;; All NES ROMs have a 16 byte header that describes how the ROM ;; works; specifically, how many banks of 16kB PRG (program) code, ;; how many 8kB banks of CHR data, which mapper to use for bank swapping ;; and how to perform background mirroring ;; iNES header block .byte "NES",$1A .byte $01 ;; 1 PRG ROM page (how big is an ophis page?) .byte $02 ;; 1 CHR (tile/sprite) ROM page (how big is a page?) .byte $00 ; Horizontal mirroring .byte $00 ; Mapper 0 (NROM, no bank switching) .byte $00,$00,$00,$00 ; Reserved bytes .byte $00,$00,$00,$00 ; Reserved bytes .include "defines.S" .text zp ; zero page - this begins at $00 ; all pointers should go here ;; $0000 - $000F is reserved for local variables/function args .org $0010 ; $0010 - 002F reserved for pointers .space curSpriteDataLo 1 .space curSpriteDataHi 1 .org $0030 .space curSpriteLen 1 .space playery 1 .space playerx 1 .space pad1a 1 .space pad1b 1 .space pad1select 1 .space pad1start 1 .space pad1up 1 .space pad1down 1 .space pad1left 1 .space pad1right 1 .space pad1areleased 1 .text .org $C000 ;; PRG bank code starts at 0xC000 ;; START will be called by the NES whenever the system boots ;; or when the reset button is pressed (think of _start in libc ) ;; but the fact that the NES looks at "START" is only because we ;; specified it in bank 1 at 0xFFFA, the vector table START: SEI ;; disable IRQs (we don't have an IRQ vector) CLD ;; disable decimal mode (NES 6502 doesn't have ;; a decimal mode, please don't produce decimal ;; mode instructions, NESASM!) LDX #$40 ;; load 0x40 into X register STX $4017 ;; store what's in X to address 0x4017 ... ;; 0x4017 is the Joystick 2 port?! WTF does this ;; do?! LDX #$FF TXS ;; Move the contents of X to the stack pointer INX ;; increment X by 1, which causes overflow, so ;; now X is 0 STX $2000 ;; set PPU flag to disable NMI (0x2000 = 0) STX $2001 ;; set PPU flag to disable rendering (0x2001=0) STX $4010 ;; disable APU IRQs, no audio _START_vblankwait: BIT $2002 ;; Bitwise AND the accumulator (LDA) with mem ;; at 0x2002, and set the Zero, Sign & Overflow ;; flags accordingly. 0x2002 is the PPU status ;; register; when 0x2002 has bit 7 set, we are ;; in vblank, so this is how we check for it. BPL _START_vblankwait ;; Until the sign bit is set, loop here. Wait ;; for vblank. _START_clearmem: ;; Hey look, it's the longest memset() ever! LDA #$00 STA $0000, x ;; store 0 to (LDX) + 0x0000 ... but X should ;; be 0 at this point (see START where we INX), ;; so why aren't we just using zero-page ;; addressing? ;; ... that's what I thought at first, before STA $0100, x ;; I realized that I'm looking at a loop: STA $0200, x STA $0400, x ;; for ( x = 0; x < 256 ; x++) STA $0500, x ;; *(0x0100 + x) = 0; STA $0600, x ;; .... STA $0700, x ;; the INX and BNE at the bottom are the "; x++)" ;; This clears the zero page (0000-00FF, the ;; stack (0100-01FF), the entirety of main RAM ;; (0200-07FF) LDA #$FE ;; These two are clearing all of the sprite STA $0200, x ;; OAM; previous tutorial had this at 0300, ;; which may have been wrong. We don't HAVE ;; to reserve this range for OAM; we could ;; just manually poke bits into the PPU, but ;; that is 3-4x (or more) slower than reserving ;; 256 bytes for an OAM copy that we DMA into ;; the PPU on every vblank/NMI INX ;; X is already 0 so this should do X=1, ;; and the Zero and Sign flags should both go 0 BNE _START_clearmem ;; "; x++)", loop back to clrmem until X rolls _START_vblankwait2: BIT $2002 ;; copy paste going to happen