This paper critiques and extends Lamport’s taxonomy of asynchronous registers, [8], [9]. This extended taxonomy is used to characterise Simpson’s 4-slot asynchronous communication mechanism (ACM), [15], [16], [17], [18], [19]. A formalisation of the Lamport atomic property and Simpson’s original 4-slot implementation is given in the PVS logic [12]. We prove that the 4-slot is atomic using Nipkow’s retrieve relation proof rules, [10], [11], [7]. A description is given of the formal proofs, which have been discharged in the PVS theorem prover [13].