A boot counter kept in the GSU's battery-backed Game Pak RAM: switch the console off and on, the number is one more.

A Super FX cartridge has one RAM, the GSU's Game Pak RAM at $70:0000; with a battery it is also the save memory (header $15, size at $FFBD). The sram module addresses it like any other save memory. The block lives at $E000, above the two 16 KB framebuffers a game that presents frames would use (chips/superfx_game_skeleton). The GSU is never started: its program here is a single STOP, linked because a Super FX build carries exactly one.

This is the console row for the Super FX save path (docs/HARDWARE_VERIFICATION.md, row 26); on luna the chain j_gsu_save_boot1.toml → k_gsu_save_boot2.toml asserts 1 then 2 across a battery file.

SNES Concepts

Game Pak RAM: the GSU's work RAM and the battery-backed save memory
Keep the save block out of the framebuffers the GSU draws into
A magic word marks a written block; a fresh RAM holds garbage, not zeros

Build & Run

make -C examples/chips/superfx_save

testing/bin/luna run examples/chips/superfx_save/superfx_save.sfc --until-frame 120 --screenshot out.png

Modules

console, dma, background, text, sram (LIB_MODULES), plus superfx added by USE_SUPERFX := 1; USE_SRAM := 1 (header $15: GSU + battery).