Salmon Run - Atari 800 Disassembly

Salmon Run is a 1982 Atari 800 game where you play as a salmon swimming upstream to spawn. You dodge rocks, waterfalls, and predators like bears and fishermen while controlling your speed and leaping from the water. The game features distinctive water sound effects generated by the POKEY chip that go beyond what seems like it should be possible on this relatively primitive machine.

For more background on the game and its creator, see this Ars Technica article.

Screenshots

The Sound-Graphics Linkage

After completely disassembling Salmon Run (4,443 lines of 6502 assembly), the disassembly revealed something interesting: you can run the sound code without the graphics, but it's not exactly the same.

How It Works

The Display List Interrupt (DLI) - the code that runs during screen drawing - also handles:

Fish sprite animation
Color changes
Collision detection
Wave/splash effects
and the stream sound frequency updates

The code path through the DLI varies based on game state, which means the time between sound updates changes dynamically. When you move the fish, the sound texture shifts. When waves splash, the timing changes.

Technical Analysis

POKEY Sound Chip Configuration

Register	Value	Description
AUDCTL	$00	Standard 64kHz poly clock
AUDC1	$A4	Pure tone (distortion $A), volume 4
AUDF1	RANDOM & $1F	Random 0-31, updated via DLI
AUDC2	$A0	Silent (volume 0) during idle

The DLI Code (Excerpt)

; SOUND UPDATE (lines 2615-2622 of disassembly)
LDA RANDOM          ; Get hardware random number
AND #$1F            ; Mask to 0-31
STA AUDF1           ; <-- This creates the stream texture!
CMP #$0E            ; If frequency >= 14...
BCS L5026           ; ...skip channel 2
LDA RANDOM
AND #$3F
STA AUDF2           ; Channel 2 (silent during idle)

; After sound update: 400+ lines of game logic that
; affect timing between the next AUDF1 update!

; CHECK FISH STATE
LDA L06A2           ; Fish movement flag
BEQ L505C           ; If zero, skip fish code
JMP L5406           ; DIFFERENT CODE PATH = DIFFERENT TIMING!

; CHECK WAVE/SPLASH
LDA L3FF2           ; Wave counter
BNE L5066
LDA L3FF3           ; Splash counter
BEQ L5069
JMP L5110           ; SPLASH PATH = DIFFERENT TIMING!

Key Findings

Memory Efficiency: No separate sound driver needed
CPU Efficiency: Sound updates happen "for free" during display processing
Organic Feel: Sound subtly varies with gameplay
Possible to imitate but perfect replication requires the original graphics: You can't just "rip" the sound - you'd have to rip the entire game