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From: ralf@b.gp.cs.cmu.edu (Ralf Brown)
Newsgroups: comp.os.msdos.programmer
Subject: Re: SUMMARY:  MSDOS memory FREE functionc
Message-ID: <10886@pt.cs.cmu.edu>
Date: 28 Oct 90 02:34:26 GMT
References: <1990Oct28.001029.19179@watdragon.waterloo.edu>
Organization: Carnegie-Mellon University, CS/RI
Lines: 32

In article <1990Oct28.001029.19179@watdragon.waterloo.edu> dvadura@watdragon.waterloo.edu (Dennis Vadura) writes:
}Summary of MSDOS FREE and memory coalescing.  A while back I posted
}a request if anyone knows how to get DOS FREE (int 21, ah=0x49) to coalesce
}adjacent blocks.  I received several responses and based on those come to these
}conclusions.  DOS FREE on its own does NOT coalesce adjacent free memory blocks.
}(in my opinion this is stupid).  DOS ALLOC (int 21, ah=0x48) of a block
}whose size is 0xffff seems to do the trick.  FREEING that resulting block has
}the effect of coalescing all previously allocated but now free adjacent blocks.

I've since looked at the actual DOS memory management code, and can state that
generic MSDOS 3.30 works as follows:
	no coalescing on FREE
	all possible blocks coalesced on *any* ALLOC
	free blocks immediately following a block being RESIZEd are coalesced

Also, the coalescing on ALLOC is done by calling a coalesce routine on each
free block, to coalesce any immediately following free blocks (same code that
is called by the RESIZE coalescing).  Since the DOS memory chain is not
doubly-linked, omitting the coalescing on FREE is an overall performance
win, since it would be necessary to walk the memory chain to find the
preceding memory block (which might already be free and need to be coalesced).
ALLOC walks the entire memory chain anyway....

Note: if the alloc of FFFFh paragraphs is ever successful, your memory chain
must be corrupted, since that is 1M-16 bytes....  DOS doesn't allocate any
memory if there is no sufficiently large block available.  Thus there is no
need for the call to FREE after the alloc of FFFFh paragraphs.
-- 
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