Contra

References

1. ^Touching a weapon item in the US NES Contra adds 1,000 points and dispatches on the item's type: R sets the rapid-fire bit, M, F, S, and L replace the gun (keeping rapid fire only when the new gun is the one held), B starts an invincibility timer at $80 ($90 on level 7), and the falcon destroys the enemies on screen. A death resets the gun to the default.Codevermiceli/nes-contra-us
   How we know

   6884pick_up_weapon_item:
   ⋮
   6886    lda #$0a                     ; a = #$0a
   6887    sta $00                      ; set score to add to player as #$0a (1,000 points)
   ⋮
   6895    lda ENEMY_ATTRIBUTES,y       ; get weapon item attributes
   6896    and #$07                     ; keep bits 0-2 (attributes)
   6897    beq @set_rapid_flag          ; set rapid flag for weapon if attribute is #$00
   6898    cmp #$05                     ; check for b weapon (barrier). Gives invincibility
   6899    bcc @compare_and_set_weapon  ; branch if less than #$05 (MFSL)
   6900    beq @set_invincibility_timer ; branch for barrier weapon
   6901    jsr destroy_all_enemies      ; falcon weapon effect - destroy all enemies
   ⋮
   6912@set_invincibility_timer:
   6913    lda #$80          ; set duration of the b weapon effect
   ⋮
   

   5150init_player_dec_num_lives:
   5151    jsr init_player_and_weapon ; initialize player variables to 0
   

   5338init_player_and_weapon:
   5339    lda #$00                ; a = #$00
   5340    sta P1_CURRENT_WEAPON,x ; reset current player's weapon
   

   Checked against the evidence by its author, an AI model, 2026-10-21.

   Checked by a second AI model, 2026-10-21.

   In the claim ledger

2. ^ abWhile the title screen scrolls in, the US NES Contra compares each new d-pad, A, or B press with a ten-entry table, Up Up Down Down Left Right Left Right B A. A wrong press sets the count to $FF, which stops the check until the title starts over. The full sequence sets a flag, and a new game then gives each player 29 lives in reserve instead of 2: 30 lives instead of 3.Codevermiceli/nes-contra-us
   How we know

   750game_routine_01:
   751    jsr konami_input_check                   ; check if current input is part of Konami code (30-lives code)
   

   930konami_input_check:
   931    ldy KONAMI_CODE_NUM_CORRECT    ; load the number of successful inputs of Konami code
   932    bmi konami_code_exit           ; if #$ff (invalid Konami input), exit
   933    lda CONTROLLER_STATE_DIFF      ; buttons pressed (only care on input change so held button doesn't affect code)
   934    and #$cf                       ; only care about input from d-pad and A/B buttons (not select nor start)
   935    beq konami_code_exit           ; if no input detected, exit
   936    cmp konami_code_lookup_table,y ; compare with Konami code sequence at index y
   937    beq konami_input_index_correct ; on success, goto konami_input_index_correct
   938    lda #$ff                       ; incorrect sequence for Konami code
   939    sta KONAMI_CODE_NUM_CORRECT    ; since incorrect set KONAMI_CODE_NUM_CORRECT (number of successful inputs) to $ff
   ⋮
   945    cpy #$0a                    ; Konami code is 10 inputs, compare against how many successfully entered
   946    bcc konami_code_exit        ; Konami code not yet fully entered, exit
   ⋮
   

   716game_routine_00:
   ⋮
   720    sty KONAMI_CODE_NUM_CORRECT     ; initialize konami check to #$00 (see konami_input_check)
   

   1165init_player_lives:
   1166    lda #$02                  ; start off with #$02 lives
   1167    ldy KONAMI_CODE_STATUS    ; 30-lives code switch ($01 = code activated)
   1168    beq init_player_num_lives ; if KONAMI_CODE_STATUS is not set, then just set 2 lives
   1169    lda #$1d                  ; KONAMI_CODE_STATUS active so set lives to #$1d (29 decimal)
   ⋮
   1174    sta P1_NUM_LIVES,x          ; if X is 1, then set P2_NUM_LIVES to accumulator (a is either #$02 or #$1d)
   

   Checked against the evidence by its author, an AI model, 2026-10-21.

   Checked by a second AI model, 2026-10-21.

