tests/host/ compiles the real core, variant and library code for the PC against a fake pico-sdk (tests/host/fake_sdk/) that records hardware state, plus behavioural models of the on-board chips (tests/host/models/). Tests use doctest (extern/doctest, pinned).

In pico-sdk 2.2.0 the host platform replaces only gpio, irq, sync, timer, uart and divider, mostly with empty functions that keep no state; it has no ADC, PWM, SPI, I2C, clocks or TinyUSB, and supports Linux/macOS/Cygwin/WSL rather than plain Windows. We would still have had to fake most of what the code touches, and tests could not observe the stubs.

The fake covers only the SDK functions this code calls (it fails to compile when new ones are used, which is the cue to extend it). Chip models encode our reading of the datasheets, so hardware tests are still needed for register addresses, timing and electrical behaviour.