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program.asm
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;===============================
; (C)2017, Sjors van Gelderen
;===============================
;********************************
; Constants
;********************************
PRG_COUNT = 1
CHR_COUNT = 1
MIRRORING = %0001
;********************************
; iNES header
;********************************
.db "NES",$1A ; ID of the header
.db PRG_COUNT ; 1 PRG-ROM block
.db CHR_COUNT ; 1 CHR-ROM block
.db $00|MIRRORING ; Mapper 0 with vertical mirroring
.dsb 9,$00 ; Padding
;********************************
; PRG bank
;********************************
.base $10000-(PRG_COUNT*$4000)
Reset
SEI ; Disable IRQs
CLD ; Disable decimal mode
LDX #$40
STX $4017 ; Disable APU frame IRQ
LDX #$FF
TXS ; Set up stack
INX
STA $2000 ; Disable NMI
STX $2001 ; Disable rendering
STX $4010 ; Disable DMC IRQs
JMP AwaitVerticalBlankDone
AwaitVerticalBlank
BIT $2002
BPL AwaitVerticalBlank
RTS
AwaitVerticalBlankDone
JSR AwaitVerticalBlank ; First wait
ClearMemory
LDA #$00
STA $0000,X
STA $0100,X
;STA $0200,X
STA $0300,X
STA $0400,X
STA $0500,X
STA $0600,X
STA $0700,X
LDA #$FE ; Better check what this value is
STA $0200,X
;STA $0300,X
INX
BNE ClearMemory
JSR AwaitVerticalBlank ; Second wait
;********************************
; Variables
;********************************
.enum $0000 ; Not quite clear on this yet
;;; Background
bg_offset .dsb 2
bg_boundary .dsb 1
;;; Animations
anim_address .dsb 2
anim_length .dsb 1
;;; Sprites
sprite_big .dsb 1 ; Whether or not the sprite is big (2x2 as opposed to 1x1)
sprite_source .dsb 1 ; Number of sprite to render
sprite_data_offset .dsb 1 ; Determines which byte of the transfer is written
sprite_target .dsb 2 ; Where to put the sprite
;;; Player
player_pos .dsb 2 ; Position
player_pos_saved .dsb 2 ; Position history
player_vel .dsb 2 ; Velocity
player_accel .dsb 2 ; Counts acceleration frames
player_moving .db 0 ; This flag determines if the player is moving horizontally
player_grounded .db 0 ; This flag determines if the player is on the floor
;;; Collision
collision_pos .dsb 2 ; Coordinates of the collision check
coll_nt_offset .dsb 2 ; Nametable offset
collision .dsb 1 ; Flag to be set by collision subroutine
;; blocking_tiles .db $20,$21,$30,$31
blocking_tiles .dsb 4
blocking_tiles_amount .db 4
current_blocking_tile .dsb 1
;;; Camera
camera_x .dsb 1
;;; Logic
dirty .dsb 1
.ende
;;; Predefined bytes
LDA #$20
STA blocking_tiles
LDA #$21
STA blocking_tiles+1
LDA #$30
STA blocking_tiles+2
LDA #$31
STA blocking_tiles+3
LDA #$04
STA blocking_tiles_amount
;;; Player setup
LDA #$10 ; Set initial position
STA player_pos
STA player_pos+1
LDA #$00 ; Set initial velocity
STA player_vel
STA player_vel+1
LoadPalettes
LDA $2002 ; Read PPU status to reset high/low latch
LDA #$3F
STA $2006 ; Write high byte
LDA #$00
STA $2006 ; Write low byte
LDA #$3C
STA $2007
LDX #$01
LoadPalettesLoop
LDA Palettes,X
STA $2007
INX
CPX #$1F
BNE LoadPalettesLoop
LoadNametables
LDA #<Nametable_0 ; Store nametable address
STA bg_offset
LDA #>Nametable_0
STA bg_offset+1
LDY #$00
LDX #$00
LDA #$00
STA bg_boundary ; Set initial tile loading boundary
LDA $2002 ; Read PPU status, reset high/low latch
LDA #$20
STA $2006 ; Write high byte
LDA #$00
STA $2006 ; Write low byte
LoadNametablesLoop
LDA (bg_offset),Y ; Push the current tile
STA $2007
INY
CPY bg_boundary ; Check if the phase is done
BNE LoadNametablesLoop
CPX #$03 ; Check if the first nametable is done
BEQ LoadNextNametable
CPX #$07 ; Check if the second nametable is done
BEQ LoadNametablesDone
INX ; Increment iteration counter
INC bg_offset+1 ; Increment high byte of offset
CPX #$03 ; Check if this is the last section
BEQ LoadShortBoundary
CPX #$07 ; Check if this is the last section
BEQ LoadShortBoundary