in ASM BPL _START_vblankwait2 ;; once we've gotten 1 vblank, ;; cleared mem, and gotten another vblank, ;; the PPU is ready. Wait for it. MAIN: ;; horray, here is main() _MAIN_LoadPalettes: LDA $2002 ; The PPU Memory address at $2006 expects ; the high byte of the palette address first, ; then the low byte, but we can't know ; which one it's expecting right now, so we ; read the PPU status at $2002 to reset the ; high/low latch on $2006. LDA #$3F ; we're populating the second palette, at $3F10 STA $2006 LDA #$00 STA $2006 LDX #$00 _MAIN_LoadPaletteLoop: LDA palette, x ; Loop over each index of the byte array at STA $2007 ; 'palette', store each one into the accumulator INX ; and then store the accumulator into the PPU CPX #$20 ; .. compare X to 20 (size of 'palette'), and BNE _MAIN_LoadPaletteLoop ; loop as long as the Zero flag isn't set (NE) ;; All sprites live between 0200-02FF; there are a max of 64 sprites ;; on screen, and each one has a 4 byte struct describing it. ;; *(sprite + 0) = y position ;; *(sprite + 1) = tile index (0-FF) in the pattern table for pixels ;; *(sprite + 2) = attributes. Color palette, priority, and mirroring. ;; 76543210 ;; ||| || ;; ||| ++- Color Palette of sprite. Choose which set of 4 from ;; ||| the 16 colors to use. You can select sprite colors only ;; ||| in groups of 4 on 4 byte boundaries; so you can select ;; ||| colors 0-3, 4-7, 8-11, and 12-15, but not 2-5, for ;; ||| example. Palette construction and use is an art in ;; ||| itself! ;; ||| ;; ||+------ Priority (0: in front of background; 1: behind background) ;; |+------- Flip sprite horizontally ;; +-------- Flip sprite vertically ;; *(sprite + 3) = x position ;; All the sprite OAM data is initialized at the bottom of bank 1 ;; at .org $FF00 ;; $2000 is the PPU Control register, controlled by various bitflags. ;; ;; 7654 3210 ;; |||| |||| ;; |||| ||++- Base nametable address ;; |||| || (0 = $2000; 1 = $2400; 2 = $2800; 3 = $2C00) ;; |||| |+--- VRAM address increment per CPU read/write of PPUDATA ;; |||| | (0: increment by 1, going across; ;; |||| | 1: increment by 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 (has no effect on the NES) ;; +--------- Generate an NMI at the start of the ;; vertical blanking interval (0: off; 1: on) LDA #%10000000 ; enable NMI (so we get a function call every ; vblank), and draw sprites from table 0 STA $2000 ;; The PPU mask is set at $2001, the 2nd PPU Control register, and it ;; sets one config option for every bit of the byte ;; ;; 76543210 ;; |||||||| ;; |||||||+- Grayscale (0: normal color; 1: AND all palette entries ;; ||||||| with 0x30, effectively producing a monochrome display; ;; ||||||| note that colour emphasis STILL works when this is on!) ;; ||||||+-- Disable background clipping in leftmost 8 pixels of screen ;; |||||+--- Disable sprite clipping in leftmost 8 pixels of screen ;; ||||+---- Enable background rendering ;; |||+----- Enable sprite rendering ;; ||+------ Intensify reds (and darken other colors) ;; |+------- Intensify greens (and darken other colors) ;; +-------- Intensify blues (and darken other colors) LDA #%00010000 ;; turn on sprites, no more background color STA $2001 ;; Write to PPU Control Register 2 LDX #$0 LDA #$80 STA playerx STA playery _MAIN_loop: JMP _MAIN_loop ;; Loop forever NMI: LDX #$0 LDY #$0 ;; This is almost a reusable function ... store 'sprMario' as our ;; current sprite that we're about to draw LDA sprMario STA curSpriteLen LDA #>sprMarioData STA curSpriteDataHi LDA #