Music Modification Code Suggestions for SMB 1

Hi,

I have a question: for all those of you familiar with 6502 disassembly, who can think of a music modification code where the modified code, which of course involves changing some bytes, that will fit in the same space as the original?

The code looks like this:
$90E7: 02                     ; water music
$90E8: 01                     ; ground music
$90E9: 04                     ; underground music
$90EA: 08                     ; castle music
$90EB: 10                     ; cloud music
$90EC: 20                     ; pipe intro music
$90ED: AD 70 07 - LDA $0770   ; if in title screen mode, leave
$90F0: F0 23    - BEQ $9115
$90F2: AD 52 07 - LDA $0752   ; check for specific alternate mode of entry
$90F5: C9 02    - CMP #$02    ; if found, branch without checking starting position
$90F7: F0 0D    - BEQ $9106   ; from area object data header
$90F9: A0 05    - LDY #$05    ; select music for pipe intro scene by default
$90FB: AD 10 07 - LDA $0710   ; check value from level header for certain values
$90FE: C9 06    - CMP #$06
$9100: F0 0E    - BEQ $9110   ; load music for pipe intro scene if header
$9102: C9 07    - CMP #$07    ; start position either value $06 or $07
$9104: F0 0A    - BEQ $9110
$9106: AC 4E 07 - LDY $074E   ; load area type as offset for music bit
$9109: AD 43 07 - LDA $0743
$910C: F0 02    - BEQ $9110   ; check for cloud type override
$910E: A0 04    - LDY #$04    ; select music for cloud type level if found
$9110: B9 E7 90 - LDA $90E7,Y ; otherwise select appropriate music for level type
$9113: 85 FB    - STA $FB     ; store in queue and leave
$9115: 60       - RTS

I suggest that this to be a music on demand thing, where you can choose one song to be played as background music for the whole game save for the pipe intro and cloud themes, and, this particular modification is to be treated a la the bank of code on the arcade version of Donkey Kong where the so-called "kill screen" bug is developed. What kind of instructions should I use in the case of Super Mario Bros.? There are opcodes such as AND, NOP, CLC, CLV, et al.

For instance, you could erase the code that loads the area type as the music bit offset, and instead replace that with code for which you could load the world # into the Y register (think LDY $075F) and then to have it check to see if the world # is greater than 1, and then force it back to 1 if so.

Thank you,



Ben Edge
 

If you want one song for the entire game, just NOP the whole thing $90ED-9112.
At $9111, LDA #(value from table at $90E7 that corresponds to desired music).

I'm a little confused about what you want to do.  I'm guessing (from the code you selected) that you want to change which bg music plays somehow.  Can you be more specific about what change you want to make?

I'm a little confused about what you want to do.  I'm guessing (from the code you selected) that you want to change which bg music plays somehow.  Can you be more specific about what change you want to make?

One example that I previously mentioned was how I could get background music by world number rather than by area type. The default music selection judgment is by area type.

~Ben

I'm a little confused about what you want to do.  I'm guessing (from the code you selected) that you want to change which bg music plays somehow.  Can you be more specific about what change you want to make?

One example that I previously mentioned was how I could get background music by world number rather than by area type. The default music selection judgment is by area type.

~Ben

If you wanted to load the bg music based on world number (a value 0-8?), expand the table to have 9 values, and then load the world number in Y and use it as an index into the table.

Code: [Select]

;01 = ground
;02 = water
;04 = underground
;08 = castle
;10 = cloud
;20 = pipe intro
$90E7: 01                     ; World 0, ground music.  I assigned music randomly for an example
$90E8: 01                     ; World 1
$90E9: 01                     ; World 2
$90EA: 02                     ; World 3
$90EB: 08                     ; World 4
$90EC: 10                     ; World 5
$90ED: 10                     ; World 6
$90EE: 04                     ; World 7
$90EF: 02                     ; World 8

$90F0    AD 70 07 - lda $0770          ;Check menu screen
$90F3    F0 09?  -  beq $90FD          ;And skip if menu
$90F5    AC 5F 07 - ldy $075F          ;Load World number
$90F8    B9 E7 90 - lda $90E7, y       ;grab the corresponding song
$90FB    85 FB -    sta $FB            ;queue it up
$90FD    60 -       rts                ;quit
$90FE+  NOP

Expanding your table makes the code start at $90F0 instead of $90ED, so you will have to update all jumps or subroutine calls to $90ED.  To avoid this you could stick the table after the code instead.

It might be easier to leave the music code alone and instead go into the level data and change each level's "area type".