JMP LoadNametablesLoop
LoadNextNametable
LDA #<Nametable_1 ; Store address of next nametable
STA bg_offset
LDA #>Nametable_1
STA bg_offset+1
INX ; Account for missed INX
LDY #$00 ; Reset registers and boundary
LDA #$00
STA bg_boundary
LDA $2002 ; Read PPU status, reset high/low latch
LDA #$24
STA $2006 ; Write high byte
LDA #$00
STA $2006 ; Write low byte
JMP LoadNametablesLoop
LoadShortBoundary
LDA #$C0 ; Set the boundary to 192 tiles more
STA bg_boundary
JMP LoadNametablesLoop
LoadNametablesDone
LoadAttributes
LDA $2002 ; Read PPU status, reset high/low latch
LDA #$23
STA $2006 ; Write high byte
LDA #$C0
STA $2006 ; Write low byte
LDX #$00
LoadAttributesLoop
LDA Attributes,X
STA $2007
INX
CPX #$40 ; Check if end of first table was reached
BEQ LoadNextAttributes
CPX #$80 ; Check if end of second table was reached
BEQ LoadAttributesDone
JMP LoadAttributesLoop
LoadNextAttributes
LDA $2002 ; Reset high/low latch
LDA #$27 ; Set address of second attribute table
STA $2006
LDA #$C0
STA $2006
JMP LoadAttributesLoop
LoadAttributesDone
JMP StartSpriteTransferDone
StartSpriteTransfer
LDA #$00 ; Reset data offset
STA sprite_data_offset
LDA #$00
STA $2003 ; Set the low byte (00) of the RAM address
LDA #$02
STA $4014 ; Set the high byte (02) of the RAM address, start the transfer
RTS
StartSpriteTransferDone
JMP DrawSpriteDone
DrawSprite
LDX #$00
LDY sprite_data_offset ; Load the saved offset
DrawSpriteLoop
CPX #$00 ; Write Y coordinate
LDA sprite_target+1
BNE +
CLC
SBC #$01 ; Scanline correction
+
STA ($0200),Y
INY
LDA sprite_source ; Write tile number
STA ($0200),Y
INY
LDA #%00000000 ; Color palette 0, no flipping
STA ($0200),Y
INY
LDA sprite_target ; Write X coordinate
CPX #$03
BNE ++
CLC
SBC #$01 ; Fix weird glitch on last tile X
++
STA ($0200),Y
INY
LDA sprite_big
CMP #$00 ; Possibly redundant?
BEQ DrawSpriteLoopDone ; Sprite isn't big (0)
INX
CPX #$04 ; Finished rendering 4 segments of big sprite
BEQ DrawSpriteLoopDone
CPX #$02 ; Account for bottom row
BNE +++
LDA sprite_source ; Move one row down
;; CLC
ADC #$0D
STA sprite_source
LDA sprite_target ; Reset X
;; CLC
SBC #$0F
STA sprite_target
LDA sprite_target+1 ; Increase Y
;; CLC
ADC #$07
STA sprite_target+1
+++
CPX #$03 ; Weird drawing bug fix for last tile X
BNE ++++
INC sprite_target
++++
INC sprite_source
LDA sprite_target
;; CLC
ADC #$08
STA sprite_target
JMP DrawSpriteLoop
DrawSpriteLoopDone
TYA
STA sprite_data_offset ; Save the offset
RTS
DrawSpriteDone
JMP ReadControllerDone
ReadController
LDA #$01 ; Initiate read
STA $4016
LDA #$00
STA $4016
LDX #$00 ; Reset X
STX player_moving ; Reset moving flags
;; STX player_moving+1
ReadController1Loop
LDA $4016
AND #%00000001 ; Check if the button is pressed
BEQ NotPressed
;; CPX #$00 ;A
;; BNE ReadADone
;; LDA #$00
;; STA player_vel
;; STA player_vel+1
;; JMP NotPressed
;; ReadADone
CPX #$04 ; Up
BNE ReadUpDone
LDA player_grounded ; Check if the player is already jumping
BEQ NotPressed
LDA #$00 ; Set moving vertically(jumping) flag
STA player_grounded
LDA #$FA ; Set jump speed
STA player_vel+1
JMP NotPressed
ReadUpDone
;; CPX #$05 ; Down
;; BNE ReadDownDone
;; LDA #$01 ; Set vertically moving flag
;; STA player_moving+1
;; INC player_accel+1 ; Increase acceleration frames
;; LDA player_accel+1
;; CMP #$05
;; BNE NotPressed
;; LDA #$00 ; Reset acceleration frame counter
;; STA player_accel+1
;; LDA player_vel+1 ; Check if velocity isn't already at max
;; CMP #$02
;; BEQ NotPressed ; Possibly incorrect branch condition
;; INC player_vel+1 ; Increase velocity
;; ReadDownDone
CPX #$06 ; Left
BNE ReadLeftDone
LDA #$01 ; Set horizontally moving flag
STA player_moving
INC player_accel ; Increase acceleration frames
LDA player_accel
CMP #$05
BNE NotPressed
LDA #$00 ; Reset acceleration frame counter
STA player_accel
LDA player_vel ; Check if velocity isn't already at max
