A Short Description of the Pascal Heap Manager

 The memory manager manages a contiguous area of memory as a heap, using a
boundary tag technique.  The  area can be expanded anytime, but the expansion
must be contiguous.
 The heap is completely subdived into objects, which can be any size >=3 words.
Unallocated blocks kept in doubly linked lists.  Adjacent objects on the
free list are combined to minimize fragmentation.

free storage list looks like:
	word 0:		[tag bit][size of block],,[link to next block]
	word 1:					  [link to previous block]
	.
	.
	.
	word N-1:	[tag bit][size=N]	,,0
allocated storage looks the same, except that there are no links to
other blocks.

  The tag bits allow DISPOSE to determine if it should do compaction
without searching the free list, and also make it possible to
check for various errors, like killing things twice.


ENTRY POINTS:

NEW(size)	return a pointer to a block of exactly "size"

		Algorithm: Search the free list for the first item which
			   is either exactly the right size, or at least
			   3 words larger.

			   If none found, expand heap by enough, try again.

			   If larger, split the block into two; one of
			   which is the right size, and DISPOSE the remainder.

			   Return the exact size block.

DISPOSE(pointer,size)
		Return a block to free storage

		Algorithm:
			Check tag words for consistency with
			"pointer", "size", and each other.
			Die violently if not all OK.

			Check tag words of previous and subsequent
			blocks in memory.  For any that are on
			the free list (as determined by the tag bits):
			unlink and merge with the block being returned.

			Link newly freed block onto the END of the free list.
			This implies that returned chunks tend to be used in
			round robin fashion.

CASIZE()	Check the validity of the heap's data base for any
		inconsistency that is implied by the bookkeeping scheme.
		If all OK, return USED,,FREE

		Data Base:
			AVAIL: BLOCK 1	; first entry on free list
			       BLOCK 1	; tail of free list
			BEGMEM:BLOCK 1	; pointer to block allocated at lower
					;  boundary of the heap
			ENDMEM:BLOCK 1	; pointer to block allocated at upper
					;  boundary of the heap
		Algorithm:
			Starting at BEGMEM until ENDMEM
			Check each object's tags for mutual consistency.
			Check that no two adjacent blocks are on the free list
			Check that we remain <= ENDMEM
			end.