Format of DESQview system memory block header:
Offset	Size	Description	)
 00h	WORD	segment of next header or 0000h
 02h	WORD	segment of previous header or 0000h
 04h	WORD	size of block in paragraphs, including header
 06h	BYTE	availability flag (00h in use, 01h free)
Note:	this header is located one paragraph before the memory block proper
SeeAlso: #00423