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NES

Architecture

NES Console

  • 6502 CPU
    • 16 bit address bus
    • 64 kB address space
      • 0x0000 - 0xFFFF
    • Four register
      • Accumulator
        • perform maathematical operations
        • compare the value stored in it
      • X , Y increment register
        • basically those are used for storing values such as counters
      • Status
        • can not write
        • but can read from it to get certain value like if the accumulator overflows and it will put a status there saying that your value has overflowed
  • APU ( Audio Processing Unit )
  • PPU ( Picture Processing Unit )
    • 16 KB address space
  • Lockout chip

Game Cartridge

  • CHR ( Charater Memory ) ROM or RAM
    • where all your sprite data is stored
  • PRG ( Program Memory ) ROM
    • where your game code is stored
  • WRAM ( optional ) and battery
  • Lockout chip

When NES is on ,

  • PRG ROM copied to CPU
  • CHR copied to PPU
  • CPU write to the PPU
    • to modify the background and the sprites

CPU / PPU address space

from CPU

  • buttom 2KB internal RAM
    • which can be used for things like variables
  • next 2KB ?
    • PPU ports , use for writing to the PPU
  • on the top of it, APU and control ports
  • WRAM
  • Cartridge ROM
  • at the very end of it , NMI / RESET / IRQ vectors
    • NMI , for every frame that gets displayed to the screen , that's going to called and then from there it's going to run all of the code that modified your sprites and backgrounds and upateds the game engine.
    • RESET , gets called whenever the nes starts up , beasts up, and reset button is pressed
    • IRQ , i haven't used that , it uesd for special circumstances

from PPU

  • 1st 2KB ,
    • the pattern tables , and that's basically all the sprite data
  • name tabels
    • basically the arrangement of all the sprites on the screen that creats the background
  • attribute tabels
    • determines the colors that are used to paint the background
    • you have multiple name tables and attribute tables , so you can switch between these to give you different backgourds in your game
  • sprite palette , backgroud palette
    • you have limited number of colors you can choose from , and that is stored in these palette variables

6502 Assembly

  • Directives
    • Assembler commands
    • Start with a period
  • Labels
    • Used to organize code
    • Like a BASIC line number (for GOTO)
    • Not intented and followed by a colon
  • Opcodes - Program instructions - Indented

6502 Opcodes

you should take special considerations if your values are going to add to more than 256

  • Load / Store
    • Load: LDA , LDX, LDY
    • Store: STA , STX, STY
  • Math
    • Add: ADD , CLC
    • Subtract : SBC, SEC
    • Increment: INC, DEC , INX , INY , DEX , DEY
    • Shift : ASL , LSR
  • Comparision
    • CMP , CPX , CPY
  • Control
    • JMP
    • BEQ , BNE

Assembly Starter

\ C 6502 Desc
declare int num; .rsset $0000 where your variable in the memory address
vars num .rs 1 reserved 1 byte space
ASSIGN num = 42 LDA #$2A 
     |               |   STA num

IF | if (num==5) | LDA num | | CMP #$05 | | BNE Done | | ; do something | | Done: PROCEDURE | void MyMethod() { | MyMethod: | // do somthing | : do something | } | RTS | ... | ... | MyMethod(); | JSR MyMethod INCREMENT | x = 10; | LDA #$0A | x ++ ; | STX | y = 32 ; | INX | y-- ; | LDA #$20 | | STY | | DEY WHILE LOOP | x = 0 ; | LDA #$0 | while (x<255) { | STX | // do something | Loop: | x ++ | ; do something | } | INX | | CPX #$FF | | BNE Loop INTERRUPT | void Draw() { | NMI: METHODS | // draw code | ; modify sprites | } | ; modify background | void Update() { | ; update code | // update code | RTI | } |

Sprite

  • Can be created with programs like YY-CHR
  • Each sprite tile is 8x8 pixels
  • Each sprite tile can only have 4 unique colors
    • actually 3 colors and 1 transparency layer
  • Objects , such as mario , are usually composed of multiple sprite tiles
  • All sprites must fit on a single spritesheet

Palettes

  • Less than 60 total colors
  • PPU IO ports : $2006 , $2007
    • you write to the palette table by using PPU $2006, $2007 ports
  • Sprite palette: $3F10
  • Background palette : $3F00

Background

  • 32x30 tiles (256 * 240)
  • Tiles are 8x8 pixels
  • Four unique colors for every 2x2 tiles
  • Background is static , but can be scrolled
  • Text is usually background tiles

Controller Input

  • Call load on Controller port
    • LDA $4016 (player #1)
    • LDA $4017 (player #2)
  • Each call returns the status of the next button
    • A,B,Select, Start , Up, Down, Left, Right

Playing Sounds

  • APU 5 channels
    • Square 1, 2 -- Hollow sound
    • Triangle -- Smooth sound
    • Noise - Explosions
    • DMC - prerecorded, like voices
  • Write to $4015 to enable sounds
  • $4000 to $4003 modify sound properties

NES ROM Image

  • Generate with NES assemble
    • NESASM3
  • Play with NES emulator
    • FCEUXD
  • Tools exist for playing on actual NES hardware