Some fun messing around with 6502 assembly on the Atari 2600 (VCS).
This isn't quite a game; its more of a screw around project I kicked around.
At this point its only a skeleton and has the following:
- Scoreboard and timer BCD display
- Player movement with joysticks
- Graphics and color binary loading
- Reset button
- A bit of black and white TV support
- Random numbers via Linear Feedback Shift Register (LFSR)
I really wanted to do more with this. But, truthfully I just don't feel like working on it anymore. Its time for this to go on the shelf.
Yeah...this is all I have after ~2 weeks of messing around. This stuff is difficult for me.
- Arrow keys - joystick
- Spacebar - fire
- F2 - reset
- F3 - color TV
- F4 - black and white TV
- PAUSE - pause emulator
- Full black and white TV support
- Player and enemy projectiles
- Basic pathfinding for Enemy
- Difficulty switches increase enemy speed
- Random generated playfield obstacles
+---------------------------+
| TIA Registers | $00-$7F
+---------------------------+
| PIA RAM | $80-$FF
+---------------------------+
| ??? |
+---------------------------+
| PIA Ports and Timer | $280-$297
+---------------------------+
| ??? |
+---------------------------+
| Cartridge ROM | $F000-$FFFF
+---------------------------+
+-------------------------------------------------+ <-----+
| Vertical Sync | 3 lines |
| | |
+-------------------------------------------------+ |
| Vertical Blank | 37 lines |
| | |
+--------------------+----------------------------+ +-- 262 lines
| Horiz. | Visible | |
| Blank | Frame | 192 lines |
| | <-- 160 color clocks --> | |
+--------------------+----------------------------+ |
| Overscan | 30 lines |
| | |
+-------------------------------------------------+ <-----+
|-------------- 228 color clocks ----------------|
|--------------- 76 CPU cycles -----------------|
Main reference: https://atariage.com/forums/profile/3056-spiceware/ . This guy is absolutely incredible with 6502 asm.
- 6502 instruction set with cycle counts - https://www.masswerk.at/6502/6502_instruction_set.html
- Atari 2600 killer hacks - https://atariage.com/forums/topic/71120-6502-killer-hacks/page/3/?tab=comments#comment-1071314
.byte $2C- "skip 2 bytes" pseudo-instruction (ABS BIT)
- Counting cycles on Atari 2600 - https://www.randomterrain.com/atari-2600-memories-guide-to-cycle-counting.html
- DASM Macro Assembler - https://github.com/munsie/dasm
- Introduction to Atari2600 - https://www.randomterrain.com/
- Sprite ASM Generator - https://alienbill.com/2600/playerpalnext.html
- Stella Emulator - https://stella-emu.github.io/
- Unofficial 6502 opcodes - https://wiki.nesdev.com/w/index.php/Programming_with_unofficial_opcodes
dcp $0F- decrement at memory address and compare with accumulator