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nes-contra-us/docs/Bugs.md

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Overview

This document outlines the bugs discovered in the code during the disassembly process.

1. Missing Animation Entering Water

When animating the player falling into water, a comparison was missed which causes sprite_18 to not be used for the animation sequence. This bug also exists in the Japanese version as well as Probotector. It's clear from the code that there was an attempt to show sprite_18 (frame 3 in the table below) as part of the animation sequence. sprite_18 is the same sprite as when the player is in water and presses the d-pad down button.

Frame 1 Frame 2 Frame 3 (missing) Frame 4
0 1 2 3 
@set_enter_water_sprite:
  ...
  lda PLAYER_WATER_TIMER,x ; load water animation timer
  beq ...                  ; if timer has elapsed, branch
  cmp #$0c                 ; see if timer is greater than or equal to #$0c
  bcs ...                  ; branch if timer >= #$0c (keep current sprite)
  lda #$73                 ; PLAYER_WATER_TIMER less than #$0c, update sprite
  sta PLAYER_SPRITE_CODE,x ; set player sprite (sprite_73) water splash
  cmp #$08                 ; !(BUG?) always branch, doesn't compare to PLAYER_WATER_TIMER
                           ; but instead compares against #$73
                           ; no lda PLAYER_WATER_TIMER,x before this line, so part of splash animation is missing
  bcs ...                  ; branch using sprite_73
  lda #$18                 ; dead code - a = #$18 (sprite_18) water splash/puddle
  sta PLAYER_SPRITE_CODE,x ; dead code - set player animation frame to water splash/puddle

2. Accidental Sound Sample In Snow Field

After defeating the boss UFO (Guldaf), before calling the method level_boss_defeated, the developers forgot to load into the accumulator the appropriate sound code, usually #$57 for sound_57. So bytes in bank 7 are interpreted as an audio sample incorrectly and a short DMC channel audio clip is played.

level_boss_defeated uses whatever is in the accumulator as the sound code to play. Since the accumulator wasn't set, it has the previous value of #$ff (see boss_ufo_routine_0b). This is interpreted by the audio engine as a DMC sound code that doesn't exist.

The audio engine will incorrectly read bytes from sound_table_00 as the dmc data below.

  • sampling rate #$03 (5593.04 Hz), no loop
  • counter length #$77
  • offset #$97 --> $c000 + (#$97 * #$40) --> $e5c0
  • sample length #$19

The data that is interpreted as a DPCM-encoded DMC sample is $e5c0, which is in bank 7's collision_box_codes_03 data. This bug does not exist in the Japanese version of the game because the level_boss_defeated is not responsible for playing the boss defeated sound. This bug does exist in Probotector.

Extracted sound sample: sound_ff.mp3

Note that the boss defeated audio (sound_55) is still played because the enemy defeated routine is set to boss_ufo_routine_09 (see enemy_destroyed_routine_05). boss_ufo_routine_09 plays sound_55.