Drowning during bosses

An anonymous reader asks:

I don't think you've talked about it yet so I'd like to ask a few things related to the Hydrocity Act 1 boss in S3A.

Sure, go ahead.

Why does it use the Act 2 boss BGM instead?

loc_47DBA:                                      loc_69F64:
    move.l  #Obj_HCZ_MinibossLoop,(a0)              move.l  #Obj_HCZ_MinibossLoop,(a0)
    moveq   #$19,d0                                 moveq   #$2E,d0
    jsr     (Play_Sound).l                          jsr     (Play_Sound).l
                                                    move.b  #$2E,($FFFFFF91).w

locret_47DC8:                                   locret_69F78:
    rts                                             rts

Because for some reason, in S3A the act 1 boss requests the act 2 boss music.

Why, if you let the air countdown start and then get out of the water, does the S&K Act 1 boss BGM start to play? It's seems to be the only time where track 18 (I think it's 18) is used. (Apparently this glitch also happens in Act 2 but I haven't tried)

Because, to put it mildly, the code that's responsible for resuming a level's music after having previously switched to the drowning theme is a brittle piece of crap:

Player_ResetAirTimer:
    cmpi.b  #$C,$2C(a1)
    bhi.s   loc_1744C
    cmpa.w  #Player_1,a1
    bne.s   loc_1744C
    move.w  (Level_music).w,d0
    btst    #1,$2B(a1)
    beq.s   loc_17430
    move.w  #$2C,d0

loc_17430:
    tst.b   (Super_Sonic_Knux_flag).w
    beq.w   loc_1743C
    move.w  #$2C,d0

loc_1743C:
    tst.b   (Boss_flag).w
    beq.s   loc_17446
    move.w  #$18,d0

loc_17446:
    jsr     (Play_Sound).l

loc_1744C:
    move.b  #$1E,$2C(a1)
    rts

Let's go over what this code does. First, the current level's music is loaded from the Level_music RAM variable, which as we saw before, is pulled from the master playlist, based on the current zone and act. This value is then stored in the d0 register, from which it will eventually be read by the Play_Sound function called at loc_17446.

Before it can reach loc_17446 though, the value must survive a gauntlet, which begins by checking bit 1 of the player's status_secondary bitfield, which is set when the player is invincible. If the player is currently invincible, the contents of the d0 register are replaced with the value $2C, which is the ID for the invincibility music track.

Next, if the Super_Sonic_Knux_flag is set, the value of the d0 register is once again replaced with $2C, the ID for the invincibility music track. Note that this check is redundant: the player is always invincible when in their Super form.

Finally, the Boss_flag is checked. If the player is currently fighting a boss, then the game should continue playing boss music, and therefore the d0 register is overwritten with the value $18, which as the commenter duly notes, is the theme used by act 1 bosses in Sonic & Knuckles. No effort is made to pick specific themes for each boss, despite the fact that there are only two bosses in the entire game where the player can trigger the countdown music.

This is annoying for several reasons. In Sonic 3, both bosses play the act 2 boss theme, so they could've just made the code set d0 to $19 and be done with it: at least it would always resume the right track! Meanwhile, as we can see from the code near the start of this post, Sonic & Knuckles actually saves the current boss track to RAM address $FF91, but nothing in the game ever makes use of this value.

Why complicate matters, though? Just check the current act and use that information to pick one song or the other.

It's possible to drown during the score tally but in S3&K this was apparently fixed. Why does it happen and how it was fixed?

The question presupposes a falsehood: the only way to drown during the level results is by running out of air just as the results object slides into view, which works equally well in both Sonic 3 and Sonic & Knuckles.

In practice, however, the level results object prevents this by restoring both player objects' air reserves (at offset $2C of their SSTs) right as the stage clear theme begins playing:

Obj_LevelResultsWait:
    tst.w   $2E(a0)
    beq.s   loc_2CF4C
    subq.w  #1,$2E(a0)
    cmpi.w  #$121,$2E(a0)
    bne.s   locret_2CF8E                        ; Play after eh, a second or so
    move.b  #$1E,($FFFFB02C).w                  ; Reset air for Hydrocity
    move.b  #$1E,($FFFFB076).w
    moveq   #$29,d0
    jmp     (Play_Sound).l                      ; Play level complete theme

The title card object then repeats the process, ensuring the player always begins the second act with a full set of lungs.

Thanks in advance.

Thank you for the questions!