CMP #$FE
BEQ NotPressed ; Possibly incorrect branch condition
DEC player_vel ; Increase velocity
ReadLeftDone
CPX #$07 ; Right
BNE ReadRightDone
LDA #$01 ; Set horizontally moving flag
STA player_moving
INC player_accel ; Increase acceleration frames
LDA player_accel
CMP #$05
BNE NotPressed
LDA #$00 ; Reset acceleration frame counter
STA player_accel
LDA player_vel ; Check if velocity isn't already at max
CMP #$02
BEQ NotPressed ; Possibly incorrect branch condition
INC player_vel ; Increase velocity
ReadRightDone
NotPressed
INX
CPX #$08
BNE ReadController1Loop
ReadController1LoopDone
LDA player_vel ; Check if the player is moving
CMP #$00
BEQ NotMoving
LDA player_moving ; Reduce horizontal speed if necessary
BNE NotMoving
LDA player_vel
CMP #$FE
BCC ++
INC player_vel ; If <0 increase
JMP NotMoving
++
CMP #$03
BCS NotMoving
DEC player_vel ; If >0 decrease
NotMoving
;; LDX #$00 ; Reset X
;; ReadController2Loop
;; LDA $4017
;; INX
;; CPX #$08
;; BNE ReadController2Loop
;; ReadController2LoopDone
RTS
ReadControllerDone
;;; Could certainly be compressed
JMP MapCollisionDone
MapCollision
LDA #$00 ; Reset collision flag
STA collision
LDA #<Nametable_0 ; Store nametable address
STA coll_nt_offset
LDA #>Nametable_0
STA coll_nt_offset+1
;;; Horizontal collision coordinate
LDA collision_pos ; Extract least significant hex
AND #%00001111
TAX
LDA collision_pos ; Extract most significant hex
AND #%11110000
LSR A
LSR A
LSR A
LSR A
ASL
CPX #$08 ; Determine sub-tile
BCC +
CLC
ADC #$01
+
STA collision_pos ; Store X coordinate of NT query
;;; Vertical collision coordinate
LDA collision_pos+1 ; Extract least significant hex
AND #%00001111
TAX
LDA collision_pos+1
AND #%11110000 ; Extract most significant hex
LSR A
LSR A
LSR A
LSR A
ASL
CPX #$08 ; Determine sub-tile
BCC ++
CLC
ADC #$01
++
STA collision_pos+1 ; Store Y coordinate of NT query
LDX #$FF ; Determine NT offset according to Y
-
INX
CPX collision_pos+1 ; Until Y has been accounted for
BEQ +++
LDA coll_nt_offset ; Load least significant byte of offset
CLC
ADC #$20 ; Add a row of 32 tiles (hex 20)
STA coll_nt_offset
BCC -
INC coll_nt_offset+1 ; If we exceeded the limit, INC the most significant byte of offset
JMP - ; Continue the loop
+++
LDX #$FF
--
INX ; For each blocking tile
CPX blocking_tiles_amount
BEQ ++++
LDA blocking_tiles,X ; Load the current tile to check against
STA current_blocking_tile
LDY collision_pos ; Check if the tile is blocking
LDA (coll_nt_offset),Y
CMP current_blocking_tile
BNE -- ; No collision, continue the check
LDA #$01 ; Set collision flag
STA collision
++++
RTS
MapCollisionDone
JMP PlayerUpdateDone
PlayerUpdate
LDA player_grounded ; Apply gravity if not touching the floor
BNE +
INC player_vel+1
CMP #$05
BCC +
DEC player_vel+1
+
;; INC $0020
;; INC player_accel+1 ; Process and check accel timer
;; LDA player_accel+1
;; CMP #$02
;; BCS +
;; LDA #$00 ; Reset accel timer
;; STA player_accel+1
;; INC player_vel+1 ; Increase downward velocity
;; LDA player_vel+1
;; STA $0020
;; CMP #$05 ; Restrict gravity force
;; BCC +
;; DEC player_vel+1
;; +
LDA player_vel
STA $0020
LDA player_pos ; Save current position
STA player_pos_saved
LDA player_pos+1
STA player_pos_saved+1
LDA player_pos ; Add horizontal velocity
CLC
ADC player_vel
STA player_pos
LDA player_pos ; Perform collision check BOTTOM LEFT
STA collision_pos
LDA player_pos+1
ADC #$20
STA collision_pos+1
JSR MapCollision
LDA #$01
CMP collision
BNE +++
LDA player_pos_saved ; Reset position
STA player_pos
;; JMP PlayerCollisionDone
+++
LDA player_pos+1 ; Add vertical velocity
CLC
ADC player_vel+1
STA player_pos+1
LDA player_pos ; Perform collision check BOTTOM LEFT
STA collision_pos
LDA player_pos+1
ADC #$20
STA collision_pos+1
JSR MapCollision
LDA #$01
CMP collision
BNE +++
LDA #$01
STA player_grounded ; Player is touching some kind of floor
