6502 Assembly programming for the Nintendo Entertainment System (NES) and Famicom

The NES was the first games console I owned, compared to the 8 bits I was used to (Like the CPC) it's incredible high speed, and smooth scrolling were really impressive. While it's games tend to be limited to platformers, it's hardware was very decent for the time, and in it's later years, it's low cost made it an easy choice as a home games system. These days the main 'pulling point' for new developers is the huge number of sales the system had, and its presence in pretty much every territory, meaning that if you want to get into retro development, and get noticed, the NES or Famicom (as it was called in Japan) is a great computer to look at! The NES CPU is based on the 6502, and it's almost the same, however it has no BCD (Binary Coded Decimal) mode, however it's generally the same as the 6502

Cpu Ricoh 2A03 (6502 based) 1.79mhz Ram 2k  Vram 2k Resolution 256x240 Sprites 64 onscreen - 8x8 or 8x16 size Tilemap 32x30... max 256 unique patterns Colors 24 onscreen (4x4 for tiles, 4x3 for sprites) Sound chip 2 pulse, 1 triangle, 1 noise, 1 pcm The UK NES PSU outputs 9V AC the Japanese Famicom PSU outputs 10V DC - CENTER NEGATIVE BE CAREFUL! adaptors that output AC are very rare and will kill most systems - and Center negative was common for 80's Japanese systems, but is very rare these days - YOU HAVE BEEN WARNED!

ChibiAkumas Tutorials

Technical resources

The screens on consoles usually do not work like they do on computers like the BBC Graphics are not just 'bytes' in a memory address...  The screen is made up of a 'Tile Layer' and a 'Sprite Layer' To explain Tiles and sprites, lets look at our imaginary game shown to the right 'The Super Yuusha Siblings', on a theoretical game system the 'GameChibi'... Just to be very clear, we looking at this as a concept, not the actual layout of the NES! Looking at our example, We have a level with some grass, blocks, and some collectable 'stars'... our hero, Yume is controlled by the player...
The screen is made up of the Tile layer, and the Sprite Layer, Usually Sprites are drawn above the Tiles... but sometimes they may be drawn below. It's also possible we could use sprites for the stars.. but sprites are very limited, so the object doesn't move, then tiles will do the job... we can even animate the stars by switching the tile between different patterns
The tile array on the systems we'll be looking at is made up of 8x8 tiles... the array is a 'grid' of these tiles, so the tiles must line up, a block cannot be at a 'half way boundary' Tiles are defined by a number (usually 0-255)... we define the bitmap (image) data for that tile (we'll call it a pattern), then tell the hardware what positions in the tile array to use that pattern. In our example, the black background is pattern 0 ... the blocks are pattern 1... the grass is pattern 2... and the stars are pattern 3 Our 'GameChibi' console has a tile array of 8x8... and 64 bytes is used to define the tile grid... So to define the stars, we need to set memory locations 10,20 and 15 of the tile array to byte '3' A real system usually has a tile array bigger than the screen (maybe just by one row and one column)... this is to allow smooth scrolling of the screen, where two tiles are 'half shown' Now in the case of our 'GameChibi' system, with it's 64 tile screen, and 256 pattern definitions, we could just set every visible tile to a different pattern, and treat the screen as a plain bitmap again... We can do that on the MSX1, Unfortunately, many systems do not have enough tile patterns to do this, and it's often too slow anyway... we really have to work with the system in the 'way it wants' to get good results.
Because our system is using hardware sprites, we have to design our game sprites in a way that can be drawn with the hardware sprites... for example lets look at our Yume sprite... if our 'GameChibi' used 8x8 hardware sprites, we would have to use 48 of them to make this image!... we can save 6 (marked green)... these have no data, so we can just not draw them... When it comes to moving our character, the software will have to move the hardware sprites all together, so the user does not realise they are made up of many sprites!... on systems with more onscreen colors than sprite colors, two or more sprites may be overlapped to make the sprite appear more colorful Most systems will have one color (usually 0) which marks the transparent colour. but there is a problem! with software sprites on a bitmap screen, we can draw as much as we want, it will just get slow.... but with hardware sprites, we have a fixed limit of how many sprites can be shown onscreen at once! sounds bad? well actually it's worse, even though a system like the NES can show 64 sprites onscreen, there can only be 16 on a line... if more than 16 appear on the same line, some will flicker, or not appear... there's nothing we can do about it, we just have to design our game to avoid this problem!

The example above will give us 8k of graphics RAM via Mapper 2

The rest of our rom layout depends on the number of Program and Character rom banks we have

Joypad

The NES has 2 joysticks, but 2 extra pads can be added to the Famicom external port... Unlike many systems, we can't read from one port to get all the keys in one go. We need to read each button one at a time, and build up a byte representing all the buttons in our joypad.
First we need to 'Strobe' the joypad, by writing 1 to bit $4016, then we need to read in from $4016 and $4017 repeatedly to get all the bits of the Joypad.

Lightgun

The NES Name Table defines the Tilemap's patterns... one byte per 8x8 tile defines the number of the tile... the name table is 32x30 tiles in size, so spans from $2000-$23BF Note: Name Table 2,3 ($2800-$3000) are not available on an standard NES , they will only be available if your cartridge has Extra Ram! Color's are defined  by the 'Attribute table'... effectively, each square block of 2x2 tiles (16x16 pixels) have to use the same color palette... and each block of 4x4 tiles (32x32 pixels) are defined by a single byte (2 bits per block - for 4 possible palettes) Lets take the example to the right, with different 16 tiles making up a grid of 32x32 pixels- each areas palette would be defined by a single byte in the way below:  7  6  5  4  3  2  1  0 D D C C B B A A

The NES has 64 sprites, Sprites on the NES are defined by 256 bytes of OAM memory- 4 bytes per sprite

Sprites can either be 8x8 or 8x16, when they are 8x16, the top half of the sprite is taken from the patterns at $0000, and the bottom half is taken from $1000

The byte is selected by setting the OAM-address with memory location $2003 - effectively with 4x the sprite number...  then by writing the 4 bytes to $2004 (the OAM address autoincs)

The first visible pixel is at (X,Y) pos (0,8)