Is there a way to limit Fast RAM used by Amiga OS 3.1, i.e. if A500 with 030 MMU detects full 64MB in address ranges 0x40000000 to 0x44000000. Instead, can I choose to map only 8MB in lower ranges at system boot?

2 Answers 2


You can create a small C or assembly program which performs an AllocAbs call (with location & size parameters), to reserve absolute memory and put it first thing in your startup sequence.

If it succeeds, it blocks the memory you've allocated at the chosen address.

Oh you don't even have to write it as it already exists:


Blockmem is a small utility which takes two memory addresses, and allocates all of the memory between them, preventing the system (and OS friendly programs) from using that area of memory.

it was designed to prevent the OS from using faulty memory, but you can use it for whatever you want.


It's generally not possible to do this. Memory boards will add their memory into the OS during power-on using Amiga AutoConfig or a similar mechanism. Changing the memory configuration after boot is difficult if not impossible as the OS and apps may already be using some of that memory.


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