   In the claim ledger

3. ^A new game of the US NES Contra gives each player 2 lives in reserve (3 in all) and the game 3 continues. A death with none in reserve puts that player in game over, and each game over spends a continue; with none left, the CONTINUE/END choice is skipped.Codevermiceli/nes-contra-us
   How we know

   1143init_score_player_lives:
   ⋮
   1146    lda #$03           ; a = #$03
   1147    sta NUM_CONTINUES  ; continues remaining
   

   1165init_player_lives:
   1166    lda #$02                  ; start off with #$02 lives
   

   5150init_player_dec_num_lives:
   ⋮
   5153    lda P1_NUM_LIVES,x         ; load player number of lives
   5154    beq @set_game_over         ; branch if no more lives
   5155    dec P1_NUM_LIVES,x         ; decrement player number of lives
   ⋮
   5158@set_game_over:
   5159    lda #$01                  ; a = #$01
   5160    sta P1_GAME_OVER_STATUS,x ; game over state of player (1 = game over)
   

   3289show_game_over_screen:
   ⋮
   3301    dec NUM_CONTINUES                         ; subtract from number of continues remaining
   3302    bmi @no_continues_remaining               ; if run out of continues, jump
   3303    lda #$0e                                  ; a = #$0e (continue  end)
   

   Checked against the evidence by its author, an AI model, 2026-10-21.

   Checked by a second AI model, 2026-10-21.

   In the claim ledger

4. ^The US NES Contra awards its first extra life at 20,000 points and each later one 30,000 points past the last threshold, and caps lives at 99.Codevermiceli/nes-contra-us
   How we know

   1177    lda #$c8                    ; set default high score. #$c8 is 200 decimal
   1178    sta EXTRA_LIFE_SCORE_LOW    ; starting score for extra life (20,000)
   

   1283; every time an extra life is obtained, #$12c is added to points required to get next extra life (300 decimal, 30,000 in game score)
   ⋮
   1291    lda #$2c                    ; a = #$2c (12c = 300 decimal) (high byte will be set to #$01 a few lines down)
   ⋮
   1294    adc EXTRA_LIFE_SCORE_LOW,y  ; add 300 decimal to low byte of score required to get extra life
   

   1310    inc P1_NUM_LIVES,x          ; if X is $01, then set P2 number of lives
   1311    lda P1_NUM_LIVES,x          ; load incremented number into memory
   1312    cmp #$63                    ; compare to #$63 (99 decimal)
   1313    bcc @set_num_lives          ; can't have more than 99 lives
   1314    lda #$63                    ; a = #$63 (63 = 99 decimal)
   

   Checked against the evidence by its author, an AI model, 2026-10-21.

   Checked by a second AI model, 2026-10-21.

   In the claim ledger

5. ^Each completed loop of the US NES Contra and the strongest gun either player holds both shorten the timer between random soldiers: from the level's start value (144 on level 1), 40 comes off per loop, counting up to 3, and 5 per point of weapon strength (S 3, M and L 2, F 1, the default 0).Codevermiceli/nes-contra-us
   How we know

   1736adjust_generation_timer:
   1737    jsr get_soldier_gen_timer_for_lvl
   1738    sta SOLDIER_GENERATION_TIMER      ; soldier generation timer
   1739    ldy GAME_COMPLETION_COUNT         ; load the number of times the game has been completed
   1740    beq @adjust_timer_for_weapon      ; branch if haven't yet beat the game any times
   1741    tya
   1742    cmp #$04                          ; compare GAME_COMPLETION_COUNT to #$04
   1743    bcc @continue                     ; branch if GAME_COMPLETION_COUNT < #$04
   1744    ldy #$03                          ; maximum multiplier of increase chance for generating soldier is #$03
   ⋮
   1747    lda #$28          ; lower soldier generation timer
   ⋮
   1752    ldy PLAYER_WEAPON_STRENGTH ; load player weapon strength (damage strength)
   1753    beq @exit                  ; don't generate if player's weapon strength is #$00
   ⋮
   

   3270    inc GAME_COMPLETION_COUNT       ; increment game completion count, used mainly to increase enemy difficulty every play-through
   

   3920set_frame_scroll_weapon_strength:
   ⋮
   3926    sta PLAYER_WEAPON_STRENGTH ; clear player weapon strength
   

   4096set_player_weapon_strength:
   ⋮
   4102    lda weapon_strength,y         ; load how strong the weapon is
   4103    cmp PLAYER_WEAPON_STRENGTH    ; compare against current weapon strength
   4104    bcc @exit                     ; exit and do not lower player's current weapon strength
   4105    sta PLAYER_WEAPON_STRENGTH    ; store current weapon strength code (#$00-#$03)
   ⋮
   4111weapon_strength:
   4112    .byte $00 ; Regular = Weak
   4113    .byte $02 ; M = Strong
   4114    .byte $01 ; F = Medium
   4115    .byte $03 ; S = Very Strong
   4116    .byte $02 ; L = Strong
   

   Checked against the evidence by its author, an AI model, 2026-10-21.

   Checked by a second AI model, 2026-10-21.

