Acmlm's Board - I2 Archive - Rom Hacking - 6502 question
| User | Post |
HyperLamer
Posts: 1988/8210 |
One thing you might find is that the routine is after data (uncommon in newer games), so above it is just garbage. Also, you might be partway into a routine. You described a JSR nearby that jumps just past the RTS, that probably means you're in part of a routine which is executed conditionally from the routine itself. (If you can understand what I mean...  ) |
MathOnNapkins
Posts: 833/2189 |
If the part after the JSR doesn't look like code that is a hint that you should follow that JSR, as the return address probably got pulled from the stack, otherwise, there's usually no need to worry. |
Sokarhacd
Posts: 547/1757 |
well, I guess that isnt so bad then...well, I guess im off to see what i can do with this...thanks again. |
Dish
Posts: 167/596 |
Following up JSRs isn't really all that important.... at least not from my experience. The routine being jumped to will almost always return in one form or another (either by doing an RTS, or JMPing to another routine that RTS, or something similar). The only way a JSR routine wouldn't return is if the return address is pulled from the stack... something like:
JSR someroutine
someroutine:
PLA
PLA ;ditch return address
RTS
though code that does that isn't exactly common. So I wouldn't beat yourself up for not noticing JSRs. Chances are they won't affect the routine you're looking at. |
Sokarhacd
Posts: 545/1757 |
yeah, I expected them to get pretty complicated...and actually, looking at this one again, it actually is pretty big....I noticed it does this:
it has an RTS, but there is a beq up a little bit, that goes past the rts, and it goes into another part that has 2 JSR's....so its actually more complicated then I first thought...
here is what I copied from fceud.
$832A:BD 50 04 LDA $0450,X @ $0466 = #$02
$832D:38 SEC
$832E:E5 12 SBC $12 = #$01
$8330:9D 50 04 STA $0450,X @ $0466 = #$02
$8333:B0 05 BCS $833A
$8335:A9 00 LDA #$00
$8337:9D 50 04 STA $0450,X @ $0466 = #$02
$833A:A5 22 LDA $22 = #$07
$833C:C9 0B CMP #$0B
$833E: D0 10 BNE $8350
$8340:A5 F9 LDA $F9 = #$08
$8342:C9 13 CMP #$13
$8344: D0 0A BNE $8350
$8346:A9 02 LDA #$02
$8348: DD 50 04 CMP $0450,X @ $0466 = #$02
$834B:90 03 BCC $8350
$834D:9D 50 04 STA $0450,X @ $0466 = #$02
$8350:BD B8 05 LDA $05B8,X @ $05CE = #$00
$8353:29 80 AND #$80
$8355:09 08 ORA #$08
$8357:9D B8 05 STA $05B8,X @ $05CE = #$00
$835A:EC 46 01 CPX $0146 = #$00
$835D: D0 23 BNE $8382
$835F:AD 32 01 LDA $0132 = #$00
$8362:10 1E BPL $8382
$8364:BD 50 04 LDA $0450,X @ $0466 = #$02
$8367:85 BF STA $BF = #$00
$8369:BD B8 05 LDA $05B8,X @ $05CE = #$00
$836C:29 80 AND #$80
$836E:09 30 ORA #$30
$8370:9D B8 05 STA $05B8,X @ $05CE = #$00
$8373:BD 00 03 LDA $0300,X @ $0316 = #$8E
$8376:C9 9E CMP #$9E
$8378: D0 08 BNE $8382
$837A:BD 50 04 LDA $0450,X @ $0466 = #$02
$837D:18 CLC
$837E:69 0E ADC #$0E
$8380:85 BF STA $BF = #$00
$8382:BD 50 04 LDA $0450,X @ $0466 = #$02
$8385: D0 4B BNE $83D2
$8387:9D B8 05 STA $05B8,X @ $05CE = #$00
$838A:BC 00 03 LDY $0300,X @ $0316 = #$8E
$838D:20 00 DC JSR $DC00 //LOL missed this one also...gotta be more careful.
$8390:BD 28 05 LDA $0528,X @ $053E = #$90
$8393:29 FD AND #$FD
$8395:9D 28 05 STA $0528,X @ $053E = #$90
$8398:B9 E3 84 LDA $84E3,Y @ $84E4 = #$00
$839B:F0 21 BEQ $83BE
$839D:9D 00 03 STA $0300,X @ $0316 = #$8E
$83A0:A9 00 LDA #$00
$83A2:9D 08 04 STA $0408,X @ $041E = #$C0
$83A5:9D 68 04 STA $0468,X @ $047E = #$00
$83A8:9D 80 04 STA $0480,X @ $0496 = #$5D
$83AB:9D 98 04 STA $0498,X @ $04AE = #$00
$83AE:9D B0 04 STA $04B0,X @ $04C6 = #$00
$83B1:9D C8 04 STA $04C8,X @ $04DE = #$00
$83B4:9D E0 04 STA $04E0,X @ $04F6 = #$00
$83B7:9D F8 04 STA $04F8,X @ $050E = #$00
$83BA:9D 10 05 STA $0510,X @ $0526 = #$00
$83BD:60 RTS
$83BE:A9 19 LDA #$19
$83C0:20 46 F4 JSR $F446 // here
$83C3:A9 00 LDA #$00
$83C5:9D 08 04 STA $0408,X @ $041E = #$CD
$83C8:A9 9D LDA #$9D
$83CA:9D 00 03 STA $0300,X @ $0316 = #$3E
$83CD:A9 27 LDA #$27
$83CF:20 BE F6 JSR $F6BE //and here
$83D2:60 RTS
lol I missed 3 JSR's....I definently gotta pay more attention...so actually that routine is pretty damn big.
|
MathOnNapkins
Posts: 830/2189 |
If you get a JSR or JMP to a routine, and you look behind the jump point and there is no RTS, it may be the case that the code you jumped too is part of a larger routine. Always try to be aware of that. And yeah the ends of routines can get complicated. I don't know if you'll encounter it but there are at least 4 ways to get out of a routine. One is a jump. The other is pulling numbers off the stack, putting some new ones on from a jump table, and then hitting RTS which will give a location other than the original return location. It's a crafty kind of jump table. There are also JSRs that never return, one of the most confusing things I encountered. What they do is jump to another routine that jumps somewhere else using the return address on the stack. |
Sokarhacd
Posts: 544/1757 |
thanks, thats pretty much what I thought and it looks like this routine doesnt have any jumps, and its not that big really, but if I got it right it goes from $832A-$83BD I think. and theres a few bne's in there, but they dont go past the RTS, so I think thats the whole thing.. |
Dish
Posts: 166/596 |
Usually, but not always. They may end with a JMP command, and a routine may contain several RTS commands (some of which may be skipped by branching). Typically though... for small routinse... yes, it'll end with an RTS and usually start right after an RTS. This is no real 'rule' though... it's just a common trend. |
Sokarhacd
Posts: 543/1757 |
I have a question about 6502 asm, im looking at the enemy health routine in Megaman 4, and I was wondering, if im looking at some specific routine, will it alwasy begin right after an RTS command, and end at an RTS? im asking because im not sure where the routine begins and ends, and also, i read some of the fceud tuts on dragoneyestudios and they all say to copy stuff beginning after an RTS above, to on below...im thinking im right but I just wanted to make sure. |
|
hukka
