Mega Man 6 Weapon Refill Patch

This patch for Mega Man 6 makes weapon energy refill upon death in the whole game. A detailed description can be found on the RomHacking page of this hack, as well as an alternative download link. In this README you will find further information on how this patch was made and reference material.

Tools

• The FCEUX NES emulator, for disassembly, hex editing, memory viewing, testing and even breakppoints.
• LunarIPS, to create a patch file.

Process

As the other hacks, first the weapon energy value offsets were acquired. The refilling process was a bit more tricky because of how the values behaved in Mega Man 6 internally, luckily the in-game procedure was found. After studying and testing the procedure a method was found to call it safely and restore all weapon energy through it. It turned out no values had to be modified and passed to it. However it did reset registers. Because it is unknown which memory is safe to use for free values, it was decided to use the NES Stack to store the registers and restore them from it after the procedure finished.

The patch was tested a lot to ensure its loaded on every instance that the weapons have to be refilled according to the requirements.

References

In order to write the code a lot of knowledge was required. Here I list the sources specifically used for this project. DarkSamus993 provided information on the in-game routine for refilling weapon energy:

; rom addr = $077A4F
1D:BA3F:A5 51     LDA $0051    ; stageID
1D:BA41:C9 09     CMP #$09     ; Mr. X's Fortress, Stage 2
1D:BA43:B0 33     BCS $BA78    ; don't refill weapons for the final stages
1D:BA45:A0 0B     LDY #$0B
1D:BA47:B9 79 BA  LDA $BA79,Y  ; load boss index [00 00 00 01 01 01 02 02 02 03 03 03]
1D:BA4A:AA        TAX
1D:BA4B:BD 92 06  LDA $0692,X  ; load boss beaten bits (RAM addr $0692 - $0695)
1D:BA4E:85 00     STA $0000
1D:BA50:B9 85 BA  LDA $BA85,Y  ; load stage flags [01 02 04 01 02 04 01 02 04 01 02 04]
1D:BA53:25 00     AND $0000
1D:BA55:F0 09     BEQ $BA60    ; check if boss beaten
1D:BA57:B9 91 BA  LDA $BA91,Y  ; load weapon index [03 07 07 02 01 01 04 08 08 05 06 06]
1D:BA5A:AA        TAX
1D:BA5B:A9 1B     LDA #$1B
1D:BA5D:9D 88 06  STA $0688,X  ; refill weapon energy
1D:BA60:88        DEY
1D:BA61:10 E4     BPL $BA47
1D:BA63:AD 92 06  LDA $0692    ; items collected byte #1
1D:BA66:2D 93 06  AND $0693    ; items collected byte #2
1D:BA69:2D 94 06  AND $0694    ; items collected byte #3
1D:BA6C:2D 95 06  AND $0695    ; items collected byte #4
1D:BA6F:29 04     AND #$04
1D:BA71:F0 05     BEQ $BA78    ; branch if all beat letters not collected
1D:BA73:A9 1B     LDA #$1B
1D:BA75:8D 91 06  STA $0691    ; refill beat weapon energy
1D:BA78:60        RTS

A big thanks for the people behind these to make 6502 programming more accessible.

• An excellent reference for quickly looking up the value of an opcode.
• More detailed information on the opcodes that made me understand what was required for the patch.
• Basic ROM information that was required to understand the mapper.