Arnold comes with a collection of tests which are used to verify it's operation against real CPC, Plus and expansions.
These tests aim to test every feature and bug of the hardware including rarely used ones.
It also attempts to test against show the differences with the ICs used by the Amstrad which came from different manufacturers (8255, CRTC etc)
* Acid tests: [http://www.cpctech.org.uk/test.zip Acid tests]