This direct link between code and component is the difference between a working Speccy and a pile of spare parts.
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If you want, I can provide:
An unpublished ROM used by engineers for hardware verification. It occupies only 8KB and is a common target for those looking for the "original" diagnostic feel. Sinclair Test Program (1985): zx spectrum test program link
Deep RAM soak tests, keyboard, and interrupts. This direct link between code and component is