   In the claim ledger

6. ^On the side-scrolling levels of the US NES Contra the screen moves only while a player passes X 128 (176 when two are playing), by that player's speed, and not while the other player is left of X 33; on the boss's screen it takes over and scrolls on its own to reveal the boss.Codevermiceli/nes-contra-us
   How we know

   5164set_frame_scroll_if_appropriate:
   ⋮
   5178    lda SPRITE_X_POS,x                     ; load player X position
   5179    cmp horizontal_scroll_point_tbl,y      ; load horizontal scroll point position
   5180    bcc @exit                              ; if not yet reached point to initiate horizontal scroll, exit
   5181    cpy #$02                               ; see if both players are active
   5182    bne @set_horizontal_level_frame_scroll ; if both players aren't active, begin setting scroll
   ⋮
   5186    lda SPRITE_X_POS,y                     ; load other player's X position
   5187    cmp #$21                               ; compare other player to left edge of screen
   5188    bcc @stop_player_x_velocity            ; don't scroll if other player is preventing it by being too far to the left
   ⋮
   5196    jsr set_boss_auto_scroll            ; starts the auto scroll to reveal boss if at right screen, otherwise do nothing
   ⋮
   5202    lda PLAYER_X_VELOCITY,x             ; load player X velocity
   5203    adc #$00                            ; incorporate any velocity overflow from INDOOR_TRANSITION_X_ACCUM
   ⋮
   

   5228horizontal_scroll_point_tbl:
   5229    .byte $80,$80,$b0
   

   6552set_boss_auto_scroll:
   ⋮
   6557    cmp LEVEL_SCREEN_NUMBER        ; screen number
   6558    bne @exit                      ; exit if not on the appropriate screen to start auto scroll
   ⋮
   6563    lda auto_scroll_timer_tbl,y    ; load the appropriate auto scroll timer for the level
   6564    sta AUTO_SCROLL_TIMER_00       ; start auto scroll to reveal the boss
   

   Checked against the evidence by its author, an AI model, 2026-10-21.

   Checked by a second AI model, 2026-10-21.

   In the claim ledger

7. ^ abA jump in the US NES Contra sets the 16-bit vertical speed to $FB.$F0, a rise of 4.06 px/frame ($FC.$90, 3.44 px/frame, in the indoor base levels), and the jump routine adds $23 to it every airborne frame, 0.137 px/frame², without reading a button: the jump's height doesn't depend on holding A.Codevermiceli/nes-contra-us
   How we know

   4533set_jump_status_and_y_velocity:
   ⋮
   4545    lda LEVEL_LOCATION_TYPE            ; 0 = outdoor; 1 = indoor
   4546    lsr
   4547    lda #$fb                           ; a = #$fb (-5)
   4548    ldy #$f0                           ; y = #$f0 (.94)
   4549    bcc @set_y_velocity                ; branch if outdoor level (use Y velocity -5.94)
   4550    lda #$fc                           ; indoor level, (Y velocity -4.56), set fast velocity to #$fc (-4)
   4551    ldy #$90                           ; set fractional Y velocity to #$90 (.56)
   ⋮
   4554    sta PLAYER_Y_FAST_VELOCITY,x  ; set Y fast velocity to a
   4555    tya                           ; transfer fractional velocity to a
   4556    sta PLAYER_Y_FRACT_VELOCITY,x ; set Y fractional velocity to a
   

   4708handle_jump:
   4709    lda PLAYER_Y_FAST_VELOCITY,x ; load player Y fast velocity
   4710    bmi @check_collision_above   ; branch if player is still ascending
   ⋮
   4747@apply_gravity:
   4748    jsr apply_gravity                   ; increments Y fractional velocity by #$23 (applying gravity)
   ⋮
   4756@set_y_pos:
   4757    jsr player_jumping_set_y_pos ; set player Y position based on PLAYER_JUMP_COEFFICIENT and velocity
   

   5114player_jumping_set_y_pos:
   ⋮
   5123    lda PLAYER_JUMP_COEFFICIENT,x ; load player's jump modifier (alters height of jump)
   5124    clc                           ; clear carry in preparation for addition
   5125    adc PLAYER_Y_FRACT_VELOCITY,x ; add to player's fractional Y velocity
   5126    sta PLAYER_JUMP_COEFFICIENT,x ; update player's jump modifier (alters height of jump)
   5127    lda SPRITE_Y_POS,x            ; load player's Y position
   5128    adc PLAYER_Y_FAST_VELOCITY,x  ; add (subtract) the fast Y velocity (with jump coefficient overflow)
   

   5139apply_gravity:
   5140    clc
   5141    lda PLAYER_Y_FRACT_VELOCITY,x ; load player's Y fractional velocity
   5142    adc #$23                      ; add #$23 to Y fractional velocity (.1367 decimal)
   

   Checked against the evidence by its author, an AI model, 2026-10-21.

   Checked by a second AI model, 2026-10-21.

   In the claim ledger