INT 21 - COMBI-disk v1.13 - CONTROL COMBI-disk
        AX = 4405h
        BL = drive number (00h = default, 01h = A:, etc) for RAM disk
        CX = number of bytes to write
        DS:DX -> buffer containing command packet (see below)
Return: CF clear if successful
            AX = number of bytes actually transferred
        CF set on error
            AX = error code (01h,05h,06h,0Dh) (see AH=59h)
SeeAlso: AX=4404h"COMBI"

Format of command packet:
Offset  Size    Description
 00h    WORD    version
 02h    BYTE    command code
                80h flush cache
                81h change options byte
                82h shrink memory
                83h expand memory
                84h get dirty block information
                85h reset write errors
                86h reset counters
---command code 81h---
 03h    BYTE    new options byte (see below)
---command code 82h---
 03h    WORD    number of KB to release
---command code 83h---
 03h    WORD    number of KB to expand
---command code 84h---
 03h    DWORD   -> buffer for block info (see below)
---command code 85h---
 03h    DWORD   block ID
---command code 86h---
 03h    BYTE    which counters to reset
                bit 0: hard disk read counts
                bit 1: hard disk write counts
                bit 2: RAM disk read/write counts
Note:   multiple commands may be placed in a single packet by stringing
          together as many command/argument pairs as desired

Bitfields for options byte:
 bit 0  cache off
 bit 1  cache frozen
 bit 2  write caching enabled
 bit 3  delayed writing disabled
 bit 5  fix memory allocation (no XMS lending)
 bit 6  no 'sector not found' error

Format of block info:
Offset  Size    Description
 00h    DWORD   block ID
 04h    BYTE    bitmask of valid sectors in block
 05h    BYTE    bitmask of dirty sectors in block
 06h    BYTE    last error returned by BIOS
 07h    BYTE    number of errors

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