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OS_Mal should always be called with A=0. Usage of this option is not recommended and should never be used. When MM_MUL was set during pool opening in OS_Mop, slot selection of allocation can be forced by setting slot number in A (1-3) ORed with MA_SLT.
Allocation internals
Chunk and page allocation begins from the latest bank of the slot and goes downward. Block allocation begins at the first bank and goes upward. It avoids fragmentation (big blocks inside the file system). In the same principle, MM_EXC forces in priority most free banks and MM_MUL fills the slot downward. Memory allocation for the AZ module assembler is a good example, I performed some tests.
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