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1 user currently in Rom Hacking:  hukka |
| Acmlm's Board - I2 Archive - Rom Hacking - im looking for a... |
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ziratha Ninji Level: 24 Posts: 9/231 EXP: 77808 For next: 317 Since: 06-29-04 Since last post: 4 days Last activity: 17 days |
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| hi , i intend to learn asm, ive gotten some faqs, and readmes from various websites but it all means zilch if i cant get to the asm, so i need a dissasembler, preferable a fairly good one at preferably compatible with snes level stuff. I then need to be able to get to the asm code. so if i need a program for that, ill need to know, aka if its a .xxx extension i need to know what opens .xxx . finally and less immediatly i need a assembler, only if the disaseembler doesn't assemble. oh, and any learning guides you all happen to know about i would appreciate. if you could even tell me the utilities you all use, that would be a start. thanks for anything you post, also if this has been covered in a post i missed, please point me to the said post. but ive only seena few passing mentions of nesdev and another site. |
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NightHawk Bob-Omb Level: 39  Posts: 516/621 EXP: 374743 For next: 30028 Since: 03-26-04 From: Switzerland Since last post: 432 days Last activity: 339 days |
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| Okay, first off, look here for disassemblers and assemblers for whatever you're dealing with (if it's not there, try google). Assembler source documents are basically just regular text documents (like you'd create with Notepad, for instance); they only have a different extension to signal their purpose. And again, for general tutorials, the link I gave earlier should have enough to at least get you started (and then you can come back with more specific questions  ). |
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ziratha Ninji Level: 24 Posts: 10/231 EXP: 77808 For next: 317 Since: 06-29-04 Since last post: 4 days Last activity: 17 days |
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| ok thanks, any ideas are welcome. | |||
d4s Panser Level: 29  Posts: 52/325 EXP: 142151 For next: 5734 Since: 03-23-04 Since last post: 13 days Last activity: 1 day |
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| the best way to learn assembler is learning by doing. personally, ive also started with disassembling a small rom(slideshow) and reassembling it. however, this is far more complicated than it may seem. some issues include: the memory/accu and x/y settings. you can change the size of the accumulator and the x/y registers for 8 or 16bits, thats a byte or a word(two bytes) on the fly. while this may be useful for programmers, it often causes disassemblers to misinterpret given data. example pseudocode: Blahblah: REP $#30 ; set accumulator, memory, x and y regs to 16bit LDA $80 ; load two bytes(16bit) from adress wram adress $80 into accumulator STA $81 ; store accumulator in wram adress $81 and $82 BRA Wurst ; branch to routine Wurst ;************************************************************* ;problem ;************************************************************* SomeRoutine: LDA $#45 ; load 8bit value 45 into accumulator JSR $00A9 ; jump to subroutine RTS ;return ;************************************************************* Wurst: SEP $#20 ; set accumulator/memory to 8bit REP $#10 ; set x/y regs to 16bit BRA SomeRoutine ; branch always to SomeRoutine the problem here is that the size of the accumulator is changed during execution. disassemblers either have a fixed setting for it during disassembly or have some basic tracing features, but still, they get some data wrong. the above code example would probably look like this when disassembled: Blahblah: REP $#30 ; set accumulator, memory, x and y regs to 16bit LDA $80 ; load two bytes(16bit) from adress wram adress $80 into accumulator STA $81 ; store accumulator in wram adress $81 and $82 BRA Wurst ; branch to routine Wurst ;************************************************************* ;problem ;************************************************************* SomeRoutine: LDA $#4520 ; the disasm thinks the accu is still 16bits and misinterprets the LDA #$00 ; jsr ($#20 in hex) as the second byte to load with the above ; instruction. the LDA #$00 ($#A900) actually was the adress ; the deleted jsr instruction should have jumped to. RTS ;return ;************************************************************* Wurst: SEP $#20 ; set accumulator/memory to 8bit REP $#10 ; set x/y regs to 16bit BRA SomeRoutine ; branch always to SomeRoutine do you see the problem here? the disassembler cant know if a hex value is an instruction, an adress or some other data. it can only try to guess the cpu status bits by preceding code, but this doesnt work out %100 in most cases and leaves you wondering where the problem could be. i ended up tracing the rom in an emulator to a textfile and then comparing the data with the disassemblers output to see where it made mistakes. also, you have to add the labels again if you want the routine to be flexible (aka .org it everywhere you want) and this can be a bit tedious, ecspecially if offsets or bank change are hardcoded. try to get nevikstis starterkit first (http://nesdev.parodius.com). i would recommend to use the assembler included there, wla-65816, its pretty flexible. (edited by d4s on 07-29-04 04:32 AM) (edited by d4s on 07-29-04 04:36 AM) |
