show-sprite.asm

.segment "HEADER"
  .byte "NES", $1A ; Constant
  .byte 2 ; 2 x 16KB PRG ROM
  .byte 1 ; 1 x 8KB CHR ROM

.segment "CHARS"
  ;-------------------------------------------------------------
  ; CREATE SPRITES
  ;-------------------------------------------------------------
  ; The following creates one sprite. The following two bitmaps
  ; are overlayed to create one sprite, where each pair of bits
  ; in the same position in the two bitmaps are added to result
  ; in the following way:
  ; 00 : transparant
  ; 01 : color 1 from palette
  ; 10 : color 2 from palette
  ; 11 : color 3 from palette
  .byte %00011000  ; 0240 1-48:    **
  .byte %00111110  ; 0241 1-48:   *****
  .byte %01100000  ; 0242 1-48:  **
  .byte %00111100  ; 0243 1-48:   ****
  .byte %00000110  ; 0244 1-48:      **
  .byte %01111100  ; 0245 1-48:  *****
  .byte %00011000  ; 0246 1-48:    **
  .byte %00000000  ; 0247 1-48:

  .byte %00011000  ; 0248 1-49:    **
  .byte %00111110  ; 0249 1-49:   *****
  .byte %01100000  ; 024A 1-49:  **
  .byte %00111100  ; 024B 1-49:   ****
  .byte %00000110  ; 024C 1-49:      **
  .byte %01111100  ; 024D 1-49:  *****
  .byte %00011000  ; 024E 1-49:    **
  .byte %00000000  ; 024F 1-49:

.segment "VECTORS"
  .word 0, 0, 0, nmi, reset, irq

.segment "STARTUP" ; avoids warning

.segment "CODE"

  ; First of two waits for vertical blank to make sure that the
  ; PPU has stabilized
  vblankwait1:
      BIT $2002
      BPL vblankwait1
  vblankwait2:
      BIT $2002
      BPL vblankwait2

  ; setup PPU
  LDA $2002 ; read PPU status to reset the high/low latch to high

  ;-------------------------------------------------------------
  ; SET PPUCTRL
  ;-------------------------------------------------------------
  ; see: https://wiki.nesdev.com/w/index.php/PPU_registers#PPUCTRL
  LDA #%10000000
      ; ||||||||
      ; ||||||++- Base nametable address
      ; ||||||    (0 = $2000; 1 = $2400; 2 = $2800; 3 = $2C00)
      ; |||||+--- VRAM address increment per CPU read/write of PPUDATA
      ; |||||     (0: add 1, going across; 1: add 32, going down)
      ; ||||+---- Sprite pattern table address for 8x8 sprites
      ; ||||      (0: $0000; 1: $1000; ignored in 8x16 mode)
      ; |||+----- Background pattern table address (0: $0000; 1: $1000)
      ; ||+------ Sprite size (0: 8x8; 1: 8x16)
      ; |+------- PPU master/slave select
      ; |         (0: read backdrop from EXT pins; 1: output color on EXT pins)
      ; +-------- Generate an NMI at the start of the
      ;           vertical blanking interval (0: off; 1: on)
  STA $2000        ; PPUCTRL

  ; Set PPUMASK
  LDA #%00010000
      ; ||||||||
      ; |||||||+- Greyscale (0: normal color, 1: produce a greyscale display)
      ; ||||||+-- 1: Show background in leftmost 8 pixels of screen, 0: Hide
      ; |||||+--- 1: Show sprites in leftmost 8 pixels of screen, 0: Hide
      ; ||||+---- 1: Show background
      ; |||+----- 1: Show sprites
      ; ||+------ Emphasize red*
      ; |+------- Emphasize green*
      ; +-------- Emphasize blue*
  STA $2001

  ;-------------------------------------------------------------
  ; SET UP THE COLOUR PALETTE FOR SPRITES
  ;-------------------------------------------------------------
  ; Setting up the colour palette for sprites is done by writing
  ; the four colours a sprite can use to VRAM address $3f10-$3f13
  ; First let the PPU know what address in VRAM the CPU wants to
  ; write to by first setting the high byte of the staring address
  ; (3F) and the low byte (10) to address $2006 (PPUADDR).
  LDA #$3F         ; PPUADDR
  STA $2006        ; write the high byte of $3F10 address
  LDA #$10         ; PPUADDR
  STA $2006        ; write the low byte of $3F10 address

  ; Then write the four colours that make up the palette to $2007
  ; (PPUDATA) one by one. Colour information:
  ; https://nesdoug.com/2015/11/19/5-a-little-color/
  LDA #$02         ; dark blue
  STA $2007        ; writes to PPU $3F10
  LDA #$16         ; red
  STA $2007        ; writes to PPU $3F11
  LDA #$0B         ; dark green
  STA $2007        ; writes to PPU $3F12
  LDA #$24         ; light purple
  STA $2007        ; writes to PPU $3F13

  ; store sprite information in $0200 - $0202
  LDA #$50         ; Y position of top of sprite
  STA $0200
  LDA %00000010    ; tile number. 0 means first tile from the CHARS segment
     ; ||||||||
     ; |||||||+- Bank ($0000 or $1000) of tiles
     ; +++++++-- Tile number of top of sprite (0 to 254; bottom half gets the next tile)
  STA $0201
  LDA #%00000000   ; attributes
      ; ||||||||
      ; ||||||++- Palette (4 to 7) of sprite
      ; |||+++--- Unimplemented
      ; ||+------ Priority (0: in front of background; 1: behind background)
      ; |+------- Flip sprite horizontally
      ; +-------- Flip sprite vertically
  STA $0202
  LDA #$50         ; X position of left side of sprite.
  STA $0203

  forever:
    JMP forever

; INTERUPTS
  nmi: ; triggered on vblank (1/60th of a second)
    ; Start DMA transfer. This will copy from $0200 to $02FF to the internal PPU OAM
    ; See: http://wiki.nesdev.com/w/index.php/PPU_registers#OAM_DMA_.28.244014.29_.3E_write
    LDA #$00
    STA $2003  ; set the low byte (00) of the RAM address in OAMADDR
    LDA #$02
    STA $4014  ; set the high byte (02) of the RAM address in 4014, start the transfer
    RTI

  irq:
    RTI

  reset:
    SEI        ; ignore IRQs
    CLD        ; disable decimal mode
    LDX #$40
    STX $4017  ; disable APU frame IRQ
    LDX #$ff
    TXS        ; Set up stack
    INX        ; now X = 0
    STX $2000  ; disable NMI
    STX $2001  ; disable rendering
    STX $4010  ; disable DMC IRQs

    ; Optional (omitted):
    ; Set up mapper and jmp to further init code here.

    ; If the user presses Reset during vblank, the PPU may reset
    ; with the vblank flag still true.  This has about a 1 in 13
    ; chance of happening on NTSC or 2 in 9 on PAL.  Clear the
    ; flag now so the @vblankwait1 loop sees an actual vblank.
    BIT $2002