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Physical Memory Pools (a.k.a. PMPs) are a type of Dynamic Area introduced in RISC OS 5.23 that allow for full control over the logical to physical mapping of memory that is allocated to the PMP.
When you create a regular dynamic area, you must specify its maximum logical size. This controls how much of the logical address space should be reserved for the dynamic area. It also places an implicit limit on how much physical memory can be allocated to the area – the number of physical RAM pages which are allocated to the area is always the same as the number of logical RAM pages.
When you create a PMP, you specify separate physical and logical limits. This can be useful when you have a large amount of data but you only need to access a small amount of it at a time – you can create a PMP with a large physical size but a small logical size, and use the logical area as a window to peer into the data contained in the physical pages.