Activities carried out during the Assembly course for Atari 2600 with the 6052 processor. First of all, thank you Gustavo Pezzi for taking my fear out of the Assembly language during the course and making me understand such important concepts of computer architecture. It was certainly one of the best courses I ever took to understand concepts learned during Computer Engineering faculty here in Brazil. My professors at the university always made me afraid to program in Assembly. During the course, I understood that the language is not as complicated as I thought. Thanks again, Gusttavo. The concepts passed on in the course helped me a lot to connect concepts in embedded systems.
First, choose an IDE of your choice. During the course, I used VSCode. Gustavo, in turn, used Vim. After that, download the DASM assembler for 8-bit architectures available in the references link. On Linux, for example, rename the dasm.Linux.x86 file and copy it to the path:
$ sudo cp dasm /usr/local/bin
This command allows you to use the DASM assembler from anywhere via the terminal. Through this brief presentation, it will be possible to compile all the programs through the Makefiles existing in each of the folders. After that, download the Stella emulator to run the programs. The emulator can be downloaded using the references below. After that, run the installation command:
$ sudo dpkg -i stella_6.2.1-1_amd64.deb
The example above was an installation for the Ubuntu 20.04 system. For installation on other systems, refer to the emulator reference manual. If you don't prefer to run the codes on the machine, you can also run them in online IDEs like Javatari or 8bitworkshop.
The file vcs.h contains useful definitions of important memory space addresses. All we need to do is to add include "vcs.h" in beginning of our main assembly files. If we also add include "macro.h" we can call some useful macros that will make our lives easier. The file macro.h can be downloaded here.
| Address | Type |
|---|---|
| 0000-002C | TIA (write) |
| 0030-003D | TIA (read) |
| 0080-00FF | RIOT (RAM) |
| 0280-0297 | RIOT (I/O,Timer) |
| F000-FFFF | Cartridge (ROM) |
The specifications say that the Atari 2600 has 128 bytes of RAM memory… so how come the page zero addresses go from $00 to $FF? The Atari 2600's memory is indeed 128 bytes, and our page zero is 256 addresses long ($00-$FF). The 6502 chip has a 16-bit address bus giving it 256 available addresses (0 to 255). These addresses inform the CPU on where in memory a byte of data is located. But since the process of locating data is handled by the processor and not the actual memory (RAM), we end up with two different values. The Atari 2600 only had 128 bytes of RAM but had the ability to register the location of up to 256 total bytes of data. The remaining memory address locations are given to the TIA (read/write) and the RIOT (I/O,Timer).
After several lessons of Assembly language for 6502, it is proposed to carry out a final project. The project can be consulted in the archive bomber.asm. In this project, a bomber game focused on the logic of language is carried out. Below is a gif of the final project.
As can be seen, the jet is able to move in all possible directions and also launches shots. The bomber appears in random positions defined in the code. It is not possible to predict its position immediately. The top left on the top is the score that the player makes when shooting the bomber and on the right is the execution time (simulated). A game over situation is simulated when the jet hits the bomber and fills the screen in red. After that, the scoreboard is restarted.
Also remembering that sounds can be enabled in the game. The sound mimics a jet's turbine. The higher the jet is, the higher the sound will be. Uncomment the code blocks below:
- 1st Code snippet:
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;; Calculations and tasks performed in the VBlank ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; lda JetXPos ldy #0 jsr SetObjectXPos ; set player0 horizontal position <jump subroutine> lda BomberXPos ldy #1 jsr SetObjectXPos ; set player1 horizontal position <jump subroutine> lda MissileXPos ldy #2 jsr SetObjectXPos ; set missile horizontal position <jump subroutine> jsr CalculateDigitOffset ; calculate scoreboard digits lookup table offset jsr GenerateJetSound ; configure and enable our jet engine audio sta WSYNC sta HMOVE ; apply the horizontal offsets previously set lda #0 sta VBLANK ; turn off VBLANK
- 2st Code snippet:
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;; Generate audio for the jet engine sound based on the jet y-position ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;; The frequency/pitch will be modified based on the jet current y-position. ;; Normally, the TIA audio frequency goes from 0 (highest) to 31 (lowest). ;; We subtract 31 - (JetYPos/8) to achieve the desired final pitch value. ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; GenerateJetSound subroutine lda #3 sta AUDV0 ; set the audio volume register lda #8 ; white noise (explosions) sta AUDC0 ; set the audio control register to white noise lda JetYPos ; loads the accumulator with the jet y-position lsr lsr lsr ; divide the accumulator by 8 (using right-shifts) sta Temp ; save the Y/8 value in a temp variable lda #31 sec sbc Temp ; subtract 31-(Y/8) sta AUDF0 ; set the audio frequency/pitch register rts
- Pikuma: 6502 Assembly Language for the Atari 2600
- DASM: 8-bit macro assembler
- Instructions and Opcodes for 6502
- Stella: A multi-plataform Atari 2600 VCS emulator
- Javatari: The online Atari 2600 emulator
- 8-bit code IDE in your browser
- Editor for atari 2600 graphics
- Asymetric Playfield TIA timing
- Gopher 2600 VCS Emulator
- AtariAge forum
- C64-Wiki