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loadingNOW Red Paragoomba Level: 14  Posts: 11/61 EXP: 10382 For next: 2689 Since: 07-15-04 From: Silent Hill Since last post: 136 days Last activity: 3 days |
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| learning by doing is best. It's gerereally recommented to start with nes because you have a nice debugger and the instuctionset is not that complex (i can't comment on that because i didn't start with 6502 becaue I came from a pc-reversing background). @d4s: actually datasecure's ida does quite a good job there (assumes everything is data, once you type c it converts the line to code and goes on until it does not see any more code - this works very well in 8086 and PPC sets. it does work as well if you're working on arm/thumb tho but it's always possible to correct mistakes manually) |
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Euclid Cheep-cheep Level: 23 Posts: 87/193 EXP: 65528 For next: 2195 Since: 03-15-04 From: Australia Since last post: 24 days Last activity: 7 days |
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| i got 2 words to add. this site |
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ziratha Ninji Level: 24 Posts: 11/231 EXP: 77808 For next: 317 Since: 06-29-04 Since last post: 4 days Last activity: 17 days |
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| ok heres some updates/ questions please dont hate me due to my obvious stupidity oi. ok update i dled a whole bunch of stuff. but so far i cant find much useful maybe due to the fact that i most of them either dont accept .nes input or dont have texts that tell how to get the dissasembler to do what i want to do, maybe i need to find the home site for the programs? i found an actual nes dissassmbler, and decided on a rom to disassemble, i dissasemmbled it and then went over to an assembler and it wouldn't assemble, i figured it would have some errors but 3000????? wow anyway i tried a snes assembler and it only got 1000 errors so...? new questions: about assembly/deassembly can i use a snes dissasmbler on a nes type file? if so, will it be able to be assembled back to the .nes format? im not entirely sure on this one. when i dissassemble something like a .nes file shouldn't it the asm file be the same size as the .nes file? cause my asm file ended up smaller...  i got the snes starter kit, but am having problems with it, it wants a .obj file with my asm to compile, where/how do i get a . obj file? about asm itself if i change the asm to make it assemble thereby changing the code, and then make a asm fix/addon will this work on other copies of the rom? Because if the asm was changed due to my fixes, wouldn't that make my roms slightly differant from the origional copy? and possible cause problems patching? what the heck is the comand dc.b, i found a large number of this command in my asm code, (i think its asm, it looks like it and its .asm) but i could find no mention of this code in any of my multitude of faqs/walkthrughs so thats whats happened so far, im getting a very basic understanding of some of the commands and such . thanks again for any responses and please feal free to ridicule me for my fKitten Yiffers so far, so long as you also post somthing usefull. :-p |
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NightHawk Bob-Omb Level: 39 Posts: 537/621 EXP: 374743 For next: 30028 Since: 03-26-04 From: Switzerland Since last post: 432 days Last activity: 339 days |
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| Okay, first off with the tools... if you're hacking a .nes ROM, use (dis)assemblers for the NES, not the SNES, and vice versa. Second, disassemblers don't really work very well for what you're trying to do... finding one that will give you code that can be re-assembled can be a major pain, depending on your platform. And no, the ASM file shouldn't be the same size as the NES ROM... it should be much larger (instructions for the NES' CPU are 1 - 3 bytes in size, but the disassembly will usually have a line of about 20 characters - bytes - per instruction). And about those errors... the reason that the SNES assembler is giving you fewer errors, might be because you used a disassembler that, while going through the data sections (remember that code/data look the same to a disassembler; only very advanced ones can tell the difference), decoded some bytes as instructions for the SNES' CPU, which will of course give you errors if you try to use an NES assembler on the results. Changing the asm just to make it assemble shouldn't actually change the binary at all; if it does, something was done wrong. "DC.B" means "DeClare Byte": it simply reserves one byte of space with the value following. Did that make any sense to you?  If not, feel free to ask more questions. |
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ziratha Ninji Level: 24 Posts: 12/231 EXP: 77808 For next: 317 Since: 06-29-04 Since last post: 4 days Last activity: 17 days |
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| OK, THANSK IM GONNA HAFTA DOWNLOAD a new nes assembler/dissambler thanksfor the help. |
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