When supported by the driver, this subfunction can be used to obtain hardware-dependent status and availability information that is not provided by other MS-DOS function calls.Transfers control data from a block-device driver directly into an application program's buffer. The length and contents of the data are specific to each device driver and do not follow any standard format. This function does not necessarily result in any input from the physical device.
Call with:
AH = 44H
AL = 04H
BL = drive code (0 = default, 1 = A, 2 = B, etc.)
CX = number of bytes to read
DS:DX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
AX = bytes transferred
and buffer contains control data from device driver
If function unsuccessful
Carry flag = set
AX = error code
Notes:
When supported by the driver, this subfunction can be used to obtain hardware-dependent status and availability information that is not provided by other MS-DOS function calls.
Block-device drivers are not required to support IOCTL Subfunction 04H. If this subfunction is requested and the driver does not have the ability to process control data, control returns to the program with the carry flag set and error code 0001H (invalid function) in register AX.
Example:
Read a control string from the block-device driver for drive C into the buffer buff.
buflen equ 64 ; length of buffer
ctllen dw ? ; length of control string
buff db buflen dup (0) ; receives control string
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mov ax,4404h ; function & subfunction
mov bl,3 ; drive C = 3
mov cx,buflen ; buffer length
mov dx,seg buff ; buffer address
mov ds,dx
mov dx,offset buff
int 21h ; transfer to MS-DOS
jc error ; jump if read failed
mov ctllen,ax ; save control string length
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