Format of SMARTDRV control block:
Offset	Size	Description	)
 00h	BYTE	function code (see #01522)
---functions 00h-03h,06h,07h---
 no additional fields
---function 04h---
 01h	BYTE	write caching control action
		00h turn off write-through
		01h turn on write-through
		02h turn off write buffering (also flushes cache)
		03h turn on write buffering (also flushes cache)
---function 05h---
 01h	WORD	number of timer ticks between cache flushes
---function 08h---
 01h	BYTE	new flush-on-reboot flag (00h off, 01h on)
---function 0Ah---
 01h	BYTE	full-track writes are
		00h not cached
		01h cached
---functions 0Bh,0Ch---
 01h	WORD	number of 16K pages by which to increase/reduce cache size
---function 0Dh---
 01h	DWORD	new address to which to chain on INT 13
Note:	the previous address is not preserved