LDA player_pos_saved+1
STA player_pos+1
LDA #$00
STA player_vel+1
+++
PlayerCollisionDone
;; LDA player_pos ; Perform collision check TOP LEFT
;; STA collision_pos
;; LDA player_pos+1
;; STA collision_pos+1
;; JSR MapCollision
;; LDA #$01
;; CMP collision
;; BNE +
;; LDA player_pos_saved ; Reset position
;; STA player_pos
;; LDA player_pos_saved+1
;; STA player_pos+1
;; +
;; LDA player_pos ; Perform collision check TOP RIGHT
;; ADC #$10
;; STA collision_pos
;; LDA player_pos+1
;; STA collision_pos+1
;; JSR MapCollision
;; LDA #$01
;; CMP collision
;; BNE ++
;; LDA player_pos_saved ; Reset position
;; STA player_pos
;; LDA player_pos_saved+1
;; STA player_pos+1
;; ++
;; LDA player_pos ; Perform collision check BOTTOM LEFT
;; STA collision_pos
;; LDA player_pos+1
;; ADC #$20
;; STA collision_pos+1
;; JSR MapCollision
;; LDA #$01
;; CMP collision
;; BNE +++
;; LDA player_pos_saved ; Reset position
;; STA player_pos
;; LDA player_pos_saved+1
;; STA player_pos+1
;; +++
;; LDA player_pos ; Perform collision check BOTTOM RIGHT
;; ADC #$10
;; STA collision_pos
;; LDA player_pos+1
;; ADC #$10
;; STA collision_pos+1
;; JSR MapCollision
;; LDA #$01
;; CMP collision
;; BNE ++++
;; LDA player_pos_saved ; Reset position
;; STA player_pos
;; LDA player_pos_saved+1
;; STA player_pos+1
;; ++++
RTS
PlayerUpdateDone
JMP PlayerDrawDone
PlayerDraw
LDA #$01 ; Big sprite
STA sprite_big
LDA #$00 ; Set tile
STA sprite_source
LDA player_pos ; Set position
STA sprite_target
LDA player_pos+1
STA sprite_target+1
JSR DrawSprite ; Actually draw sprite 1
LDA #$02 ; Set tile
STA sprite_source
LDA #$30 ; Set position
STA sprite_target
STA sprite_target+1
JSR DrawSprite ; Actually draw sprite 2
RTS
PlayerDrawDone
PPUCleanUp:
LDA #%10010100 ; Enable NMI, sprites from pattern table 0
STA $2000
LDA #%00011110 ; Enable sprites, background, disable clipping left
STA $2001
LDA #$00
STA $2005 ; Reset scrolling
STA $2005
Forever ; Wait until NMI occurs
LDA dirty ; Check if dirty
BEQ Forever
DEC dirty ; Set clean
JSR ReadController
JSR PlayerUpdate
JMP Forever
NMI ; This is the time to do all drawing
LDA #$01 ; Set dirty flag
STA dirty
JSR StartSpriteTransfer
JSR PlayerDraw
RTI
IRQ
RTI
;********************************
; Data
;********************************
Palettes
.incbin "palette.dat"
Nametable_0
.incbin "nametable_0.nam"
Nametable_1
.incbin "nametable_1.nam"
Attributes
.db %01010101, %01010101, %01010101, %01010101, %01010101, %01010101, %01010101, %01010101
.db %01010101, %01010101, %01010101, %01010101, %01010101, %01010101, %01010101, %01010101
.db %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111
.db %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111
.db %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111
.db %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111
.db %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000
.db %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000
.db %01010101, %01010101, %01010101, %01010101, %01010101, %01010101, %01010101, %01010101
.db %01010101, %01010101, %01010101, %01010101, %01010101, %01010101, %01010101, %01010101
.db %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111
.db %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111
.db %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111
.db %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111, %11111111
.db %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000
.db %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000, %00000000
;********************************
; Vectors
;********************************
.pad $FFFA ; First of the three vectors starts here
.dw NMI
.dw Reset
;.dw IRQ
.dw 0
;********************************
; CHR-ROM bank
;********************************
.incbin "graphics.chr" ; Includes 8KB graphics file