sgdk-multitasking

SGDK multitasking

Use time left after foreground work. The Mega Drive still has one 68000: a background task makes a long calculation interruptible; it does not make that calculation cheaper or guarantee a completion deadline.

Start from the user's game and its existing ownership and frame loop. Spare Time Lab demonstrates execution contracts, not a prescribed game, UI or architecture.

Workflow

1. Pull ewrogers/sgdk:latest and inspect its task.h, task.s, frame waits and

startup stack setup. Record versions. Online master may differ from the image.

1. First consider a small bounded step each frame. Use the SDK task when a pure

CPU routine, such as a synchronous decoder, needs to continue across frames without rewriting its internal state machine. Short jobs may not justify it.

1. Reserve foreground time first: gameplay, input, rendering preparation, audio

commands and VBlank service. Choose a deadline/fallback for late results. A busy foreground may leave the worker no useful time.

1. Keep one persistent, non-returning callback. Give it an audited stack and one

explicitly owned job. Preserve its PC/stack when holding work; TSK_stop() followed by TSK_userSet() is not pause/resume.

1. Restrict worker code to immutable inputs and private RAM. Audit every called

routine for global state and hardware access. Keep VDP, DMA, input, audio, allocation and interrupt control in the foreground. Keep ROM mappings stable.

1. Publish complete output through a documented single-CPU handshake. Cancellation

is a request until the worker acknowledges it. Keep queued DMA sources alive and immutable until the foreground knows transfers have drained.

1. Test deterministic bytes, rejected requests, cancellation boundaries, retry,

guards and publication. Build release/debug on ARM64 and AMD64. Check actual frame pacing, input, stack headroom and PAL/NTSC behavior in an emulator; console interrupt timing and audio contention still need hardware.

Read SDK execution and stacks before using TSK APIs or combining them with interrupts. Read jobs and ownership when designing the mailbox, cancellation, deadlines and output handoff.

Spare Time Lab unpacks verified APLIB data and computes a weather map while a small foreground activity continues. It includes its own source generator and build instructions; no other installed skill is required.

Boundaries

• sgdk-compression selects codecs, verifies decoded bytes and measures costs.

This skill controls when CPU work runs; ordinary unpack() remains synchronous to its caller even when that caller is a background task.

• sgdk-streaming owns caches, residency and bounded graphics uploads. A worker

can prepare bytes; the foreground streaming owner decides when to publish them.

• sgdk-ai owns detection/pathfinding algorithms; prefer bounded searches before

using spare-time execution. Never apply an obsolete result to a changed world.

• sgdk-memory, sgdk-performance, sgdk-testing own broader budgets and QA.

Supply stack/buffer costs and completion measurements, not a competing clock.

• sgdk-raster, sgdk-audio, sgdk-cartridge own raster timing, sound drivers and

bank mappings. A task is not permission to access those subsystems concurrently.

These links describe responsibilities, not mandatory companion installations.