Snake's Revenge NES (Request)

[asscock]

Game genie invincibility and cannot be detected? That would be amazing, thanks for reading!

[MasterGrand]

https://gamehacking.org/game/30882

Hope to help you.

[Dteyn]

Gyrocopter enemies don't notice you
AVOEUESL

No enemies when spotted by spotlight
ATNAVGSZ

Standing guards don't notice you
NYUETAAO

I managed to come up with a couple of codes to prevent being detected, they should help a lot. I only play-tested them on the first level though so hopefully they are good for other areas of the game too. Let me know if there are any issues with them!

The Infinite Energy code (SXKVKASA) from the DB worked for me, at least for the first level. Are there areas/levels where it doesn't work? VisitntX also posted an Invincibility code (SZEVEPSA) which is listed a bit further down on the DB.

Cheers!




Code Development Notes:
Spoiler Alert! Click to view...

Gyrocopter enemies don't notice you
AVOEUESL

To find this routine, I watched RAM in slow-mo and noticed $5B-onward gets set to new 
values when enemies are alerted. The stack indicated a return to $ACFC:

 01:8CD6:B9 17 8D  LDA $8D17,Y @ $8D18 = #$84
 01:8CD9:05 5B     ORA $005B = #$01
>01:8CDB:85 5B     STA $005B = #$01

.... (tracing back to return address)

 01:ACF9:20 9B 88  JSR $889B
>01:ACFC:BD 84 04  LDA $0484,X @ $048B = #$7D

I tried a few things and one was to try canceling the routine at $889B with an RTS.

Lucky guess! Now the copters won't spot you. They will however be alerted if you
bump into them.

889B:60:BD = AVOEUESL

No enemies when spotted by spotlight
ATNAVGSZ

Again, I set a write breakpoint on $5B and triggered a spotlight. Then followed the 
code backward:

 01:8C76:A5 5B     LDA $005B = #$01
 01:8C78:09 04     ORA #$04
>01:8C7A:85 5B     STA $005B = #$01
 01:8C7C:A9 50     LDA #$50
 01:8C7E:85 5E     STA $005E = #$78
 01:8C80:AD E3 05  LDA $05E3 = #$23
 01:8C83:C9 41     CMP #$41
 01:8C85:F0 03     BEQ $8C8A
 01:8C87:4C 03 8D  JMP $8D03

The return address on the stack was $862F:

 01:8629:20 1B 8D  JSR $8D1B
 01:862C:20 76 8C  JSR $8C76
>01:862F:4C 18 8A  JMP $8A18

So I tried cancelling the routine at $8C76 with an RTS, and enemies no longer spawn.
Awesome! :)

8C76:60:A5 = ATNAVGSZ

Standing guards don't notice you
NYUETAAO

Again I set a write BP for $5B. This particular routine runs when the guards spot you:

 01:80CF:A5 5B     LDA $005B = #$85
 01:80D1:29 7F     AND #$7F
>01:80D3:85 5B     STA $005B = #$85
 01:80D5:4C 18 8A  JMP $8A18

Following the return address on the stack, we go back to $D4ED:

 07:D4E1:20 2C C4  JSR $C42C
 07:D4E4:20 27 A1  JSR $A127
 07:D4E7:20 34 C4  JSR $C434
 07:D4EA:20 30 80  JSR $8030
>07:D4ED:A5 5B     LDA $005B = #$05
 07:D4EF:29 0E     AND #$0E
 07:D4F1:F0 08     BEQ $D4FB

I tried cancelling the routine at $8030 with an RTS but it caused other issues.

So looking further up the routine at $8030, the value at $55 is compared to $10,
then there is a JSR $8001 that sets the enemies in motion.

 01:803B:A5 55     LDA $0055 = #$0D
 01:803D:C9 10     CMP #$10
 01:803F:F0 03     BEQ $8044
 01:8041:20 01 80  JSR $8001
 01:8044:A5 5B     LDA $005B = #$01
 01:8046:29 80     AND #$80
 01:8048:D0 7B     BNE $80C5
 01:804A:A5 49     LDA $0049 = #$00

I tried multiple approaches to this one, the most effective way of bypassing it
was to change the compare value at $803D to something else.

803E:FF:10 = NYUETAAO