INT 21 - SMARTDRV.SYS v3.x only - IOCTL - CACHE CONTROL
        AX = 4403h
        BX = handle for device "SMARTAAR"
        CX = number of bytes to write
        DS:DX -> SMARTDRV control block (see below)
Return: CF clear if successful
            AX = number of bytes actually written
               = 0000h if control block too small for given command
        CF set on error
            AX = error code (01h,05h,06h,0Dh) (see AH=59h)
SeeAlso: AX=4402h"SMARTDRV",INT 2F/AX=4A10h/BX=0000h

Format of SMARTDRV control block:
Offset  Size    Description
 00h    BYTE    function code
                00h flush cache
                01h flush and discard cache
                02h disable caching (flushes and discards cache first)
                03h enable caching
                04h control write caching
                05h set flushing tick count
                06h lock cache contents
                07h unlock cache contents
                08h set flush-on-reboot flag
                09h unused
                0Ah control full-track caching
                0Bh reduce cache size
                0Ch increase cache size
                0Dh set INT 13 chain address
---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

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