths. 012C = 5 minute periods. 0258 = 10 minute periods (this is the one we found). 04B0 = 20 minute periods. 001E = 30 second periods (this is for the controller cheat). You can change those values to anything you want, and if you selected that particular period length in the option screen, your new "hacked" time would kick in. Well, that concludes our ROM hacking 101 class. As I said before, after you've done a few, reading an assembly trace log gets easier. Just remember that you don't need to know what everything in the trace.log means, just the part you're interested in. To be honest with you, there is still a lot of stuff in there that I don't completely understand. Get to know the basics and you'll find lots of useful stuff. Tony H.