Format of Overlay Asynchronous Control Block (OACB):
Offset	Size	Description	)
 00h	DWORD	link to next OACB, 0000h:0000h if last (filled by VLM.EXE)
 04h	DWORD	callback address or 0000h:0000h
 08h	BYTE	InUse flag (00h if complete) (set by VLM.EXE)
 09h	BYTE	flag, reserved for VLM use
 0Ah	WORD	destination VLM
 0Ch	WORD	destination function
 0Eh	WORD	temporary storage for VLM.EXE
 10h  6 BYTEs	reserved
 16h  6 DWORDs	EAX,EBX,ECX,EDX,ESI,EDI
 2Eh  4 WORDs	DS,ES,FS,GS (FS and GS not used)