PacMan Documentation - HeitorBRaymundo/861 GitHub Wiki

Overview

PacMan is a classical NES Game, very popular and loved by people all around the world. On this project we tried to create a variation of the game, mixing the concept with a popular game now a days, the Fortnite: Battle Royale. So, in the game you will have to use the keys of your keyboard to make the PacMan scape from the evil ghosts.

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The PacMan Game

• 1 Technical Documentation
  • 1.1 Graphics
  • 1.2 Game Logic
  • 1.3 Soundtrack

1. Technical Documentation

In this section we aim to explain, with more detail, how we developed the game using the Assembler 6502 language. Given more details about the choices that we took and how you could modify the code to make your adventure.

1.1 Graphics

We can divide the graphics of the game in two main aspects, the palettes colours and the sprites.

Palettes

Before putting any graphics on screen, you first need to set the color palette. There are two separate palettes, each 16 bytes. One palette is used for the background, and the other for sprites. The byte in the palette corresponds to one of the 64 base colors the NES can display. $0D is a bad color and should not be used. These colours are not exact and will look different on emulators and tv.

Sprites

Anything that moves separately from the background will be made of sprites. A sprite is just an 8x8 pixel tile that the PPU renders anywhere on the screen. Generally objects are made from multiple sprites next to each other. Examples would be Mario and any of the enemies like Goombas and Bowser. The PPU has enough internal memory for 64 sprites. This memory is separate from all other video memory and cannot be expanded.

On the code, we made loops to load the colours and palettes in the memory. We choose to create the sprites of the PacMan and ghosts directly at the ".asm" file, without creating a binary file with the loaded images. So, we could create the characters in the way we wanted.

You can define the palettes like this:

    .db $FE,$20,$11,$15, $FE,$05,$15,$25, $FE,$08,$18,$28, $FE,$0A,$1A,$2A

The loop to load thing in the memory can be made like this:

LoadPalettesLoop: 
    LDA Palette, y
    STA $2007	
    INY	
    CPY #$20
    BNE LoadPalettesLoop

And to finish, you can create the sprites in three steps. First you define the bits at the memory, like this:

SpritePacMan:
    .db %00000000
    .db %11100000
    .db %11111000
    .db %11111100
    .db %11111110
    .db %11111000
    .db %11100000
    .db %10000000

Then, you have to define the positions and attributes, what can be made like this:

Sprites:
  .db $80, $00, %00000000, $88  ; 200, 201, 202, 203
  .db $80, $01, %00000000, $80	; 204, 205, 206, 207
  .db $88, $02, %00000000, $88	; 208, 209, 20A, 20B
  .db $88, $03, %00000000, $80	; 20C, 20D, 20E, 20F

Finally, is just load at the memory.

1.2 Game Logic

The game logic is quite simple and we'll give a simple overview on how we structure it (with code 😄). In order to make easy to everyone understand what is happening, we can subdivide this topic in a few subtopics: PacMan movement, Ghosts movements and collisions.

PacMan movement

His moves are quite simple and dependent on the keys the user inputs. So, you just have to read which was the pressed key and move the character, what can be done like that:

PacUp:
    LDA #%10000000
    STA $202
    STA $206
    STA $20A
    STA $20E
    LDA #$09
    STA $201
    LDA #$0A
    STA $205
    LDA #$07
    STA $209
    LDA #$08
    STA $20D
    DEC $200
    DEC $204
    DEC $208
    DEC $20C
    LDA #10
    STA directionPacMan
    JMP FimPacMan

Ghost movement

Move the ghost is a little bit more complicated, because he is a computer character, so we have, at first, to choose a random direction, and then, move the ghost. To generate a random number you can use a code like that:

RandomSeed1:
    LDA seed1
    ASL A
    ASL A
    CLC
    ADC seed1
    CLC
    ADC #03
    STA seed1
    RTS

A trick to make the generated number inside the range you want is to make an AND max_value. So, after select the direction, you move it, doing something like that:

Ghost1Up: 	;Yellow
    DEC $210
    DEC $214
    DEC $218
    DEC $21C
    LDA #10
    JSR CheckCollisionF1P
    JMP ContinueGhost2

Collisions

Finally, we have to check if the ghost and the PacMan collide or not. To make it easy, we make the check from the ghost part of the code, doing this:

CheckCollisionF4P:
    LDA $207 
    CMP $243
    BCS justReturn4
    LDA $203
    CMP $247
    BCC justReturn4
    LDA $200
    CMP $248
    BCS justReturn4
    LDA $208
    CMP $240
    BCC justReturn4
    JMP collide

1.3 Soundtrack